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biological vocabulary-5

By admin | October 15, 2010

heterophytic a. with two kinds of spores, borne by different sporophytes.
heteroplanogametes n.plu. motile gam­etes that are unlike one another.
heteroplasia n. the development of a tissue from another of a different kind.
heteroplasm n. tissue formed in abnormal places.
heteroplasmon n. cell containing a mixture of genetically different cytoplasms.
heteroplastic a. (1) appl. grafts of unre­lated material; (2) appl. grafts between individuals of different species or genera.
heteroploid a. (1) having an extra chromo­some through non-disjunction of a pair in meiosis; (2) not having a multiple of the basic haploid number of chromosomes; (3)



n. an organism having heteroploid nuclei. heteropolymer n. polymer composed of different types of subunit.
heteropolysaccharide n. any polysac­charide made up of different types of monosaccharide subunit.
Heteroptera n. in some classi.cations, an order of insects including the water boatmen, capsids and bed bugs.
heteropycnotic a. appl. regions of chro­mosomes that remain compact and densely staining, even in interphase nuclei, when the remainder of the chromatin is more dispersed.
heterorhizal a. with roots coming from no determinate point. heterosexual a. of, or pert. the opposite sex, appl. e.g. hormones. heterosis n. (1) cross-fertilization q.v.;
(2) hybrid vigour q.v.
heterosomal a. (1) occurring in, or pert., different bodies; (2) appl. rearrangements in two or more chromosomes.
heterosporangic a. bearing two kinds of spores in separate sporangia.
heterosporous a. appl. plants that produce two kinds of spores—megaspores and microspores—by meiosis. Applies to all seed plants, some ferns and club mosses. Megaspores give rise to the female gametophyte, microspores to the male gametophyte, both much reduced in these plants. n. heterospory.
heterostemonous a. with unlike stamens.
heterostrophic a. coiled in a direction opposite to normal.
heterostyly n. condition in which indivi­duals within a species differ in the length of style in their .owers, as in primroses with their pin-eyed (long-styled) and thrum-eyed (small-styled) .owers. Anthers in one type of .ower are at the same level as stigmas in the other, thus ensuring cross-pollination.
a. heterostylic, heterostylous.
heterosynapsis n. pairing of two non­homologous chromosomes.
heterosynaptic facilitation see pre­synaptic faciliation.
heterotaxis n. abnormal or unusual arrange­ment of organs or parts.
heterotetramer n. a protein composed of four subunits and having more than one type of subunit.
heterothallic a. appl. cell, thallus or mycelium of alga or fungus which can only undergo sexual reproduction with a mem­ber of a physiologically different strain. cf. homothallic. see also mating type.
heterotic a. pert. cross-fertilization, appl. vigour: hybrid vigour.
heterotopic a. in a different or unusual place, appl. transplantation of tissue or organ.
heterotopy n. (1) displacement; (2) abnor­mal habitat.
heterotrichous a. (1) having two types of cilia; (2) having a thallus consisting of prostrate and erect .laments, as in certain algae.
heterotrimer n. a protein composed of three subunits and having more than one type of subunit.
heterotrimeric G proteins class of guanine-nucleotide binding proteins com­posed of three subunits: a, ß and .. They are associated with the cytoplasmic face of the plasma membrane of mammalian cells and are involved in transmitting signals from certain cell-surface receptors to intracellular pathways. In the inactive state, the a subunit has GDP bound. On activation, the a subunit binds GTP in place of GDP and dissociates from the ß. subunit. The GTP-bound a subunit is usu­ally, but not always, the effector subunit. The a subunit contains intrinsic GTPase activity which converts the bound GTP to GDP, eventually inactivating the subunit, which then reforms the heterotrimeric G protein. see also Gi, Gp, Gq, Gs, G protein-coupled receptors, GTPase-activating pro­tein, guanine-nucleotide exchange factors, gustducin, small G proteins, transducin.
heterotroph n. organism requiring organic compounds as a carbon source. a. heter­otrophic. cf. autotroph. see also chemo­organotroph, mixotroph, photoheterotroph.


heterotropous a. pert. ovule with micropyle and hilum at opposite ends in a plane parallel with that of placenta.
heterotypic a. (1) pert. mitotic division in which daughter chromosomes remain united and form rings; (2) pert. fusion of membranes from different intracellular compartments; (3) heterophilic q.v.
heterotypical a. appl. genus comprising species that are not truly related.
heteroxylous a. appl. wood containing vessels and .bres as well as tracheids.
heterozygosis n. (1) formation of a zygote by two genetically different gametes; (2) the condition of being heterozygous.
heterozygosity n. proportion of heter­ozygotes for a given locus in a population.
heterozygote n. heterozygous organism or cell. alt. hybrid. cf. homozygote.
heterozygote advantage the case where the heterozygote for a given pair of alleles is of superior .tness than either of the two homozygotes.
heterozygous a. appl. diploid organism, or cell or nucleus, that has two different alleles at a given gene locus. alt. hybrid. cf. homozygous.
heuristic a. appl. methods of problem-solving that use past experience and a trial-and-error approach.
HEV high endothelial venule q.v.
hexa-pre.x derived from Gk hex, six, signifying having six of, arranged in sixes.
hexacanth a. having six hooks.
hexactinal a. with six rays.
hexactine n. a sponge spicule with six equal and similar rays meeting at right angles.
Hexactinellida n. class of Porifera, the glass sponges or hexactinellid sponges, typic­ally radially symmetrical with a skeleton of large six-rayed spicules of silica, often fused to form a three-dimensional network.
hexactinian a. with tentacles or mesen­teries in multiples of six, appl. certain coelenterates.
hexacyclic a. having .oral whorls consist­ing of six parts.
hexaene n. sponge spicule like a trident but with six branches.
hexagynous a. (1) having six pistils or styles; (2) with six carpels to a gynoecium.
hexamer n. (1) a protein with six subunits;
(2) a sequence of six nucleotides or six amino acids. hexamerous a. occurring in sixes, or
arranged in sixes. hexandrous a. having six stamens. hexapetaloid a. with petaloid perianth of
six parts. hexapetalous a. having six petals. hexaphyllous a. having six leaves. hexaploid (1) a. having six sets of chromo­
somes; (2) n. an organism having six times
the haploid chromosome number. hexapod a. having six legs. hexapterous a. having six wings or wing­
like expansions.
hexarch a. (1) appl. stele having six alternating xylem and phloem groups; (2) having six vascular bundles.
hexasepalous a. having six sepals. hexaspermous a. having six seeds. hexasporous a. having six spores. hexastemonous a. having six stamens. hexaster n. a hexactine spicule in which
the rays branch and produce star-shaped structures. hexastichous a. having the parts arranged in six rows.
hexokinase n. enzyme that catalyses the phosphorylation of glucose and some other hexose sugars. EC 2.7.1.1.
hexosamine n. amino sugar in which the sugar is a hexose. Examples are gala­ctosamine, glucosamine.
hexosaminidase n. enzyme which cata­lyses the cleavage of a terminal hexose amino sugar from compounds such as gangliosides.
hexosan n. polysaccharide made of linked hexose subunits. Examples are starch, gly­cogen, inulin, cellulose.
hexose n. monosaccharide containing six carbon atoms (formula C6H12O6). Examples are glucose, fructose, galactose, mannose.
hexose monophosphate shunt pentose phosphate pathway q.v.
Hfr strain bacterial strain in which the F factor is integrated into the chromosome, leading to an increased frequency of transfer of chromosomal genes.
Hg symbol for the chemical element mer­cury q.v. HGF hepatocyte growth factor q.v.
HGH human growth hormone.
HGPRT hypoxanthine guanine phospho ribosyltransferase q.v.
HI haemagglutination inhibition q.v.
hiatus n. any large gap or opening.
hibernaculum n. a winter bud.
hibernal a. of the winter.
hibernating glands former term for brown adipose tissue q.v.
hibernation n. the condition of passing the winter in a resting state of deep sleep, when metabolic rate and body temperature drop considerably. Only a few small mammals,
e.g. some rodents, hedgehogs, bats, and other small insectivores, undergo a “right” hibernation. Obligate hibernators enter hibernation spontaneously as the result of a circannual behavioural rhythm. Facultative hibernators enter hibernation when food becomes scarce and temperatures drop below a certain level. Related conditions include winter torpor in reptiles and winter lethargy in larger mammals, e.g. bears, badgers, skunks and racoons. see also aestivation.
hidden Markov model (HMM) type of statistical model that is used in bio­informatics e.g. to construct large multiple alignments that include very distantly related sequences.
hidrosis n. sweating, perspiration.
hiemal a. pert. winter, appl. aspect of a community.
hiemilignosa n. monsoon forest composed of small-leaved trees and shrubs which shed their leaves in the dry season.
hierarchy n. (1) see dominance systems;
(2) a natural classi.cation system in which organisms are grouped according to the number of characteristics they have in common and ranked one above another.
high-density lipoproteins (HDL) group of lipoproteins found in blood plasma, which are rich in phospholipids and cholesterol. The protein component is synthesized in the liver.
high endothelial venule (HEV) blood capillary found in lymphoid tissues, whose walls are composed of high endothelial cells.
high-energy bond a misleading term denoting a chemical bond whose breakage releases a large amount of free energy, such as the bond between the two terminal phos­
phate groups in ATP.
highly repetitive DNA see repetitive DNA.
high mobility group proteins HMG pro­teins q.v.
high-performance liquid chromato­graphy (HPLC) type of column chromato­graphy using small-particle media and a mobile phase pumped through at a con­stant rate, used for analytical separations.
hilar a. of or pert. a hilum.
hiliferous a. having a hilum.
Hill coef.cient (nH) a number obtained from equilibrium binding experiments that gives information on the number of bind­ing sites for a ligand present on a protein and about whether they show cooperativity.
Hill reaction the reaction showing that isolated chloroplasts could, on illumina­tion, cause the reduction of suitable elec­tron acceptors such as ferricyanide to ferrocyanide and generate oxygen, .rst demonstrated by Robert Hill in 1939.
hilum n. (bot.) (1) scar on ovule or seed where it was attached to ovary; (2) nucleus of a starch grain; (3) (zool.) notch, open­ing or depression in an organ, usually where a blood vessel or nerve enters. alt. hilus.
hindbrain n. cerebellum, pons and medulla oblongata. That part of the brain concerned with basic body activities independent of conscious control, such as regulation of muscle tone, posture, heartbeat, respiration and blood pressure. alt. rhombencephalon.
hind-gut (1) outgrowth of the yolk sac ex­tending into tail-fold in human embryo;
(2)
posterior part of alimentary tract;

(3)
proctodaeum q.v. hind-kidney metanephros q.v. hinge cells large epidermal cells which,
by changes in turgor, control rolling and unrolling of a leaf.
hinge ligament tough elastic substance that joins the two parts of a bivalve shell.
hinge region .exible part of an IgG or IgA antibody molecule, joining the antigen-binding arms to the stem.
hinge tooth one of the projections found on the hinge line, or line of articulation, of a bivalve shell.
hinoid a. with parallel veins at right angles to midrib, appl. leaves.
hip n. (1) (bot.) common name for the type of pome fruit produced by some members of the Rosaceae (e.g. roses); (2) (zool.) region of articulation of vertebrate hindleg with trunk. see also coxa.
hip girdle pelvic girdle q.v.
hippocampal commissure tract of .bres connecting the two cerebral hemispheres in the region of the hippocampal areas.
hippocampal formation brain region con­sisting of hippocampus and dentate gyrus.
hippocampal gyrus subiculum q.v.
hippocampus n. area in centre of cerebral hemisphere, lying around the thalamus and just above the corpus callosum. It is thought to be vital for learning and memory. Hurt to the hippocampus is associated with amnesia.
hippomorphs n.plu. group of the Perisso­dactyla including the extinct brontotheres and the horses (family Equidae).
hippuric acid benzoyl glycine, a constituent of the urine of herbivorous animals.
Hippuridales n. order of dicots, land, marsh or water plants, comprising the families Gunneraceae (gunnera), Haloragaceae and Hippuridaceae (mare’s tail).
hirsute a. hairy; (1) appl. birds, covered with hair-like feathers; (2) having stiff, hairy bristles or covering.
hirsutidin n. a blue anthocyanin pigment.
hirudin n. protein obtained from buccal secretions of leech, which inhibits action of thrombin on .brinogen, preventing clot­ting of blood.
Hirudinea n. class of carnivorous or ectoparasitic annelids, commonly called leeches, which have 33 segments, ciru­moral and posterior suckers and usually no chaetae.
His histidine q.v.
His’ bundle band of muscle .bres, with nerve .bres, connecting auricles and ven­tricles of heart. alt. atrioventricular bundle.
hispid a. having stiff hairs, spines or bristles.
histamine n. amine synthesized from histidine by decarboxylation and involved in producing in.ammation. It is produced by mast cells and is responsible for many of the symptoms of allergies. It causes contraction of smooth muscle of airways, and dilation of blood vessels, causing them to become leaky. Histamine is also produced by some neurons and acts as a neurotransmitter in the central nervous system.
histidine (His, H) n. amino acid with a basic side chain, constituent of protein, possibly essential in the human diet, pre­cursor of histamine.
histidine kinase see protein histidine kinase.
histioblast n. an embryonic cell of sponges.
histiogenic histogenic q.v.
histioid a. like a web.
histiotypic a. appl. uncontrolled growth of cells in tissue culture. cf. organotypic.
histoblast nests small groups of cells (histoblasts) in abdomen of dipteran larvae which develop into the adult epidermal structures of the abdomen at metamorphosis.
histochemistry n. the study of cells and tissues esp. in respect of their staining properties. a. histochemical. see also immunohistochemistry.
histocompatibility antigens cell-surface proteins that determine the acceptance or rejection of a tissue when transplanted into another individual of the same or another species. see MHC molecules, minor his­tocompatibility antigens.
histocompatible a. appl. tissue that is not rejected if transplanted into another indi­vidual. n. histocompatibility.
histogen n. zone of tissue in apical mer­istems in plants from which new tissue develops.
histogenesis n. development of tissues.
histogenic a. producing tissues.
histogram n. type of graphical representa­tion in which data are grouped in some way and represented as a set of columns, the height of each column being the amount or frequency of the data item in the group.
histoid histioid q.v.
histoincompatible a. appl. tissue that is rejected if transplanted into another indi­vidual. n. histoincompatibility.
histology n. the study of the detailed structure of living tissue, by staining and microscopy.
histolysis n. the dissolution of tissues.
histone n. any one of a set of simple basic proteins (H2A, H2B, H3, H4), rich in arginine and lysine, which are bound to DNA in eukaryote chromosomes to form nucleosomes q.v. Histone H1 binds to the linker DNA between nucleosomes.
histone acetylation the post-translational covalent addition of acetyl groups to spe­ci.c lysine residues in the amino-terminal tails of histone proteins. Acetylation of his­tone tails in chromatin modi.es the func­tional state of the chromatin by attracting additional proteins to the modi.ed site. Histones can also be acetylated on certain lysine residues before their incorporation into nucleosomes. This acetylation is only temporary and helps to mark newly syn­thesized chromatin.
histone acetyltransferase (HAT) enzyme that carries out histone acetylation, using acetyl-CoA as the acetyl donor. see also histone deacetylase.
histone deacetylase (HDAC) any of a class of enzymes that removes acetyl groups from acetylated lysines on histone proteins in chromatin, thus altering the functional state of the chromatin. see also histone acetyltransferase.
histone-like proteins small basic proteins present in prokaryotes, some of which are associated with bacterial DNA.
histone methylation the covalent post­translational addition of one or more methyl groups to lysine residues in the amino-terminal tails of histone proteins in chromatin. Additional proteins are attracted to the modi.ed site, altering the functional state of the chromatin.
histone methyltransferase enzyme that carries out histone methylation, using S­adenosylmethionine as the methyl donor. alt. histone methylase.
histone modi.cation covalent modi­.cation of histones in chromatin by the addition of acetyl groups, methyl groups or phosphoryl groups, which changes the properties of the chromatin in ways that affect the expression of the genes it contains.
histoplasmosis n. fungal disease of humans caused by systemic infection with the yeast Histoplasma capsulatum.
histotrophic a. pert. or connected with tissue formation or repair.
histotypic a. according to cell type, appl. aggregation of dissociated cells.
histozoic a. living within tissue, appl. the trophozoite stage of certain sporozoan parasites.
HIV human immunode.ciency virus q.v.
hives urticaria q.v.
HIV receptor the CD4 cell-surface protein on T cells, which acts as a receptor for entry of the human immunode.ciency virus (HIV).
HLA complex human leukocyte antigen complex. see major histocompatibility complex, MHC molecules. HLA-A, HLA­B, HLA-C are the human MHC class I molecules, HLA-DP, HLA-DQ, HLA-DR are the human MHC class II molecules.
HLH helix-loop-helix q.v.
H line H band q.v.
HMG proteins high mobility group proteins, the non-histone proteins in chromatin that have high mobility on electrophoresis.
HMM heavy meromyosin. see meromyosin.
hnRNA heterogeneous nuclear RNA q.v.
hoary n. greyish-white, having a frosted appearance.
hock n. in horses and other hoofed mam­mals, the joint on hindleg corresponding to the tarsal joint.
Hogness box TATA box q.v.
holandric a. transmitted from male to male through Y chromosomes, appl. sex-linked characters.
holandry n. having the full number of testes, as two pairs in oligochaete worms.
Holarctica n. zoogeographical region com­prising the Nearctic and Palaearctic regions.
holcodont a. having the teeth in a long continuous groove.
holdfast n. adhesive region by which an organism can attach itself to a surface, used esp. for the adhesive disc by which members of the brown seaweeds attach themselves to rocks.
holistic a. appl. explanations that attempt to clarify complex phenomena in terms of the properties of the system as a whole.
n. holism. cf. reductionist.
Holliday junction the four-armed structure formed between two double-stranded DNA molecules at the point of crossing-over during recombination. alt. Holliday intermediate.
holobasidium n. a single-celled basidium. cf. heterobasidium, phragmobasidium.
holobenthic a. living on sea bottom or in depths of sea throughout life.
holoblastic a. pert. cleavage of fertilized egg in which cleavage furrow extends throughout the whole egg. cf. discoidal, meroblastic, super.cial.
holobranch n. a gill in which gill .laments are borne on both sides.
holocarpic a. (1) having fruit body formed by entire thallus; (2) appl. parasitic fungi without rhizoids or haustoria, living in host cell.
Holocene n. recent geological epoch fol­lowing Pleistocene, started ca. 10,000 years ago. alt. Recent.
holocentric a. appl. chromosomes having a “diffuse centromere” so that when frag­mented each part of the chromosome behaves at mitosis as though it possesses a centromere.
holocephalian a. pert. cartilaginous .shes of the subclass Holocephali, the rabbit .shes, with crushing teeth, a whip-like tail and an operculum covering the gills. alt. chimaeras, rat-.sh.
holocephalous a. appl. a rib with a single head.
holochroal a. having eyes with globular or biconvex lenses closely crowded together, so that cornea is continuous over whole eye.
holocrine a. appl. glands whose secretion is accompanied by complete breakdown of the secretory cells, e.g. sebaceous glands.
holocyclic a. pert. or completing alternation of sexual or parthenogenetic generations.
holoenzyme n. complete, fully functional enzyme molecule, consisting of the enzymatic subunit (apoenzyme) and any prosthetic group, cofactor, or regulatory or accessory protein subunits required for full regulated function.
hologamodeme n. group of freely inter­breeding individuals of the same taxon in a local area.
hologamy n. (1) condition of having gam­etes similar to somatic cells; (2) fusion between mature individuals as in some protozoans.
holognathous a. having jaw in a single piece.
hologynic a. transmitted directly from fe­male to female, appl. sex-linked characters.
holomastigote a. having one type of .agellum scattered evenly over the body.
holometabolous a. appl. the orders of insects that undergo a full metamorphosis, with a four-stage life history (egg, larva, pupa, adult). They are the Neuroptera (alder.ies, lacewings), Mecoptera (scor­pion .ies), Trichoptera (caddis .ies), Lepidoptera (butter.ies and moths), Coleoptera (beetles), Strepsiptera, Hyme­noptera (ants, bees and wasps), Diptera (two-winged .ies) and Siphonaptera (.eas) (see individual entries).
holomictic a. appl. lakes that are strati.ed seasonally, because of thermal differences. cf. meromictic.
holomorph n. all the possible forms of a particular fungus. cf. anamorph, teleomorph.
holomorphosis n. regeneration in which the entire part is replaced. a. holomorphic.
holoparasite n. parasite that cannot exist independently of its host, or on a dead host.
holophyte n. any green, phototrophic, inde­pendent plant.
holophytic a. (1) autotrophic q.v.; (2) phototrophic q.v.
holoplankton n. organisms that complete their life-cycle in the plankton.
holopneustic a. with all spiracles open for respiration.
holoptic a. with eyes touching or nearly touching on top of head.
holoschisis n. division of the nucleus by constriction without the formation of chromosomes or a spindle and without the breakdown of the nuclear membrane.
holosericeous a. (1) completely covered with silky hairs; (2) having a silky lustre or sheen.
holostean, holosteous a. having a bony skeleton, appl. .shes.
Holostei, holosteans n., n.plu. group of bony .shes present from the Mesozoic but now represented only by the garpike and bow.n.
holostomatous a. with mouth of aperture entire.
holostylic a. appl. type of jaw suspension in which the palatoquadrate is fused with the cranium without involving the hyoid arch, typical of rabbit-.shes.
holosystolic a. pert. a complete systole.
Holothuroidea, holothurians n., n.plu. class of sausage-shaped echinoderms com­monly called sea cucumbers. They have minute skeletal plates embedded in the .eshy body wall.
Holotrichia, holotrichans n., n.plu. group of ciliate protozoans having no obvious zone of composite cilia around the mouth, and swimming by cilia distributed all over the body.
holotype type specimen q.v.
holozoic a. obtaining food in the manner of animals, by ingesting food material and then digesting it.
holozygote n. zygote containing the entire genomes of both uniting cells.
homaxial a. built up around equal axes.
homeobox n. nucleotide sequence .rst identi.ed in homeotic genes in Drosophila and present in developmental genes in a wide range of other organisms. It encodes a DNA-binding sequence, the homeodomain.
homeodomain n. DNA-binding protein domain which is encoded by the homeobox sequence, and which is found in many gene-regulatory proteins involved in development.
homeologous chromosomes homoeo­logous chromosomes q.v.
homeoprotein n. protein containing a homeodomain.
homeosis n. the transformation of one part into another, as in the modi.cation of antenna into a leg in the Drosophila mu­tant Antennapedia, or of petal into stamen in some plant mutants. alt. homoeosis, metamorphy, metamorphosis.
homeostasis n. (1) maintenance of the constancy of internal environment of the body or part of body; (2) maintenance of equilibrium between organism and environment; the balance of nature. a. homeostatic. alt. homoeostasis.
homeotely n. evolution from homologous parts, but with less close resemblance.
homeothermic homoiothermic q.v.
homeotic a. (1) appl. mutations that trans­form part of the body into another part; (2) appl. genes identi.ed by these mutations. alt. homoeotic.
homeotic selector gene see Hox genes.
home range territory q.v.
homing n. the selective entry of leukocytes into different sites in the body, mediated by interactions between homing receptors on the leukocyte and cell adhesion mol­ecules on the vascular endothelial cells at these sites.
hominid n. member of a human (Homo spp.) or human-like (e.g. Australopithecus) spe­cies characterized by upright posture and other features distinguishing it from the ape lineage (the pongids).
Hominidae, hominids n., n.plu. family of primates that comprises right humans (Homo spp.) and human-like hominids (Australopithecus spp.).
hominoid a. having similarities to humans, appl. African apes and various ape-like fossils, as well as early hominids.
Hominoidea, hominoids n., n.plu. sup­erfamily of primates that includes the families Hominidae (humans and human­like hominids), Pongidae (fantastic apes) and Hylobatidae (gibbons).
homiothermic homoiothermic q.v.
Homo the genus of right men, including several extinct forms (H. habilis, H. erectus, H. neanderthalensis) and modern man, H. sapiens, who are or were prim­ates characterized by completely erect stature, bipedal locomotion, reduced den­tition, and above all by an enlarged brain size.
homo-pre.x from Gk homo, the same, indicating e.g. similarity of structure, from the same source, of similar origins, con­taining similar components.
homoacetogen n. bacterium that produces acetate as the sole product of sugar fer­mentation or from H2 + CO2.
homoacetogenic fermentation fermen­tation of fructose to acetic acid.
homoallelic a. appl. allelic mutant genes which have mutations at the same site, so that intragenic recombination cannot yield a functional gene.
homobasidiomycetes n., n.plu. basidiomycete fungi producing their basidiospores on typically club-shaped, non-septate basidia (homobasidia or holobasidia). They comprise the mush­rooms and toadstools, bracket fungi, coral fungi, puffballs, earthstars, stinkhorns and bird’s nest fungi. alt. Hymenomycetes. cf.
heterobasidiomycetes.
homoblastic a. arising from similar cells.
homocarnosine n. a dipeptide, ala-.­aminobutyric acid, found chie.y in brain.
homocarpous a. bearing only one kind of fruit.
homocellular a. composed of cells of one type only.
homocercal a. appl. type of tail .n in which vertebral column ends before it, and the upper and lower lobes are more-or-less equal.
homochlamydeous a. having the outer and inner perianth whorls alike, not dis­tinguishable as calyx and corolla.
homochromous a. of one colour, appl. .orets of a composite .owerhead.
homochronous a. occurring at the same age or period, in successive generations.
homocysteine n. an amino acid, not found in protein, an intermediate in the biosynthesis of methionine.
homocytotropic antibody IgE q.v.
homodimer n. protein composed of two identical subunits.
homodont a. having teeth all alike, not differentiated.
homodromous a. (1) having the genetic spiral alike in direction in stem and branches; (2) moving or acting in the same direction.
homoecious a. occupying the same host or shelter throughout the life-cycle.
homoeo-homeo-q.v.
homoeo box homeobox q.v.
homoeologous a. partially homologous, appl. genetically and evolutionarily related chromosomes from different genomes within a heterogenomic polyploid or from related species.
homoeostasis homeostasis q.v.
homoeotic homeotic q.v.
homofermentation n. fermentation of glu­cose or other sugar resulting in lactic acid as the sole product.
homogametic sex the sex possessing a pair of homologous sex chromosomes and therefore producing gametes all of one sex. In mammals it is the female, which is XX. In birds, reptiles and lepidopterans the homogametic sex (ZZ) is the male.
homogamy n. (1) inbreeding due to some type of isolation; (2) condition of having .owers all alike; (3) having stamens and pistils mature at same time.
homogangliate a. having ganglia sym­metrically arranged.
homogenate n. cell extract obtained by breaking open cells and releasing their contents.
homogeneous a. composed of identical or similar components. alt. homogenous. homogenetic a. having the same origin.
homogenic a. pert., or having different genes.
homogenic incompatibility in fungi, the inability of genetically similar individuals to fuse.
homogenization n. mechanical disruption of tissue so that cells are ruptured and their contents released.
homogenous (1) appl. parts or organs that are similar due to descent from a common ancestral type; (2) homogeneous q.v.
homogentisate n. intermediate compound in the degradation of the amino acids phenylalanine and tyrosine. In the inherited enzyme de.ciency alcaptonuria, it accu­mulates in urine and turns it black on exposure to air.
homogeny n. correspondence between parts due to common descent.
homoiomerous a. appl. lichens in which the algal cells are honestly evenly distributed throughout the thallus.
homoiosmotic a. appl. organisms with constant internal osmotic pressure.
homoiothermic a. appl. animals that maintain a more-or-less constant body temperature regardless of external temper­ature variations, e.g. birds and mammals. Although virtually all homoiotherms are also endothermic, the two terms are not synonyms and describe different aspects of thermoregulation. n. homoiotherm. cf. poikilothermic.
homokaryon n. hypha or mycelium having more than one haploid nucleus of identical genetic constitution.
homokaryotic a. having genetically iden­tical nuclei in a multinucleate cell, or in different cells of a hypha.
homolactic fermentation fermentation that results in lactic acid as the sole product.
homolateral a. on, or pert. the same side.
homolecithal a. appl. eggs having small, evenly distributed yolk.
homolog alt. spelling of homologue.
homologous a. (1) appl. structures or other attributes in different species that re­semble each other because of origin by common descent; (2) appl. chromosomes in a diploid organism which contain the same sequence of genes but are derived from different parents, and which pair with each other at meiosis; (3) appl. genes determining the same character; (4) appl. DNA or protein sequences that have some degree of similarity to each other because they have been derived from a common ancestral sequence by divergent evolution;
(5) appl. structures having the same phylogenetic origin but not necessarily the same .nal structure or function, e.g. wings and legs in insects; (6) (immunol.) allogeneic q.v.
homologous end-joining a repair process for double-strand breaks in DNA, in which DNA recombination mechanisms enable the repair of the hurt helix by refer­ence to a homologous part of a sister chromatid or homologous chromosome.
homologous recombination (1) recom­bination between two DNAs of identical or very similar sequence, as in the recom­bination that occurs between homologous chromosomes at meiosis; (2) technique of targeted gene disruption in which a chromosomal gene is disrupted by the introduction into the cell of a mutant copy of the gene, which then undergoes recombination with the chromosomal gene, replacing it with the mutant copy.
homologue n. (1) any structure of similar evolutionary and developmental origin to another structure, but serving different functions; (2) one member of a pair of homologous chromosomes; (3) one mem­ber of a pair or set of homologous DNA or protein sequences.
homology n. resemblance by virtue of com­mon descent. adj. homologous.
homology modelling a technique for pre­dicting the three-dimensional structure of a protein on the basis of the similarity of its sequence to that of a protein of known structure. alt. comparative modelling.
homomallous a. curving uniformly to one side, appl. leaves.
homomixis n. the union of nuclei from the same thallus, as in homothallism.
homomorphic a. of similar size and structure.
homomorphism n. (1) condition of having perfect .owers of only one type;
(2) similarity of larva and adult. a. homomorphic. homomorphosis n. having a newly regen­erated part like the one removed. homomultimer n. a protein consisting of two or more identical subunits.
homonomous a. (1) appl. segmentation into similar segments; (2) following the same stages or processes, as of develop­ment or growth.
homonym n. a name which has been given to two different species. When an instance is learned, the second named species must be renamed.
homo-oligomeric a. appl. proteins com­posed of several identical subunits.
homopetalous a. having all the petals alike.
homophilic a. binding like-to-like, appl. cell adhesion molecules that bind to identical molecules on other cells. cf. heterophilic.
homophyllous a. bearing leaves all of one kind.
homoplast n. organism or organ formed from similar cells, as a coenobium.
homoplastic a. (1) similar in shape and structure but not origin; (2) appl. graft made into another individual of the same species.
homoplasy n. resemblance in form or structure between different organs or organisms due to evolution along similar lines rather than common descent. a. homoplasious. alt. homoplasty, con­vergent evolution.
homopolysaccharide n. polysaccharide made of only one type of monosaccharide subunit.
Homoptera n. group of insects that includes the plant bugs, aphids, cicadas and scale insects.
homopterous a. having wings alike.
homosequential a. appl. species of Diptera with polytene chromosomes that have exactly the same banding pattern.
homoserine n. an amino acid, not found in protein, involved in the biosynthesis of methionine and threonine.
homosporous a. appl. plants producing only one type of spore by meiosis, e.g. most mosses and ferns. n. homospory.
homostyly n. the condition that all .owers of the same species have styles of the same length. a. homostylous. cf. heterostyly.
homotaxis, homotaxy n. similar assem­blage or succession of species or types in different regions or strata, not necessarily contemporaneous. a. homotaxial.
homothallic a. (1) appl. cells, thalli or mycelia of algae or fungi that can undergo sexual reproduction with a genetically similar strain, or with a branch of the same mycelium or thallus; (2) appl. strains of the yeast Saccharomyces cerevisiae in which switches of mating type take place in some individuals (and in which, there­fore, conjugation can take place between members of the same strain). see also mating type.
homotropous a. (1) turned in the same direction; (2) appl. ovules having micro­pyle and chalaza at opposite ends.
homotypic a. binding of like to like, e.g. of cell types, molecules.
homotypy n. (1) the equality of structures on both sides of the main axis of body; (2) serial homology, as of successive segments of some animals; (3) reversed symmetry.
homoxylous a. appl. wood without xylem vessels and consisting of tracheids.
homozygote n. a homozygous organism or cell.
homozygous a. appl. diploid organism, cell or nucleus that carries two identical alleles at a given gene locus. n. homozygosity. cf. heterozygous.
homunculus n. the miniature human foetus supposed to be present in sperm, according to proponents of 18th century preforma­tion theory.
honest behaviour behaviour that conveys the individual’s real intentions to another individual.
honey bee generally refers to Apis mellifera, the hive bee. see also bees.
honeydew n. (1) sugary exudate on leaves of many plants; (2) sweet liquid secreted by aphids.
honey guides nectar guides q.v.

honey-stomach in some insects, an ex­pansion of the oesophagus in the anterior part of the abdomen, serving to store ingested liquid which is regurgitated as required.
Hoogsteen base pairing non-standard base pairing that can be made between purine and pyrimidine bases.
hookworms n.plu. parasitic nematode worms that cause severe disease in humans, and including Ancylostoma duodenale and Necator americanus. Common and wide­spread in tropical areas, the larvae enter the body through the skin, and the adult worm lives in the intestine, abrading the intestinal walls and eventually causing severe anaemia and general debilitation.
hopanoids n.plu. compounds similar to sterols, found in the cell membranes of some bacteria.
hordaceous a. pert. or resembling barley.
hordein n. storage protein in barley grains.
horizon n. (1) soil layer of more-or-less well-de.ned character; (2) a layer of deposit characterized by de.nite fossil species and formed at a de.nite time.
horizontal cell type of nerve cell in retina, forming a layer with bipolar and amacrine cells and making lateral connections.
horizontal gene transfer the acquisition of genes by one species from another species. alt. lateral gene transfer.
hormogonium n. cyanobacterial .lament between two heterocysts, which propagates a new organism when it breaks away. plu. hormogonia.
hormone n. a substance that is produced by one tissue and transported to another tissue where it induces a speci.c physio­logical response.
horn n. (1) the hollow projections on the head of many ruminants, consisting of layers of keratinized epidermis laid down on a bony base; (2) any projection resemb­ling a horn; (3) anterior part of each uterus when posterior parts are united; (4) a tuft of ear feathers in owls; (5) a spine in .shes;
(6)
a tentacle in snails; (7) cornu q.v.;

(8)
keratin q.v.
hornworts n.plu. common name for mem­bers of the plant division Anthocerophyta
q.v.
horny corals another name for the gor­gonians q.v.
horological a. appl. .owers opening and closing at a particular time of day and night.
horotelic a. evolving at a standard rate.
horsehair worms Nematomorpha q.v.
horseradish peroxidase (HRP) enzyme found in the roots of horseradish and other plants, which reacts with certain substrates to leave a deposit of black granules. It is used esp. as a histochemical stain to trace axons of nerve cells.
horseshoe crabs common name for the Xiphosura, also called king crabs, a group of aquatic arthropods with af.nities with the arachnids rather than the crustaceans, and often placed in the separate class Merostomata. They have a heavily chitinized body with the cephalothorax covered by a horseshoe-shaped carapace.
horsetails common name for the Sphenophyta q.v.
host n. (1) any organism in which another organism spends part or all of its life, and from which it derives nourishment or gets protection; (2) the recipient of grafted or transplanted tissue.
host-induced modi.cation of enveloped viruses, the incorporation of host cell mem­brane material into the envelope, causing differences in the physical properties of virions propagated in different types of cell.
host range the range of different species, or cell types, that a pathogen can infect.
hotspot n. region of a gene or chromosome at which mutation or recombination is markedly increased.
housekeeping genes genes that are ex­pressed in most cell types and which are concerned with basic metabolic activities common to all cells.
Hox genes ancient family of homeobox-containing genes present in all Metazoa and which are involved in specifying a cell’s position and identity along the antero-posterior axis during development.
HPFH hereditary persistence of foetal haemo­globin q.v.
HPLC high-performance liquid chromato­graphy q.v.
HPRT hypoxanthine guanine phosphoribosyl transferase q.v.
H+
pump proton pump q.v.
HPV human papilloma virus.
HRP horseradish peroxidase q.v.
Hsp general abbreviation for heat-shock proteins.
H strand heavy strand of DNA, esp. pert. mammalian mitochondrial DNA which can be separated into H and L (light) strands on the basis of their density.
H substance complex carbohydrate antigen on red blood cells, the unmodi.ed form of the basic ABO blood group antigen, found in persons of blood group O.
5-HT 5-hydroxytryptamine (serotonin) q.v.
HTH helix-turn-helix q.v.
HTLV-I human T-cell leukaemia virus, the cause of adult T-cell leukaemia, a rare cancer.
HTLV-II human T lymphotropic virus II, a retrovirus isolated from humans but caus­ing no known disease.
hue n. colour, one of the three basic dimen­sions of perception of visible light by humans. see also brightness, saturation.
human chorionic gonadotropin (hCG) protein hormone produced by the de­veloping conceptus and placenta after implantation and which is involved in maintenance of pregnancy.
Human Genome Project international publicly-funded project that mapped and sequenced the complete human genome.
human immunode.ciency virus (HIV) a retrovirus transmitted maternally or by transfer of body .uids (sexually or by transfusion of infected blood), which infects CD4 T lymphocytes, leading to their eventual depletion and a severe immunode.ciency—bought immune de.ciency syndrome (AIDS). Two types of the virus have been found, HIV-1 and HIV-2.
humanized antibodies antibodies con­structed by genetic engineering in which a desired antigen-binding site from a mouse antibody is inserted into a human antibody.
human leukocyte antigen HLA q.v.
humeral a. pert. shoulder region.
humerus n. the bone of the upper arm, or upper part of vertebrate forelimb.
humic a. pert. or derived from humus.
humic acid fraction that precipitates from a solution of humus in weak alkali on addition of acid.
humicolous a. living in the soil. n. humicole.
humi.cation n. the production of humus in the soil by the action of microorganisms on plant and animal residues.
humin n. black insoluble residue left when humus is dissolved in dilute alkali.
humor n. any body .uid, nowadays chie.y used in connection with the .uids of the eye. see aqueous humor, vitreous humor.
humoral a. (1) appl. immunity mediated by antibodies; (2) appl. antibodies circulating in blood and lymph.
humulone n. a bitter compound obtained from hops.
humus n. black organic material of complex composition which is the end-product of the microbial breakdown of plant and animal residues in the soil.
Huntington’s disease autosomal dominant genetic disease characterized by the onset of mental and physical deterioration in middle age. It is caused by ampli.cation of trinucleotide sequences within the affected gene. alt. Huntington’s chorea.
HU protein a histone-like protein found complexed with DNA in bacteria.
Huxley’s layer the middle layer of poly­hedral cells in the inner root sheath of hair follicle.
HVR hypervariable region of antibody molecule.
hyaline a. (1) transparent or translucent;
(3) free from inclusions; (4) appl. carti­lage of smooth glassy appearance, lacking obvious .bres.
hyaline layer outer layer of matrix cover­ing surface of sea urchin blastula. It is secreted by the cortical granules of the egg on fertilization.
hyalocyte n. cell secreting the vitreous humor of eye.
hyaloid a. transparent or translucent.
hyaloid artery central artery of retina run­ning through hyaloid canal to back of lens, in foetal eye.
hyaloid canal canal running through vitre­ous body of eye, from optic nerve to back of lens.
hyaloid fossa anterior concavity of vitreous body in the eye, receptacle of lens. hyaloid membrane delicate membrane enveloping vitreous body of the eye.
hyalopterous a. having transparent wings.
hyalospore n. transparent unicellular spore in some fungi.
hyaluronans n.plu. viscous high-molecular­weight polymers of N-acetylglucosamine and glucuronic acid, abundant in connect­ive and other tissues. They act as lubricat­ing agents in synovial .uid and form the cementing substance between animal cells. alt. hyaluronate, hyaluronic acid.
hyaluronidase n. enzyme that degrades hyaluronans, produced e.g. by various pathogenic bacteria and aiding them to invade tissues. EC 3.2.1.36, r.n. hyalurono­glucuronidase.
hybrid n. (1) progeny of a cross between parents of different genotype; (2) hetero­zygote q.v.; (3) any macromolecule (esp. DNA) composed of two or more parts of different origins. v. hybridize. a. hybrid.
hybrid-arrested translation technique for identifying the cDNA corresponding to an mRNA by its ability to pair with the mRNA in vitro to inhibit translation.
hybrid cell cell formed by fusion of cells from two different species in which the chromosomes are contained in a single large nucleus. cf. heterokaryon.
hybrid cline the serial arrangement of characters or forms produced by crossing species.
hybrid DNA (1) heteroduplex DNA q.v.;
(2) DNA molecule composed of segments of different origin, as in recombinant DNA. alt. chimeric DNA.
hybrid dysgenesis the production of sterile progeny, showing chromosomal abnormalities and mutations, on crossing certain strains of the fruit .y Drosophila melanogaster. It may involve either the I-R system or the P-M system. Dysgenesis is seen in crosses of I males with R females and in crosses of P males with M females but not vice versa. see P factors, M cytotype.
hybridization n. (1) formation of a hybrid q.v.; (2) cross-fertilization; (3) DNA hybrid­ization q.v.
hybridoma n. a hybrid cell line producing monoclonal antibodies. It is formed by fusion of a single antibody-producing B cell from the spleen and a myeloma cell. The resulting cell can both multiply inde.nitely in culture and produce anti­bodies and is used to produce monoclonal antibodies for research and medical dia­gnostic procedures.
hybrid sterility sterility in an individual arising from the fact that it is a hybrid.
hybrid swarms populations consisting of descendants of species hybrids, as at borders between geographical areas popu­lated by these species.
hybrid vigour the phenomenon often seen in crosses between two pure-bred lines of plants, that the hybrid is more vigorous than either of its parents, presumably owing to increased heterozygosity. see also overdominance.
hybrid zone geographical area in which two populations once separated by a geographical barrier hybridize after the barrier has broken down.
hydathode n. epidermal structure in plants specialized for secretion or exudation of water.
hydatid n. (1) any vesicle or sac .lled with clear watery .uid; (2) sac containing encysted stages of larval tapeworms;
(3) vestige of Müllerian duct constituting
appendix of testis.
hydatiform a. resembling a hydatid.
hydatiform mole cyst-like growth arising in uterus from implantation of abnormal embryo.
Hydra small freshwater hydrozoan that has been used as an experimental animal in developmental studies.
hydranth n. an individual specialized for feeding in a hydrozoan colony.
hydrarch succession hydrosere q.v.
hydratase n. enzyme catalysing the hydra­tion of a compound by acceptance of a molecule of water, and the removal of such added water. EC sub-subgroup 4.2.1. r.n. hydro-lyase. cf. hydrolase.
hydric a. having an abundant supply of moisture. hydride ion n. a hydrogen atom with an additional electron. hydrobiology n. study of aquatic plants and animals and their environment. hydrobiont n. organism living mainly in water. hydrocarbon n. molecule composed of hydrogen and carbon only.
hydrocarpic a. appl. aquatic plants having .owers that are fertilized out of the water but submerged for development of fruit.
hydrocaulus n. the “stem” and “branches” of a colonial hydroid.
Hydrocharitales n. order of aquatic her­baceous monocots comprising the family Hydrocharitaceae (frog’s-bit).
hydrochoric a. (1) dispersed by water; (2) dependent on water for dissemination. n. hydrochory.
hydrocladia n.plu. the branches of certain hydrozoan colonies.
hydrocoel n. the water vascular system in echinoderms.
hydrocoles n.plu. animals living in water or a wet environment.
hydrocortisone n. glucocorticosteroid hormone produced by the cortex of the adrenal gland and very similar to cortisone in structure and properties. It has marked effects on carbohydrate metabolism and is an immunosuppressant. alt. cortisol.
hydrofuge a. water-repelling.
hydrogen (H) n. the lightest of all the chemical elements. In the free state it is a colourless odourless .ammable gas (H2). It is a constituent of all organic molecules and one of the essential elements for living organisms. see also deuterium, pH, tritium.
hydrogenase n. any of several enzymes that can use molecular hydrogen (H2) for the reduction of a substance, present in
e.g. the hydrogen-oxidizing bacteria.
hydrogenation n. addition of a hydrogen atom to a molecule, which is a reduction reaction. cf. dehydrogenation.
hydrogen bacteria bacteria that can use the oxidation of molecular hydrogen (H2) as their source of energy and oxygen as the electron receptor. They are mostly facultative chemolithotrophs. see also hydrogen-oxidizing bacteria.
hydrogen bond the attraction between an electronegative atom with a lone pair of electrons and a hydrogen atom that is covalently bonded to another elec­tronegative atom, e.g. –O…..H–N–. It is the strongest of the weak non-covalent intermolecular attractions and is of fantastic importance in biology as it is one of the main forces governing e.g. the folding of a protein chain into its .nal functional three-dimensional structure, and the inter­actions of proteins with each other and with small molecules such as enzyme substrates. Hydrogen bonds also hold together the two DNA strands of a DNA molecule. Hydro­gen bonding between water molecules is responsible for the high melting and boiling points of water (compared with those of
e.g. methane) and its high surface tension. hydrogen ion a proton, H+ . hydrogen ion pump proton pump q.v. Hydrogenobacteria see Aquifex–
Hydrogenobacter group.
hydrogenosome n. organelle containing hydrogenases, found in anaerobic protozoa.
hydrogen-oxidizing bacteria diverse group of aerobic and anaerobic bacteria that oxidize molecular hydrogen (H2) as their source of energy, using a variety of electron acceptors, depending on the species. Those aerobic species that use oxygen as the electron acceptor are gener­ally called hydrogen bacteria.
hydrogen peroxide H2O2, potentially toxic product produced as a by-product of the reduction of oxygen in aerobic respiration. It is degraded by catalase to water and oxygen.
hydrogen sulphide H2S, compound used as an energy source by some colourless sulphur bacteria, which oxidize it to ele­mental sulphur and then to sulphate.
hydroid n. (1) (bot.) empty water-conduct­ing cell, joined with others to form a strand of water-conducting tissue in the stems of many mosses; (2) (zool.) one of the forms of individuals in the Hydrozoa, a class of solitary and colonial coelenterates, having a hollow cylindrical body closed at one end and with a mouth at the other surrounded by tentacles. alt. polyp.
hydrolase n. any enzyme that catalyses a hydrolysis. EC group 3.
hydrological cycle water cycle q.v.
hydro-lyase hydratase q.v.
hydrolysis n. the addition of the hydrogen and hydroxyl ions of water to a molecule, with its consequent splitting into two or more simpler molecules.
hydrolytic a. pert. or causing hydrolysis.
hydrome n. any tissue that conducts water.
hydromesophyte n. aquatic plant of tem­
perate climates.
hydromorphic a. appl. soils containing excess water.
hydronasty n. plant movement induced by changes in atmospheric humidity.
hydronium ion (H3O+
) the ion made by addition of a proton (H+) to a water mol­ecule, the usual fate of protons in solution.
hydropathy plot analysis of a protein sequence to determine stretches of hydro­phobic amino acids, which, in a mem­brane protein, may indicate transmembrane regions.
hydrophilic a. water-attracting, appl. charged or polar chemical group or mol­ecule that readily forms hydrogen bonds with water, thus tending to dissolve in water. cf. hydrophobic.
hydrophilous a. pollinated by the agency of water.
hydrophily n. pollination by water.
hydrophobia n. the aversion to water that is a symptom of rabies.
hydrophobic a. water-repelling or repelled by water, appl. non-polar chemical group or molecule that cannot form hydrogen bonds with water. Hydrophobic molecules tend to aggregate in water, excluding water from between them.
hydrophoric a. carrying water, appl. canal: the stone canal in echinoderms.
hydrophyllium n. one of leaf-like trans­parent bodies arising above and partly covering the sporosacs in a siphonophore.
hydrophyte n. (1) aquatic plant living on or in the water; (2) aquatic perennial her­baceous plant in which the perennating parts lie in water.
hydrophyton n. a complete hydrozoan colony.
hydroplanula n. stages between planula and actinula in larval stages of coelenterates.
hydropolyp n. a polyp of a hydrozoan colony.
hydroponics n. cultivation of plants with­out soil in nutrient-rich water, which is usually irrigated over some inert medium such as sand.
hydropote n. a cell or cell group, in some submerged leaves, easily permeable by water and salts.
hydropyle n. a specialized area in cuticular membrane of some insect embryos, for passage of water.
hydrorhiza n. branching root-like foot of a hydroid colony, which attaches it to the substratum.
hydrosere n. a plant succession originating in a wet environment.
hydrosinus n. an extension of the mouth cavity in some cyclostomes.
hydrosoma, hydrosome n. the conspicu­ously hydra-like stage in a coelenterate life history.
hydrosphere n. the part of the planet which is water, e.g. the oceans, rivers, lakes, streams, and including soil water.
hydrospire n. long pouches running at the side of the ambulacral grooves and acting as respiratory structures in certain echinoderms.

hydrospore n. a zoospore when moving in water.
hydrostatic a. appl. organs of .otation, as air sacs in aquatic larvae of insects.
hydrostome n. the mouth of a hydroid polyp.
hydrotaxis n. movement or locomotion in response to the stimulus of water.
hydrotheca n. cup-like extension of perisarc around individual polyps in some colonial hydrozoans, into which the polyp may withdraw.
hydrothermal vent community com­munity of organisms living around vol­canic vents in the sea .oor (hydrothermal vents) at fantastic depths. Primary producers are chie.y chemoautotrophic sulphide-oxidizing bacteria that use the energy of sulphide oxidation to .x CO2. They in­clude free-living species (e.g. Beggiatoa spp.) and intracellular bacterial symbionts living in giant vestimentiferan tube worms.
hydrotropic a. appl. curvature of a plant organ towards a greater degree of moisture.
hydroxyapatite n. hydrated calcium phos­phate (Ca10(PO4)6(OH)2), major constituent of inorganic phase of bone. Also used as a material to which double-stranded DNA will bind in various separation techniques.
3-hydroxybutyrate a ketone body, formed by reduction of acetoacetate, which can act as a substrate for cellular respiration.
hydroxycobalamin(e) vitamin B12b. see cobalamine.
hydroxyl n. chemical group (–OH) pre­sent in all alcohols, consisting of a hydrogen atom covalently linked to an
oxygen atom.
hydroxylapatite hydroxyapatite q.v.
hydroxylase monooxygenase q.v.
hydroxylysine n. hydroxylated derivative of the amino acid lysine, modi.ed after incorporation into a polypeptide chain. It is found in collagen.
hydroxyproline (Hyp) n. hydroxylated derivative of the amino acid proline, modi­.ed after incorporation into a polypeptide chain. It is found in collagen.
5-hydroxytryptamine (5-HT) amine neurotransmitter in central nervous system. It is implicated in regulation of wakeful­ness and pain sensation. Also produced by platelets and other cell types and causes constriction of blood vessels by stimulat­ing contraction of smooth muscle. alt. serotonin. see Fig. 23.
5-hydroxytryptaminergic serotonergic q.v.
Hydrozoa, hydrozoans n., n.plu. class of coelenterates with two body forms, hydroid (polyp) and medusa, generally occurring as different stages of the life-cycle. They include solitary forms such as Hydra, branching colonial forms, and the siphonophores such as the Portuguese Man o’ War which are colonies of several different types of modi.ed polyps and medusae.
hygric a. (1) humid; (2) tolerating, or adapted to, humid conditions.
hygrochasy n. dehiscence of fruits when induced by moisture.
hygrokinesis n. movement induced by a change in humidity.
hygromesophyte n. plant of temperate climates that lives in water but is not aquatic.
hygromorphic a. structurally adapted to a moist habitat.
hygropetric a. appl. fauna of submerged rocks.

hygrophanous a. as if saturated with water.
hygrophilic, hygrophilous a. inhabiting moist or marshy places.
hygrophyte n. plant that thrives in plentiful moisture, but is not aquatic.
hygroreceptor n. a specialized cell or struc­ture sensitive to humidity.
hygroscopic a. (1) sensitive to moisture;
(2) absorbing water. hygrotaxis n. movement in response to moisture or humidity. hygrotropism n. plant growth movement in response to moisture or humidity. hymen n. thin fold of mucous membrane at
mouth of vagina.
hymeniferous a. having a hymenium.
hymenium n. in ascomycete and basidiomycete fungi, the distinct layer of spore-bearing structures, asci and basidia respectively, often interspersed with bar­ren cells (paraphyses). a. hymenial. plu. hymenia.
Hymenomycetes n. group of basidiomycete fungi bearing their basidia in a well-de.ned layer (hymenium) which becomes exposed while the basidia are still immature. It comprises mushrooms and toadstools and bracket fungi.
Hymenoptera, hymenopterans n., n.plu. order of insects, including solitary and social species, comprising the ants, bees and wasps. They have two pairs of wings, and many have a pronounced waist be­tween 2nd and 3rd abdominal segments. Males are haploid and females diploid, males developing from unfertilized ova. In colonial forms, a colony usually con­tains one reproductive female (the queen), sterile female workers, a few reproductive males, and (in ants) sterile soldiers.
hyobranchial a. pert. to the hyoid and branchial arches.
hyoepiglottic a. connecting hyoid and epiglottis.
hyoglossus n. an extrinsic muscle of the tongue, arising from greater cornu of hyoid bone.
hyoid a. (1) pert. or designating a bone or series of bones lying at the base of the tongue in mammals, developed from the hyoid arch of embryo; (2) in .shes, pert. the hyoid arch or 1st gill arch.
hyoid arch the 2nd branchial arch in verte­brate embryos, which develops into the .rst gill arch in .shes.
hyoid bone in humans, a horseshoe-shaped bone lying at base of tongue.
hyoideus n. nerve supplying mucosa of mouth and muscles of hyoid region.
hyomandibular cartilage the dorsal skel­etal element of hyoid arch in .shes.
hyomental a. pert. hyoid and chin.
hyoplastron n. the 2nd lateral plate in shell of tortoises and turtles.
hyostylic n. appl. type of jaw suspension present in most modern bony .sh in which the jaws are attached to the brain case mainly by the hyomandibula, and two out of three palatoquadrate (upper jaw) con­nections to the brain case are replaced by ligaments.
hyothyroid a. pert. hyoid bone and thyroid cartilage of larynx.
Hyp hydroxyproline q.v.
hypanthium n. in some .owers, a cup-shaped extension of the margin of the receptacle to which sepals, petals and stamens are attached.
hypanthodium n. in.orescence with a con­cave capitulum on whose walls the .owers are arranged.
hypantrum n. notch on vertebra of certain reptiles for articulation with a wedge-shaped extension (the hyposphene) on neural arch of neighbouring vertebra.
hypapophysis n. a ventral extension on vertebra.
hyparterial a. situated below an artery, appl. branches of bronchi below pulmonary artery.
hypaxial a. ventral or below vertebral column, appl. muscles.
hyperacute rejection rapid and untreat­able rejection of transplanted tissue that is due to the presence in the host of pre-existing antibodies that react against antigens on the donor tissue blood vessels.
hypercholesterolaemia n. (1) raised levels of cholesterol in the blood; (2) the inherited condition familial hyper-cholesterolaemia, which in homozygotes results in a de.ciency of LDL receptors, leading to increased blood cholesterol and deposition of cholesterol in nodules in tendons, premature atherosclerosis and childhood coronary artery disease.
hyperchromism n. increased absorbance (q.v.) seen e.g. when DNA separates into separate strands.
hypercoracoid a. pert. or designating upper bone at the base of pectoral .n in .shes.
hyperdactyly polydactyly q.v.
hyperdiploid n. cell or organism which, as a result of a translocation, has more than two copies of a particular chromosome segment.
hyperfeminization n. condition of a feminized male with female characteristics exaggerated, as in small size and weight.
hyperglycaemia n. excess glucose in blood.
hyperhaploid n. cell or organism contain­ing supernumerary chromosomes.
hyper IgM syndrome immunode.ciency disease in which only IgM antibodies are made because of an inability to switch isotypes. It can be caused by several dif­ferent genetic defects.
hyperimmune n. appl. individuals with large amounts of a given antibody in their blood, appl. antiserum obtained from such an individual.
hyperimmunization n. heightened state of immunity as a result of repeated immun­ization with the same antigen.
hyperkalaemia n. abnormally high concen­tration of potassium in the blood.
hyperkinetic a. over-active.
hypermasculinization n. condition of a masculinized female with male character­istics exaggerated, as in large propor­tions, appearance of male secondary sexual characters.
hypermetamorphosis n. kind of insect life history which includes two or more differ­ent kinds of larva.
hypermorph n. a mutant allele which pro­duces a more exaggerated version of the effect of the wild-type gene.
hypermutation n. mutation occurring at a higher rate than the normal rate for that particular gene or species. see also somatic hypermutation.
hypernatraemia n. abnormally high con­centration of sodium in the blood.
hyperosmotic a. appl. a solution of higher osmotic concentration than a given refer­ence solution.
hyperparasite n. organism which is a para­site of, or in, another parasite.
hyperphagia n. increased food intake.
hyperpharyngeal a. dorsal to the pharynx. hyperpituitarism n. overactivity of the pituitary gland, resulting in gigantism.
hyperplasia n. (1) excessive development due to an increase in the number of cells; (2) an abnormal increase in cell proliferation.
hyperploid a. (1) having extra chromo­somes; (2) having too many copies of the gene in question.
hyperpnoea n. rapid breathing due to in­suf.cient supply of oxygen.
hyperpolarization n. increase in the electrical potential difference across a membrane. With regard to living cells, it indicates the inside becoming more negative with respect to the outside. cf. depolarization.
hyperpolyploid n. polyploid cell or organ­ism containing more than the normal number of chromosomes in each of its haploid sets.
hypersensitive a. (1) showing an exagger­ated or otherwise unduly sensitive response to a stimulus; (2) (immunol.) showing an inappropriate, exaggerated or uncontrolled immune response to an antigen, as in allergic reactions and anaphylactic shock.
n. hypersensitivity.
hypersensitive response in plant patho­logy, the rapid death of plant cells in re­sponse to infection with a fungal, bacterial or viral pathogen, a common defence mechanism in plants. It is generally also associated with other defence responses,
e.g. accumulation of phytoalexins and ligni.cation in neighbouring living cells. alt. hypersensitive reaction, hyper­sensitivity response.
hypersensitive site sites in chromatin where the DNA is susceptible to cleavage by DNase 1, and which are thought to rep­resent sites where the chromatin is in a different state of packing from surrounding chromatin.
hypersensitivity n. an exaggerated or intense response to a particular stimulus or substance. see also immediate hypersen­sitivity, delayed hypersensitivity, type I, II, II, IV hypersensitivity.
hyperstriatum ventralis pars caudalis in brain of birds, an integration centre for auditory and motor information in song control.
hypertelia, hypertely n. (1) excessive imitation of colour or pattern; (2) over­development of canines of babirusa, an East Indian pig, the male of which has four large tusks.
hypertension n. raised blood pressure.
hyperthermia n. rise in body temperature above normal, which is used adaptively by some animals living in hot climates as a water-conserving mechanism.
hyperthermophile n. microorganism which requires very high temperatures (>80 °C) for optimal growth. Hyperther­mophiles are found in hot springs, geysers and deep-sea hydrothermal vents. Most known hyperthermophiles are members of the Archaea. alt. extreme thermophile.
hyperthyroidism n. overactivity of the thyroid gland, with excess production of thyroid hormone, resulting in increased metabolic rate, high blood pressure, pro­trusion of the eyeballs, rapid heart rate, thinness and emotional disturbances.
hypertonia n. excessive muscle tone.
hypertonic a. having a higher osmotic pres­sure than that of another solution. If the two solutions are separated by a semi­permeable membrane water will .ow into the hypertonic solution from the other. cf. hypotonic.
hypertriploid a. appl. cells with more than three sets of chromosomes.
hypertrophic a. (1) appl. waters grossly enriched with plant nutrients; (2) appl. a structure that arises from excessive growth.
hypertrophy n. excessive growth due to increase in the size of the cells. cf. hyperplasia.
hypervariable locus DNA sequence with an exceptionally high degree of polymorphism within a population.
hypervariable regions (HVR) parts of the variable regions of immunoglobulin and T-cell receptor chains that are particu­larly variable in amino acid sequence. They contribute to the antigen-binding site. They include the complementarity-determining regions. cf. framework regions.
hypha n. tubular .lament which is the basic growth form of the vegetative phase of a fungus. Hyphal extension and branching produces the fungal mycelium. Hyphae may be continuous tubes of multinucleate protoplasm or may be partially or com­pletely subdivided along their length by transverse partitions (septa) into uninucleate or binucleate compartments. Fungal hyphae are covered by a rigid cell wall, which in most cases contains chitin as well as or instead of cellulose. Similar .lamentous vegetative structures in some algae are known as hyphae, as also are the acellular .laments of the prokaryotic actinomycetes. plu. hyphae. a. hyphal.
Hyphochytriomycota, hyphochytrids n., n.plu. phylum of freshwater protists, now generally classi.ed in the kingdoms Stramenopila or Chromista, commonly known as water moulds and formerly classi.ed as fungi. They are parasitic on algae or fungi or saprobic on plant and insect debris, growing as .ne threads and producing motile zoospores with one anterior tinsel .agellum. alt. Hypho­chytriomycetes.
hyphomycetes n.plu. fungi that bear conidia free on the mycelium.
hyphopodium n. hyphal branch with enlarged terminal cell or haustorium for attaching the hypha to its host, as in some ascomycetes.
hypnody n. the long resting period of certain larvae.
hypnogenic a. sleep-inducing.
hypo-pre.x from the Gk hypo, under. In anatomical terms often denoting situated under. In physiological and biochemical terms denoting a decrease in.
hypoachene n. achene developed from an inferior ovary.
hypobasal n. the lower segment of devel­oping ovule, which ultimately gives rise to the root.
hypobasidium n. basal part of cell of septate basidium, in which nuclei unite and which gives rise to the epibasidium from which the basidiospores are budded off.
hypobenthos n. fauna of the sea bottom below 1000 m.
hypoblast n. (1) cells lining the blastocoel cavity in mammalian blastocyst, which give rise to the yolk sac endoderm, alt. primitive endoderm; (2) lower layer of cells in developing chick blastodisc.
hypobranchial a. (1) pert. lower or 4th seg­ment of gill arch; (2) appl. space under gills in decapod crustaceans.
hypocalcaemia n. abnormally low level of calcium in the blood.
hypocalcaemic, hypocalcemic n. reduc­ing the level of calcium in the blood.
hypocarp n. .eshy modi.ed stalk of some fruits, as in cashew-apple.
hypocarpogenous a. having both .owers and fruit borne underground.
hypocentrum n. transverse cartilage that develops below nerve cord and becomes part of vertebral centrum. plu. hypocentra.
hypocercal a. having notochord terminat­ing in lower lobe of tail .n.
hypocerebral a. appl. ganglion of stomatogastric system in insects, linked to frontal and ventral ganglia, also to corpora cardiaca.
hypochile, hypochilium n. in orchid .owers, the inner or basal part of lip when in two distinct parts.
hypochondrium n. abdominal region lateral to epigastric and above lumbar. a. hypochondriac.
hypochordal a. below the notochord.

hypochromic a. paler than usual.
hypochromicity n. the decrease in optical density of a duplex DNA in comparison to the value expected from the optical density of a mixture of its constituent nucleotides in free form, caused by inter­actions between the stacked bases in the duplex. alt. hypochromism.
hypocone n. (1) posterior internal cusp of upper molar teeth; (2) the part posterior to girdle in dino.agellates.
hypoconid n. posterior cusp of lower molar on the cheek side.
hypoconule n. 5th or distal cusp of upper molar.
hypoconulid n. posterior middle cusp of lower molar teeth.
hypocoracoid a. pert. lower bone at base of pectoral .n in .shes.
hypocotyl n. that part of stem below cotyledons in plant embryo, which even­tually bears the roots.
hypodermis n. (1) in leaves, a layer of cells immediately underlying the epidermis;
(2) layer of cells, often a syncytium, under­lying the cuticle in nematode worms. a. hypodermal.
hypodiploid a. appl. cells with less than a complete diploid set of chromosomes.
hypogastric a. pert. lower abdomen.
hypogastrium n. lower central region of abdomen.
hypogeal, hypogean a. (1) living or grow­ing underground; (2) appl. germination when cotyledons remain underground.
hypogenesis n. development without occurrence of alternation of generations.
hypogenous a. growing on the under sur­face of anything.
hypoglossal nerve 12th cranial nerve, con­trols muscles of tongue and .oor of mouth. In anamniotes it is a spinal nerve.
hypoglottis n. the under part of the tongue.
hypoglycaemia n. abnormally low levels of glucose in the blood.
hypoglycaemic, hypoglycemic a. (1) appl. agents that tend to lower blood glucose level, such as insulin; (2) pert. hypoglycaemia.
hypognathous a. having the lower jaw slightly longer than the upper, with mouth­parts on the underside, appl. insects.
hypogynium n. structure supporting ovary in .owers of sedges.
hypogynous a. appl. .owers having petals, sepals and stamens attached to the recep­tacle below the ovary. n. hypogyny.
hypohaploid n. cell or organism with one or several chromosomes missing from its haploid complement.
hypohyal n. a ventral skeletal element of the hyoid arch in .shes.
hypokalaemia n. abnormally low concen­tration of potassium in the blood.
hypolimnion n. the water between the thermocline and the bottom of a lake. hypolithic a. found or living under stones.
hypomorph n. mutant allele that behaves in a similar way to the wild-type gene but has a weaker effect.
hyponasty n. the state of growth in a .attened structure when the underside grows more vigorously than the upper. a. hyponastic.
hyponatraemia n. abnormally low concen­tration of sodium in the blood.
hyponeural a. appl. system of transverse and radial nerves in echinoderms.
hyponeuston n. organisms swimming or .oating immediately under the water surface.
hyponychium n. epidermal layer on which nail rests. a. hyponychial.
hyponym n. (1) generic name not founded on a type species; (2) a provisional name for a specimen.
hypo-osmotic a. appl. a solution of lower osmotic concentration than a given refer­ence solution.
hypoparathyroidism n. condition due to decreased production or activity of para­thyroid hormone. It is characterized by a fall in blood calcium levels which leads to a wide variety of symptoms.
hypopetalous a. having corolla inserted below, and not adjacent to, gynoecium.
hypopharyngeal a. pert. or situated below or on lower surface of pharynx.
hypopharynx n. projection from .oor of mouth in dipteran insects, forming part of the food canal.
hypophragm n. lid closing opening of shell in some gastropod molluscs.
hypophyllous a. located or growing under a leaf.
hypophyseal, hypophysial a. pert. the hypophysis.
hypophysectomy n. excision or removal of the pituitary gland.
hypophysis pituitary q.v.
hypopigmentation n. lack of pigmentation.
hypopituitarism n. de.ciency of pituitary hormones, resulting in a type of infantilism.
hypoplasia n. (1) developmental de.ciency;
(2) de.cient growth. hypoplastron n. the 3rd lateral bony plate in shell of turtles and tortoises.
hypoploid a. (1) having too few copies of the gene in question; (2) lacking one or more of the chromosomes of the normal haploid set.
hypopolyploid n. polyploid cell or organ­ism lacking one or more chromosomes. hypoptilum n. a small tuft of down near
base of a feather. hypopyge, hypopygium n. clasping organ of male dipterans.
hyporachis n. the stem of aftershaft (hypoptilum) of a feather.
hyporheic zone zone around a river, esp. those with gravel beds, in which river water and its micro.ora and fauna extends as groundwater throughout the surrounding land.
hyposkeletal a. lying beneath or internal to the endoskeleton.
hyposomite n. ventral part of body seg­ment, as in certain cephalochordates such as amphioxus.
hyposperm n. the lower region of ovule or seed, below the level at which the integu­ment or testa is free from the nucellus.
hyposphene n. wedge-shaped process on neural arch of vertebra of certain reptiles, which .ts into a notch on next vertebra, in certain reptiles.
hypostasis n. case where expression of one gene is suppressed by another, non-allelic, gene. a. hypostatic.
hypostatic a. appl. a gene whose expression is prevented by another non-allelic gene.
hypostomatous a. (1) appl. leaf, having stomata on underside; (2) having mouth placed on lower or ventral side.
hypostome n. (1) conical projection con­taining mouth in hydrozoans; (2) the fold bounding posterior margin of oral aperture in crustaceans; (3) anterior and ventral part of insect head; (4) lower mouthpart in ticks, used to anchor the animal to the skin while it feeds.
hypostracum n. inner primary layer or end­ocuticle of exoskeleton in ticks and mites.
hypothalamus n. region of brain located below thalamus and forming greater part of .oor of 3rd ventricle. It secretes vari­ous peptide hormones, including releasing factors for pituitary hormones. Involved in sexual development and the control of motivated behaviour such as eating, drink­ing and sex.
hypothallus n. (1) thin layer under sporangia in slime moulds; (2) undiffer­entiated hyphal growth or marginal out­growth in lichens.
hypotheca n. younger or inner half of frustule in diatoms.
hypothecium n. the dense layer of hyphal threads below the hymenium in certain fungi.
hypothenar a. pert. the prominent part of palm of hand below base of small .nger.
hypothermia n. a drop in body temperature below normal limits, which leads to death if an external heat source is not applied.
hypothesis n. a proposed explanation of a phenomenon or of a scienti.c problem that must be tested by experiment. see also null hypothesis. cf. theory.
hypothyroidism n. condition due to underactivity of thyroid gland and de.ci­ency of thyroid hormones.
hypotonic a. having a lower osmotic pres­sure than that of another solution. If the two solutions are separated by a semi­permeable membrane water will .ow from the hypotonic solution to the other. cf. hypertonic.
hypotrematic a. appl. the lower lateral bar of branchial basket of cyclostomes.
hypotrichous a. having cilia mainly on the undersurface.
hypotrochanteric a. running below the trochanter.
hypotrophy n. (1) condition where wood grows more thickly on underside of a horizontal branch; (2) the condition where stipules or buds form on the underside.
hypotympanic a. situated below the tym­panum, pert. quadrate bone.
Hypoviridae n. family of double-stranded RNA viruses that infect fungi.
hypovirulence factor double-stranded RNA virus found in isolates of plant patho­genic fungi with reduced virulence.
hypoxanthine n. 6-oxypurine, the purine base in the ribonucleoside inosine, found chie.y in tRNA. It is similar to adenine, but with the amino group replaced by a hydroxyl group. Also found as a break­down product of purines.
hypoxanthine guanine phosph­oribosyltransferase (HGPRT) enzyme catalysing the formation of inosine monophosphate or guanine monophosphate in the minor pathway of nucleic acid biosynthesis. The HGPRT gene is often used as a genetic marker in somatic cell genetics to enable selection of cells in HAT medium. alt. HPRT.
hypoxia n. transient low levels of oxygen.
a. hypoxic. hypselodont hypsodont q.v. hypsilophodont a. having high-crowned
teeth with transverse ridges on the cheek-teeth grinding surfaces.
hypsodont a. appl. molar and premolar teeth with high crowns and small roots, as those of grazing mammals such as horses and cows.
hypural a. pert. bony structure formed by fused haemal spines of last few vertebrae, which supports the tail in certain .shes.
hyracoids n.plu. group of placental mam­mals including the hyrax, which have a rodent-like body and skull, but digits over a pad and bearing nails like elephants.
hysteranthous a. coming into leaf after .owering.
hysteresis n. (1) lag in one of two associ­ated processes or phenomena; (2) lag in adjustment of external form to internal stresses.
hysterochroic a. gradually discolouring from base to tip, appl. ageing fruit bodies.
hysterotely n. the retention or manifesta­tion of larval characteristics in pupa or imago, or of pupal characters in imago.
hysterothecium n. elongated apothecium with slits opening in moist conditions and closing in drought, as in certain fungi and lichens.
I

I (1) inosine q.v.; (2) isoleucine q.v.; (3) iodine q.v.; (4) Morisita’s index of disper­sion q.v.
IAA indole-2-acetic acid q.v.
i6Ade 6N-isopentenyladenine q.v.
IAMS International Association of Micro­biological Societies.
IAN indole acetonitrile, a naturally occur­ring auxin.
IAP islet-activating protein. see pertussis toxin.
IAPT International Association of Plant Taxonomy.
I-band, I-disc the light band at either end of a sarcomere of striated muscle, repres­enting a region of actin .laments only. The I-band is bounded at its outer edge by a dark Z-disc, the membrane to which the actin .laments are anchored at one end. see Fig. 28 (p. 419).
ICAM intercellular adhesion molecule q.v.
ICBN International Code of Botanical Nomenclature.
iccosome n. small fragment of membrane coated with immune complexes that frag­ments off follicular dendritic cells.
Ice Age Pleistocene q.v.
I-cell disease disease involving lysosomes, in which hydrolytic enzymes are secreted into the extracellular .uid instead of being packaged in lysosomes.
I-cells interstitial cells in coelenterates.
ichneumon .ies insects of the Ich­neumonidae, a family of hymenopterans that are parasitoids, laying their eggs in the larvae of other insects, especially butter­.ies and moths.
ichthyodont n. fossil tooth of .sh.
ichthyofauna n. the .shes of a particular region, area or habitat.
ichthyoid a. pert., characteristic of, or resembling, a .sh. alt. ichthyic.
ichthyolite n. fossil .sh or part of one.
ichthyology n. the study of .shes.
ichthyopterygium n. vertebrate limb when it is in the form of a .n.
Ichthyosauria, ichthyosaurs n., n.plu. group of Mesozoic aquatic reptiles with spindle-shaped body with .ns and .n-like limbs.
ichthyostegalians, ichthyostegids n.plu. primitive extinct amphibians from the Devonian–Carboniferous, having many .sh-like characteristics and sometimes con­sidered to be primitive labyrinthodonts.
ICM inner cell mass q.v.
ICNB International Committee on Nomen­clature of Bacteria.
ICNV International Committee on Nomen­clature of Viruses.
iconic memory very brief memory that stores the impression of a scene.
iconotype n. a representation, drawing or photograph of a type specimen.
icosandrous a. appl. .owers, having 20 or more stamens.
ICSB International Committee on Systematic Bacteriology.
ICSH interstitial cell-stimulating hormone. see luteinizing hormone.
ICSU International Council of Scienti.c Unions.
ICTV International Committee on Taxonomy of Viruses.
ICZN International Commission on Zoolo­gical Nomenclature.
ID50 dose of virus or other infectious agent at which 50% of the test units (e.g. animals, tissue cultures) become infected.
IDDM insulin-dependent diabetes mellitus q.v.
ideal angle in phyllotaxis, the angle be­tween successive leaf insertions on a stem when no leaf would be exactly above any lower leaf, 137°30’28..
ideomotor a. pert. involuntary movement in response to a mental image.
idiobiology n. the study of individual organisms.
idiomorph n. an alternative form of a gene­tic locus that lacks signi.cant sequence homology with its alleles.
idiopathic a. appl. diseases or conditions not preceded or occasioned by another, or by a known cause.
idiophase n. phase in secondary metabo­lism in which secondary metabolites are produced. cf. trophophase.
idiosoma n. the prosoma and opisthosoma, the body of ticks and mites.
idiotope n. antigenic determinant formed by part of the variable region of an antibody, the set of such idiotopes carried by a single antibody molecule being its idiotype.
idiotrophic a. capable of selecting food.
idiotype n. (1) the unique antigenic deter­minants formed by the variable region of an antibody molecule; (2) the total hered­itary determinants of an individual.
IEP isoelectric point q.v.
IF intermediate .lament q.v.
IF-1, IF-2, IF-3 initiation factors (q.v.) in bacterial protein synthesis.
IFN interferon (q.v.), such as IFN-a, IFN-ß and IFN-..
Ig immunoglobulin q.v.
Iga, Igb proteins that are part of the com­plete antigen receptor complex on B lymphocytes and which are involved in signalling from the receptor when antigen binds to the immunoglobulin part of the complex.
IgA immunoglobulin A q.v.
IgD immunoglobulin D q.v.
IgE immunoglobulin E q.v.
IGF-I somatomedin C q.v.
IGF-II insulin-like growth factor II q.v.
IgG immunoglobulin G q.v.
IgM immunoglobulin M q.v.
IkB protein that is complexed with the tran­scription factor NF.B, keeping it inactive in the cytoplasm.
IkB kinase (IKK) protein serine/threonine kinase that is involved in the intracellular signalling pathway that leads from a variety of receptors to the activation of the tran­scription factor NF.B.
IL-1, IL-2 etc. interleukins q.v.
Ile isoleucine q.v.
ileac, ileal a. pert. the ileum.
ileocaecal a. pert. ileum and caecum.
ileocolic a. pert. ileum and colon.
ileum n. (1) lower part of small intestine;
(2) anterior end of hind-gut in insects. iliac a. pert. or in region of ilium. iliacus n. muscle stretching from upper part
of iliac fossa to side of tendon of psoas major.
ilicium n. dorsal spine with modi.ed tip for luring prey of angler .sh (Lophiidae).
iliocaudal a. connecting ilium and tail, appl. muscle.
iliocostal a. in region of ilium and ribs, appl. muscles.
iliofemoral a. pert. ilium and femur, appl. ligament.
iliohypogastric a. pert. ilium and lower anterior part of abdomen, appl. a nerve.
ilio-inguinal a. in the region of ilium and groin, appl. a nerve.
ilio-ischadic a. pert. opening between ilium and ischium when these are fused at both ends.
iliolumbar a. in region of ilium and loins. iliopectineal a. appl. an eminence marking the point of union of ilium and pubis.
iliopsoas n. the iliacus and psoas major when considered as one muscle.
iliotibial a. appl. tract of muscle at lower end of thigh.
iliotrochanteric a. uniting ilium and trochanter of femus, appl. ligament.
ilium n. dorsal bone in each half of pelvic girdle.
Illicidales n. order of woody dicots com­prising shrubs, climbers and small trees, and including the two families Iliaceae (star anise) and Schisandraceae.
illuvial a. appl. layer of deposition and ac­cumulation below the alluvial layer in soils. alt. B-horizon.
imaginal a. pert. an imago.
imaginal discs small sacs of undiffer­entiated epithelium in body of many insect larvae, which on metamorphosis produce the adult epidermal structures appropriate to each segment. Each disc speci.es a single structure, e.g. leg disc, antennal disc, genital disc.
imago n. last or adult stage of insect meta­morphosis, the perfect insect.
imbibition n. passive uptake of water, esp. by substances such as cellulose and starch, as in uptake of water by seeds before germination.
imbricate a. overlapping, as of scales.
imbricational a. overlapping, appl. layers of enamel deposited on sides of teeth dur­ing growth.
imbrication lines parallel growth lines of dentine.
imitative a. appl. e.g. form, habit, colouring, assumed for protection or aggression when one organism imitates another.
immaculate a. without spots or marks of a different colour.
immarginate a. without a distinct margin.
immature a. (1) still not fully developed;
(2) not yet adult. cf. mature.
immediate early genes genes that are rapidly and transiently induced in the response of eukaryotic cells to agents that cause cell proliferation.
immediate hypersensitivity antibody-mediated hypersensitivity reactions that occur within minutes of exposure to anti­gen. cf. delayed hypersensitivity.
immigrant species species that migrate into an ecosystem or are introduced accid­entally or deliberately by humans.
immobilization n. the locking up of ele­ments essential for plant nutrition in or­ganic matter in soil by soil microorganisms so that the elements are not available for plant growth.
immobilized enzyme in biotechnology, puri.ed enzymes, or whole cells containing the required enzyme, which are immobil­ized by attachment to an inert solid matrix to increase the ef.ciency of enzyme use in industrial processes.
immortalized a. appl. mammalian cells that have become able to continue cell division inde.nitely, such as the cells of a perman­ent cell line which has been derived on prolonged culture from a primary tissue culture. n. immortalization.
importance value a measure of the role of a species within a community, obtained by adding together its relative density, its relative dominance and the relative fre­quency at which it is found.
immune a. appl. an individual who has produced protective antibodies and/or activated T lymphocytes and memory B and T cells against a pathogenic micro­organism and is therefore resistant to reinfection by that pathogen.
immune clearance the clearance of an antigen from the blood by formation of antibody–antigen complexes which are then scavenged by phagocytes.
immune complex complex of antibody and antigen. Deposition in tissues can cause tissue hurt and disease due to activation of the complement system by the complex.
immune disease any disease involving immunological hypersensitivity reactions or autoimmune reactions.
immune memory immunological memory
q.v.
immune response generally refers to an adaptive immune response but may also include the responses of innate immunity. Adaptive immune responses consist of two components. The production of anti­bodies circulating in the blood is known as the humoral immune response, and the production of cytotoxic and helper T cells as the cell-mediated or cellular immune response. see also adaptive immune re­sponse, allergy, antibody, antigen, B lymphocyte, clonal selection, immune sys­tem, immunity, innate immunity, plasma cell, primary immune response, secondary immune response, T lymphocyte.
immune surveillance the thought that the immune system recognizes and destroys most incipient cancer cells before they can develop. There is small evidence for this concept.
immune system the cells and tissues in vertebrates which enable them to mount a response to invading microorganisms, parasites, and other foreign substances. The immune system protects the body from infection and establishes long-lasting spe­ci.c immunity to reinfection. It is also responsible for the recognition and rejec­tion of foreign cells in tissue and organ transplantation. see also adaptive immune response, allergy, antibody, antigen, histocompatibility, immune response, immunity, innate immunity, leukocyte, lymphocyte, lymphoid, primary lymphoid organs, secondary lymphoid tissues.
immune tolerance see tolerance.
immunity n. (1) the ability to resist disease, usually referring to infectious disease. In­nate immunity against infection is due to relatively non-speci.c cellular and molecu­lar systems that attack bacteria or virus-infected cells. Speci.c immunity is a result of an adaptive immune response and may be bought naturally by previous infec­tion or is induced by vaccination with suitably treated microorganisms or their products. see also immune response; (2) in bacteria, the inability of a bacterium carrying a prophage or a plasmid to be infected with another phage or plasmid of the same type.
immunization n. the administration of an antigen (e.g. by injection) that results in a speci.c immune response against the anti­gen. alt. inoculation or vaccination, when the antigen is derived from a pathogen and confers immunity against a disease. v. immunize.
immunoaf.nity column chromato­graphy type of af.nity chromatography in which antibody bound to the column is used to purify its corresponding antigen from a mixture, or vice versa.
immunoassay n. any quantitative assay of a substance using its binding to speci.c antibody as the measuring technique.
immunoblot Western blot q.v.
immunocompetence n. the capacity to respond to antigen stimulation, the capa­city to mount an immune response. a. immunocompetent.
immunocytochemistry immunohis­tochemistry q.v.
immunode.ciency n. any de.ciency in the ability to mount an effective immune response. It may be due to various causes, such as the destruction of a class of helper T lymphocytes in AIDS, the non­production of immunoglobulin due to defects in the immunoglobulin genes, or the non-development of lymphocytes due to various genetic defects. a. immu­node.cient. see also severe combined immunode.ciency.
immunodiagnostics n. the use of anti­bodies to detect and identify e.g. viruses, bacteria, hormones, drugs.
immunodiffusion n. detection of anti­gen or antibody by formation of a line of antibody–antigen precipitate where diffus­ing antigen and antibody meet in an agar gel.
immunoelectron microscopy electron microscopy where specimens have been stained with electron-dense material (e.g. gold particles) attached to a speci.c antibody in order to highlight speci.c structures.
immunoelectrophoresis n. technique for analysing antigens in a complex mixture in which the mixture is separated by elec­trophoresis before probing with marked antibodies.
immuno.uorescence microscopy microscopical technique in which cells are stained with antibodies marked with .uorescent dyes in order to highlight speci.c structures within the cell.
immuno.uorescence test any diagnostic test that uses antibodies marked with .uorescent dyes to detect the desired antigen.
immunogen a. any substance that generates an immune response. see also antigen.
immunogenetics n. genetic analysis of the immune system.
immunogenic a. causing an immune re­sponse. see also antigenic.
immunogenicity n. the ability to provoke an immune response, appl. to antigens.
immunoglobulin (Ig) n. highly variable protein synthesized by the B cells of the immune system and the plasma cells derived from them. Immunoglobulins occur in two forms: as membrane-bound immunoglobulin on the surface of B cells, where they act as the antigen receptors, and as antibodies secreted by plasma cells after the differentiation of B cells into plasma cells during an immune response. An immunoglobulin molecule consists of two identical light chains and two identical larger heavy chains. Light and heavy chains have a variable N-terminal region (V re­gion) which varies in sequence amongst immunoglobulins with different antigen speci.cities, and a constant C-terminal region (C region) which is characteristic of the particular class or isotype to which the antibody belongs. see also antibody, B-cell receptor, C region, immunoglobulin A, immunoglobulin D, immunoglobulin E, immunoglobulin G, immunoglobulin genes, immunoglobulin M, isotype, V region.
immunoglobulin A (IgA) class of immunoglobulin containing heavy chains of isotype a. It occurs as either a monomer or a dimer. The dimer is the main anti­body in gut, and in saliva, tears, milk and colostrum.
immunoglobulin D (IgD) class of immunoglobulin containing heavy chains of isotype d. It appears as surface-bound immunoglobulin on B cells subsequent to and in conjunction with IgM. Only very small amounts are secreted and its func­tion is unknown.
immunoglobulin E (IgE) class of immunoglobulin containing heavy chains of isotype e. It is involved in allergic in.ammatory reactions and responses to intestinal parasites. It is bound by receptors on mast cells and some other leukocytes, where subsequent binding by speci.c antigens triggers release of cell granule contents.
immunoglobulin G (IgG) class of immunoglobulin containing heavy chains of isotype .. It is produced towards the end of a primary immune response and in a secondary response and is the main class of antibody circulating in blood and lymph.
immunoglobulin genes genes encoding the polypeptide chains of immunoglobu­lins. There are three gene loci: the heavy-chain locus, the . light-chain locus, and the . light-chain locus. Each locus con­sists of many similar but not identical “gene segments” encoding parts of the immunoglobulin chain. DNA rearrange­ments during lymphocyte development bring together one of each type of gene segment to form the sequence specifying a complete heavy- or light-chain gene. see C gene, D gene segment, J gene segment, V gene segment.
immunoglobulin-like domain protein do­main related in structure to the character­istic protein domains of immunoglobulins.
immunoglobulin M (IgM) class of immunoglobulin containing heavy chains of isotype µ. It is the .rst class of immunoglobulin synthesized by devel­oping B cells and is the .rst class of anti­body secreted in a primary immune response. IgM antibody is a pentamer of .ve IgM molecules held together by a joining polypeptide (J chain).
immunoglobulin receptor the antigen receptor carried on the surface of the B cells of the immune system, which is a transmembrane form of immunoglobulin. alt. B-cell receptor. see also Fc receptor, poly-Ig receptor.
immunoglobulin superfamily large superfamily of proteins that contains the immunoglobulins, T-cell receptors, and many other cell-surface proteins, such as cell adhesion molecules, that have immunoglobulin-like domains.
immunohistochemistry n. techniques for locating particular proteins in cells which use speci.c antibodies marked with an enzyme that will produce a coloured reac­tion product.
immunological memory the ability of the immune system to mount a larger and more rapid response (the secondary immune re­sponse) to an antigen already encountered, and which is mediated by long-lived T and B cells activated in the initial immune response (the primary immune response).
immunological tolerance see tolerance.
immunology n. the study of the immune system.
immunophilin n. protein that binds to immunosuppressive drugs such as cyclosporin A and tacrolimus and is in­volved in their immunosuppressive effect.
immunoprecipitation n. the detection of a protein in a mixture by binding to its speci.c antibody, leading to precipitation of the protein–antibody complex.
immunosuppression n. suppression, e.g. by radiation and by certain drugs, of the ability to mount an immune response.
immunotoxin n. (1) an antitoxin that con­fers immunity against a disease; (2) an antibody conjugated with a toxin or drug that is speci.cally targeted to ruin or react with certain cells.
IMP inosine monophosphate q.v.
imparidigitate a. having an unequal num­ber of digits.
imparipinnate a. pinnate with an odd terminal lea.et.
impedicellate a. having a very small or no pedicel (stalk).
imperfect a. appl. .owers lacking either stamens or carpels.
imperfect fungi Deuteromycetes q.v.
imperfect stage in ascomycete fungi, the asexual reproductive phase in their life history in which they bear conidia.
imperforate a. not pierced, appl. shells of foraminiferans that lack .ne pores.
impermeable a. appl. barrier, e.g. a mem­brane, that will not let a given substance pass through it. cf. permeable.
impetigo n. skin infection caused by strains of staphylococci or streptococci.
implantation n. the embedding of the fer­tilized ovum in lining of uterus.
impregnation n. transfer of sperm from male to body of female.
imprinting n. (1) a form of learning in very young animals in which a particular stimu­lus normally provided by a parent becomes permanently associated with a particular response. Examples are when a young bird learns to follow a large moving object, usually the parent bird, or when young mammals follow their mother in response to the smell of her milk; (2) genomic imprinting q.v.; (3) sexual imprinting q.v.
impulse see action potential.
inactivation n. in neurophysiology, decline of sodium current component of action potential.
inantherate a. lacking anthers.
inappendiculate a. lacking appendages.
Inarticulata n. class of brachiopods in which the shells are joined by muscles only and not by a hinged joint. cf. Articulata.
inarticulate a. (1) not segmented; (2) not jointed.
inborn error of metabolism any of a di­verse group of inherited conditions caused by a genetically determined de.ciency of a particular enzyme or production of an impaired protein. alt. enzyme de.ciency disease.
inbreeding n. (1) matings between related individuals, alt. consanguineous matings;
(2) successive crossing between very closely related individuals, in e.g. labor­atory animals, plants. This leads to the establishment of pure-breeding strains or varieties in which individuals are homozygous at a large proportion of their loci, or at selected loci.
inbreeding coef.cient (F) (1) of an indi­vidual, the probability that the pair of alleles carried by the male and female gametes that produced it are identical by descent from a common ancestor, as a result of inbreeding; (2) a measure of the reduction of heterozygosity as a result of inbreeding, given by FS = (HI – HS)/HS, where HI and HS are heterozygosity among an inbred and outbred group of individuals of the same population respectively.
inbreeding depression loss of vigour fol­lowing inbreeding, due to the expression of numbers of deleterious genes in the homozygous state.
incidence n. (1) occurrence of an event;
(2) rate at which something occurs.
incipient lethal level concentration of a toxin at which 50% of the population of test organisms can live for an inde.nite time.
incipient species populations that are diverging towards the point of becoming separate species, but which can still inter­breed although they are prevented from doing so by some geographical barrier.
incised a. with deeply notched margin.
incisiform a. shaped like incisors.
incisive a. pert. or in region of incisors.
incisors n.plu. the cutting front (premaxil­lary) teeth of mammals.
included a.(bot.) having stamens and pis­tils not protruding beyond the corolla.
included niche the case where the niche of a species occurs completely within the niche space of another species.
inclusion bodies intracellular particles such as crystals formed of e.g. viruses or proteins.
inclusive .tness sum of an individual’s own .tness plus all its in.uence on .tness in its relatives other than direct descendants.
incompatible a. (1) see incompatibility;
(2) in plant pathology, appl. an interaction between host and pathogen that does not result in disease.
incompatibility n. (1) genetically deter­mined inability to cross-fertilize (in plants) or mate (in animals) successfully; (2) the rejection of transplanted tissue of a differ­ent tissue type; (3) of plasmids, see plasmid incompatibility.
incomplete a. appl. .owers lacking sepals or petals or stamens or carpels.
incomplete dominance co-dominance q.v.
incomplete metamorphosis insect meta­morphosis in which young are hatched in general adult form (but without wings and without mature sexual organs), and develop without a quiescent (pupal) stage. cf. com­plete metamorphosis.
incomplete penetrance the lack of expression of a genetic trait in some indi­viduals possessing the genotype associated with the character.
incongruent a. not suitable or .tting, appl. surfaces of joints that do not .t properly.
incoordination n. (1) want of coordination;
(2) irregularity of movement due to loss
of muscle control.
incrassate a. thickened, becoming thicker.
incubation n. (1) the hatching of eggs by means of heat, natural or arti.cial; (2) the growth of a culture of microorganisms by keeping it for some time at an optimum temperature.
incubation period period between infec­tion and the appearance of symptoms induced by pathogenic bacteria, viruses or other parasites, during which the pathogen is multiplying.
incubatorium n. temporary pouch sur­rounding mammary area, in which the egg of the echidna, the spiny anteater, is hatched.
incudal a. pert. the incus, one of the small bones of the middle ear. incumbent a. (1) lying upon; (2) bent downwards to lie along a base. incurrent a. (1) leading into; (2) afferent;
(3) appl. ectoderm-lined canals which admit water in sponges, alt. ostia; (4) appl. inhalant siphons of molluscs; (5) appl. ostia in insect heart which admit blood.
incurvate a. curved upwards or bent back.
incus n. the middle of the chain of three small bones in the middle ear that transmit sound from the ear drum to the inner ear in mammals. It is shaped like an anvil.
indeciduate a. (1) not withering away, of sepals on fruit; (2) with the maternal part of the placenta not coming away at birth.
indeciduous n. (1) persistent; (2) not falling off at maturity; (3) everlasting;
(4) evergreen. inde.nite n. not limited in e.g. size, number. indehiscent a. appl. fruits, spores or
sporangia which do not open to release the seeds or spores, the whole structure being shed.
indel abbreviation used in molecular genetics meaning insertion or deletion.
independent assortment Mendel’s sec­ond law, which describes the fact that the segregation of alleles at one gene occurs independently of the segregation of alleles at another. This law has had to be modi.ed by the subsequent discovery of linkage,
i.e. that alleles carried close together on the same chromosome tend to be inherited together.
indeterminate a. (1) inde.nite; (2) unde­.ned; (3) not classi.ed.
indeterminate growth (1) growth from an apical meristem which forms an unre­stricted number of lateral organs such as stems, branches or shoots inde.nitely, not stopped by the development of a terminal bud; (2) inde.nite prolongation and sub­division of an axis.
indeterminate in.orescence an in.ores­cence that grows by indeterminate branch­ing because unlimited by the development of a terminal bud.
index n. (1) the fore.nger or digit next to the thumb; (2) a number or formula expressing the ratio of one quantity to another.
index case any case of an infectious dis­ease that exhibits new syndromes, types of pathogen or characteristics that indicate high potential for a new epidemic.
index fossil a fossil which typically occurs in a particular zone of a rock stratum and after which the zone is known.
index species an organism that lives only within a narrow range of environmental conditions and whose presence therefore indicates places where those conditions exist.
Indian region part of the Palaeotropical phytogeographical kingdom comprising the Indian subcontinent, Sri Lanka, the Laccadives and Maldives.
Indian subregion subdivision of the Ori­ental zoogeographical region, comprising all the Indian subcontinent south of the Himalayas except for the southern tip.
indicator species (1) species characteristic of climate, soil and other conditions in a particular region or habitat; (2) dom­inant species in a biotype; (3) species whose disappearance or disturbance gives early warning of the degradation of an ecosystem.
indifference curves iso-utility curves q.v.
indifferent species species that is not found in any particular community.
indigenous a. (1) belonging to the locality;
(2) not imported; (3) native.
indirect competition limitation of popu­lation size or fertility of two or more organisms because of competition for the same limited resource.
indirect immuno.uorescence type of immuno.uorescence test in which the .uorescent mark is attached to a secondary anti-immunoglobulin antibody, which is used to mark the primary antigen–antibody complex.
indirect transmission transmission of infection from one person to another via an intermediate living or inanimate object.
individual distance the distance around a bird in a .ock, which it defends while feeding or nesting.
individualism n. symbiosis in which the two parties together form what appears to be a single organism.
individuation n. (1) formation of inter­dependent functional units, as in a colonial organism; (2) process of developing into an individual.
Indo-Chinese subregion subdivision of the Oriental zoogeographical region, com­prising southern China and South-East Asia except for the Malayasian peninsula.
indole-2-acetic acid (IAA) a naturally occurring auxin q.v.
indole acetonitrile (IAN) a naturally occurring auxin q.v.
indoleamines n.plu. the monoamine neurotransmitters, and hormones such as serotonin and melatonin, which contain an indole group.
Indo-Malayan subregion subdivision of the Oriental zoogeographical region, comprising the Malayasian peninsula, the Indonesian islands west of Wallace’s line, and the Philippines.
Indo-Malaysian subkingdom subdivision of the Palaeotropical phytogeographical kingdom, comprising the Indian subcon­tinent and the Himalayas, southernmost China, South-East Asia, the Malaysian archipelago, Indonesia, Papua New Guinea and the Philippines. It is divided into the following regions: Continental South-East Asiatic, Indian, Malaysian.

induced a. appl. genes that are transcribed, or proteins that are synthesized, only in response to a speci.c stimulus. cf. consti­tutive. see also inducer, induction.
induced .t a model of enzyme–substrate interaction in which the substrate induces a conformational change in the active site of an enzyme on binding which enables the substrate to .t into the active site. cf. lock-and-key model.
induced mutations those occurring as a result of deliberate treatment with a mutagen. cf. spontaneous mutations.
inducer n. (1) any compound that speci.c­ally causes the synthesis of an enzyme, cf. repressor; (2) in bacteria, any small mol­ecule that speci.cally causes the expres­sion of genes specifying the enzymes required to metabolize it; (3) any chem­ical or physical stimulus that causes the expression of a speci.c gene; (4) in em­bryology, any substance produced by cells that in.uences neighbouring cells or tissues.
inducible a. (1) appl. proteins whose syn­thesis is stimulated in the presence of a speci.c agent (the inducer) which may be a chemical compound or a physical stimulus such as heat or light, cf. constitu­tive; (2) appl. genes whose transcription is similarly speci.cally stimulated.
induction n. (1) act or process of causing to occur; (2) (dev.) process in development whereby a cell or tissue directs neighbour­ing cells or tissues to develop in a particu­lar way; (3) (genet.) the speci.c synthesis of proteins in response to some stimulus, involving the speci.c activation and tran­scription of the genes encoding the required proteins; (4) (neurobiol.) lowering by one re.ex of the threshold of another, spinal induction; (5) (virol.) of viruses, the pro­duction of infectious particles from a cell carrying a provirus or prophage.
inductive stimulus an external stimulus that in.uences the growth or behaviour of an organism; an internal stimulus causing the phenomenon of induction in development.
inductor organizer q.v.
indumentum n. (1) plumage of birds;
(2) hairy covering in plants or animals. indurated a. becoming .rmer or harder. indusial a. (1) containing insect larval
cases, as certain limestones; (2) pert. an
indusium.
indusiate a. having an indusium.
indusium n. (1) (bot.) an outgrowth of epi­
dermis in plants, covering and protecting a sorus, as in ferns; (2) (mycol.) outgrowth hanging from top of stipe in some fungi;
(3) (zool.) case of a larval insect. plu. indusia.
industrial melanics dark-coloured forms of otherwise light-coloured moths and other insects that have increased in indus­trial areas since the Industrial Revolution, selected, it is thought, by the need for camou.age against soot-blackened walls and trees.
induviae n.plu. scale leaves.
inequilateral a. (1) having two sides unequal; (2) having unequal parts on either side of a line drawn from umbo to gape of a bivalve shell.
inequivalve a. having the valves of shell unequal, appl. molluscs.
inerm(ous) a. (1) without means of defence and offence; (2) without spines.
inert a. (1) physiologically inactive; (2) appl. regions of heterochromatin, in which genes are not active.
inertia n. the ability of a living system to resist being disturbed.
infection n. invasion of the tissues of the body by bacteria, viruses, fungi, protozoans, and other internal parasites. cf. infestation.
infection thread structure formed by invagination of root hair cell through which the nitrogen-.xing rhizobia enter host tissue.
infectious a. (1) capable of causing an invasion; (2) capable of being transmitted from one organism to another.
infectivity n. a quantitative measure of the ability of a virus, bacteria or other parasite to cause an infection, measured by vari­ous assays.
inferior a. below; (1) growing or arising below another organ or structure; (2) appl. plant ovary having perianth inserted around the top; (3) appl. sepals, petals or stamens attached to the top of the ovary.
inferior colliculi paired structure on dorsal surface of midbrain, which receives audi­tory information from auditory nerve. cf. superior colliculi.
inferobranchiate a. with gills under margin of mantle, as certain molluscs.
inferolateral a. below and at, or towards, the side.
inferomedian a. below and about the middle.
inferoposterior a. below and behind.
infertile a. (1) not fertile as a result of the non-production of gametes or a fail­ure of fertilization. n. infertility; (2) non-reproductive.
infestation n. invasion of the body by ectoparasites such as ticks or mites, which remain on the surface and do not enter tissues.
in.ammation n. painful swelling and red­ness of tissues caused as a result of infec­tion or tissue hurt.
in.ammatory cells cells that invade tissues and help to cause in.ammation. Examples are macrophages, mast cells, eosinophils and neutrophils.
in.ammatory mediator general term for any molecule involved in causing in.ammation. Examples are histamine, chemokines (which attract white blood cells into in.amed tissues), leukotrienes, prostaglandins and tumor necrosis factor-alpha (TNF-a).
in.ammatory reactions the cellular and molecular reactions that cause in.amma­tion. They are due chie.y to the activities of macrophages and white blood cells that enter in.amed tissues.
in.ammatory T cell Th1 cell. see helper T lymphocyte.
in.ected, in.exed a. curved or abruptly bent inwards or towards the axis.
in.orescence n. (1) .ower-head in .owering plants; (2) in mosses and liverworts, the area bearing the antheridia and archegonia.
in.orescence meristem apical shoot meristem from which an in.orescence develops.
in.uents n.plu. the animals present in a plant community, or those primarily de­pendent and acting upon the dominant plant species.
in.uenza n. respiratory disease, often severe, caused by the in.uenza virus. Pandemics of in.uenza are caused by major genetic changes in the virus. see genetic drift, genetic shift.
informational a. appl. molecules: DNA and RNA, the carriers of genetic information.
infra-(1) pre.x derived from Gk infra, below; (2) in classi.cation, denotes a group just below the status of a subgroup of the taxon following it, as in infraclass, the group below the subclass.
infra-axillary a. branching off just below
the axil. infrabranchial a. below the gills. infrabuccal a. (1) below the cheeks; (2)
below the buccal mass in molluscs. infracentral a. below a vertebral centrum. infraclass n. taxonomic grouping between
subclass and order, e.g. Eutheria (mam­mals lacking pouches) and Metatheria (marsupials which bring forth live young) in the subclass Theria.
infraclavicle n. membrane bone occurring
in pectoral girdle of some .shes. infraclavicular a. below the clavicle. infracortical a. below the cortex. infracostal a. beneath the ribs, appl.
muscles. infradentary a. below the dentary bone. infradian a. appl. biological rhythm with a
period of less than 24 hours. infrahyoid a. beneath the hyoid bone, appl.
muscles. infralabial a. beneath the lower lip. infralittoral a. (1) appl. depth zone of lake
which is permanently covered with rooted or .oating macroscopic vegetation; (2) upper subdivision of the marine sublittoral zone.
inframarginal a. under the margin, or marginal structure.
inframaxillary a. beneath maxilla, appl. nerves.
infraneuston n. small aquatic animals living on the underside of the surface .lm of water, such as some mosquito larvae.
infraorbital a. beneath the orbit of the eye.
infrapatellar a. appl. pad of stout beneath patella.
infrapopulation n. all the organisms of a single species of parasite within a single host at a particular time.
infrared radiation (IR) electromagnetic ra­diation with wavelengths from ca. 800 mm to ca. 1 mm.
infrascapular a. beneath the shoulder blade.
infraspeci.c a. (1) occurring within a species, appl. e.g. variation, competition;
(2) pert. a subdivision of a species, such
as a subspecies or variety.
infrastapedial a. beneath the stapes of ear.
infrasternal a. below the breast bone.
infratemporal a. beneath the temporal bone.
infructescence anthocarp q.v.
infundibula n.plu. (1) passages surrounded by air cells in the lung; (2) plu. of infundibulum.
infundibulum n. (1) funnel-shaped organ or structure; (2) the siphon of a cephalopod; (3) outpushing of .oor of brain that develops into the stalk of the pituitary gland; (4) conus arteriosus q.v.
infusoria, infusorians n., n.plu. term origi­nating in the 19th century for microscopic animals such as protozoans and rotifers. Until quite recently used to denote the ciliate protozoans, but now no longer in general scienti.c use.
infusoriform a. appl. ciliated free-swimming larvae of Mesozoa.
ingesta n. sum total of material taken in by ingestion.
ingestion n. mode of nutrition in which solid food is swallowed or taken into the gut or food cavity. a. ingestive. v. ingest.
Ingoldian fungi certain aquatic hyphomycetes found on submerged leaves and twigs in well-aerated water.
ingression n. the penetration of super.cial cells individually into the interior of the embryo.
ingroup n. in phylogenetic studies, the taxon under study. This is usually a group of closely related species considered to be monophyletic. cf. outgroup.
inguinal a. pert. or in region of the groin.
inhalant, inhalent a. adapted for breathing in or drawing in water, such as terminal pores of sponges and siphons of molluscs.
inheritance see heredity, Mendelian inheritance, non-Mendelian inheritance, polygenic inheritance, simple Mendelian traits.
inhibin A, inhibin B polypeptide hormones that inhibit the secretion of follicle-stimulating hormone. They are produced by the gonads.
inhibition n. (1) prevention or checking of an action, process or biochemical reaction;
(2) action of one neuron on another tending to prevent it from generating an impulse.
inhibitor n. (1) any agent which checks or prevents an action or process; (2) any substance which reversibly or irreversibly prevents the normal action of an enzyme without destroying the enzyme. Competit­ive inhibitors act by binding to the active site and preventing binding of substrate, non-competitive inhibitors act by binding to other parts of the enzyme.
inhibitory a. (1) tending to prevent a reac­tion or action, appl. e.g. stimuli, cells, com­pounds; (2) appl. neurons whose activity prevents adjacent neurons from .ring; (3) appl. neurotransmitters whose action tends to prevent neurons from .ring; (4) appl. synapses at which transmission of a signal tends to prevent .ring in the postsynaptic neuron. cf. excitatory.
inhibitory postsynaptic potential (IPSP) hyperpolarizing potential produced in a postsynaptic neuron by neurotransmitter action. It tends to prevent an action poten­tial being generated.
inion n. exernal protuberance of occipital bone.
initial n. self-renewing undifferentiated cell in a plant meristem. It produces both fur­ther initials, which remain in the meristem, and sister cells that go on to leave the meristem and differentiate.
initiation n. in animal behaviour, the re­sponse to a stimulus with behaviour that was not present before.
initiation codon the .rst codon of the protein-coding region in mRNA and the point at which translation starts. It nearly invariably speci.es methionine (in euk­aryotes) or formylmethionine (in bacteria and mitochondria). alt. initiator codon.
initiation complex (1) transcription com­plex q.v.; (2) complex of small ribosomal subunit, aminoacyl-initiator tRNA and messenger RNA that is assembled before translation commences.
initiation factor any of the accessory pro­teins needed for the initiation of transla­tion and which are involved in mRNA binding to the ribosome, initiator tRNA binding, and ribosome subunit associa­tion and dissociation. IF-2 and eIF-2 are guanine-nucleotide binding proteins. Des­ignated IF-1, IF-2, IF-3 in bacteria, eIF-1, eIF-2, . . . eIF-6 in eukaryotic cells.
initiator codon initiation codon q.v.
initiator tRNA a speci.c tRNA that picks up methionine (in eukaryotes) or formy­lmethionine (in bacteria or mitochondria) and binds to the free small subunit of the ribosome as one of the .rst steps in the formation of the initiation complex in pro­tein synthesis.
ink sac in certain cephalopods such as Sepia, a pear-shaped body in wall of the mantle cavity which contains the ink gland, secreting a black substance, ink or sepia, which is ejected as a means of defence.
inland wetland land that is covered for all or part of the year with fresh water, such as swamps, bogs, marshes, fens.
innate a. inborn, appl. behaviour which does not need to be learned.
innate immunity protection against infec­tion as a result of the activation of .xed, relatively non-speci.c defence mechan­isms. cf. adaptive immunity.
innate releasing mechanism (IRM) an instinctive mechanism in an animal which is activated to produce a response by some external stimulus.
inner cell mass (ICM) mass of cells in the mammalian blastocyst that will give rise to an embryo.
inner ear bony cavity in skull behind middle ear. It contains the membranous labyrinth, the cochlea, and the utricle and saccule connecting them. The labyrinth is concerned with balance and sensing posi­tion and consists of three .uid-.lled semi­circular canals at right angles to each other. The spirally coiled cochlea is concerned with sensing pitch of sounds.
inner mitochondrial membrane see mitochondrion.
innervate v. to supply with nerves.
innervation n. the nerve supply to an organ or part.
innervation ratio the number of muscle .bres innervated by a single motor axon.
innominate a. nameless.
innominate artery artery that gives rise to arteries in head and forelimb.
innominate bone the hip bone or lateral half of the pelvic girdle.
innominate vein vein that joins up veins from head and forelimb.
inoculation n. (1) administration of a vaccine in order to induce protective im­munity; (2) introduction of bacteria or other microorganisms, or plant and animal cells, into nutrient medium to start a new culture;
(3) introduction of pathogen into a host. inoculum n. cells, bacteria or spores used for starting off a culture.
inoperculate a. without an operculum or lid; (1) appl. spore capsules; (2) appl. .sh lacking a gill cover.
inordinate a. not in any regular arrange­ment, appl. spores in an ascus.
inorganic a. appl. material or molecules which do not contain carbon.
inorganic chemistry the chemistry of substances other than those containing carbon.
inosine (I) n. nucleoside found in tRNA, formed from adenosine by replacement of the amino group with an oxygen, often present in the .rst anticodon position where it can pair with U, C or A. The base is hypoxanthine.
inosine monophosphate (inosinic acid, inosinate) (IMP) n. a nucleotide, biosynthetic precursor to adenine mon­ophosphate and guanosine monophosphate.
inositol n. a cyclic hexahydric alcohol, occurring in various forms, of which myo-inositol is the most vital. It is a constituent of phospholipids (e.g. phosphatidylinositol) and also of phytic acid and phytin in plants.
inositol phospholipid pathway intra­cellular signalling pathway that starts with the generation of inositol trisphosphate and diacylglycerol by phospholipase C from inositol-containing phospholipids in the plasma membrane. alt. phosphoinositide pathway.
inositol trisphosphate (InsP3, IP3) inositol 1,4,5-trisphosphate, an intracellular signalling molecule produced by the action of phospholipase C on membrane phosphatidylinositol phosphate in response to stimulation of certain cell-surface receptors. Its chief effect is to stimulate the release of Ca2+ from intracellular stores such as the endoplasmic reticulum. Rais­ing the level of free Ca2+ in the cytosol activates intracellular signalling pathways, leading to the appropriate cellular response. see Fig. 24.
inquilinism n. type of symbiosis in which an animal lives in the nest of another, is tolerated by the host and shares its food.
Insecta, insects n., n.plu. very large class of arthropods, found as fossils from the Devonian onwards, containing some three-quarters of all known extant species of animal. The insects include .ies, bees and wasps, ants, butter.ies and moths, beetles, dragon.ies, grasshoppers and crickets, and many other orders. The segmented body is divided into distinct head, thorax and abdomen. The head bears one pair of antennae and paired mouthparts, and the thorax bears three pairs of walking legs and usually one or two pairs of wings. Other types of appendage may be present on the abdomen. The life history usually includes metamorphosis. alt. Hexapoda. see Appendix 3 for orders.

insecticide n. chemical that kills insects.
Insectivora, insectivores n., n.plu. large order of primitive insect-eating and omni­vorous placental mammals known from Cretaceous times to the present, including hedgehogs, moles, shrews.
insectivorous a. insect-eating, appl. cer­tain animals and carnivorous plants. alt. entomophagous. n. insectivore.
insemination n. (1) introduction of semen or sperm into female genital tract; (2) transfer of a fertilized ovum from one female to another.
inserted a. (1) attached; (2) united by natural growth, as petals to ovary.
insertion n. (1) point of attachment of organs, as of muscles, leaves; (2) point on which force of a muscle is applied;
(3) mutation in which a segment of chro­mosome or a small sequence of bases is inserted in a gene.
insertional mutagenesis the production of a mutation as a result of the insertion of a piece of DNA into a gene by in vitro gene manipulation.
insertion sequence (IS) a simple type of transposon found in bacteria, consisting of around 800–1500 bp and carrying only the genetic functions for its own transposition. Insertion sequences are also found at the ends of certain transposons.
insessorial a. adapted for perching.
insight learning learning involving reason­ing, as in humans and to a lesser extent in some animals.
insistent a. appl. hind-toe of certain birds, whose tip only reaches the ground.
in situ in the original place.
in situ hybridization technique for locat­ing the site of a speci.c DNA sequence on a chromosome, by treating mitotic cells with a radioactively or .uorescently la­belled nucleic acid probe complementary for that sequence. The probe binds to the DNA sequence and its position can be detected. The technique is also applied to detect and locate the synthesis of speci.c mRNAs in tissues. see also FISH.
insolation n. exposure to the sun’s rays. InsP3 inositol trisphosphate q.v.
inspiration n. the act of drawing air or water into the respiratory organs.
instaminate a. lacking stamens.
instar n. larva of insect or other arthropod between moults.
instinct n. behaviour that occurs as an in­evitable stereotyped response to an appro­priate stimulus, sometimes equivalent to species-speci.c behaviour. a. instinctive.
instipulate a. lacking stipules.
instructive a. appl. developmental signal that changes the course of development of a tissue, cf. permissive.
instrumental conditioning, instru­mental learning type of associative learn­ing in which the animal initially succeeds as a result of trial and error, is rewarded, and eventually learns to perform the task correctly at the .rst attempt, the right response being “instrumental” in eliciting the reward. alt. operant conditioning.
insula n. (1) triangular eminence lying deep in lateral .ssure of temporal lobe of brain;
(2) islet of Langerhans q.v.; (3) blood island q.v.
insulator elements DNA sequences that prevent the spread of heterochromatin into adjacent chromatin. They are also able to prevent an enhancer associated with one gene from in.uencing the transcription of an adjacent gene. alt. boundary elements.
insulin n. polypeptide hormone produced by ß-cells of the islets of Langerhans in the pancreas. It decreases the amount of glu­cose in the blood by promoting glucose uptake by cells and increasing the capa­city of the liver to synthesize glycogen. Its action is antagonistic to glucagon, adrenal glucocorticoids and adrenaline, and its de.ciency or reduced activity produces a raised blood sugar level and diabetes.
insulin-dependent diabetes mellitus (IDDM) type of diabetes mellitus attribut­able to non-production of insulin, and which can be treated with insulin. alt. type I diabetes, juvenile-onset diabetes.
insulin-like growth factor I (IGF-I) poly­peptide growth factor whose production is stimulated by growth hormone. Involved in post-embryonic and post-natal growth.
insulin-like growth factor II (IGF-II) polypeptide growth factor active in pro­moting growth in the embryo.
intectate a. without a tectum.
integral a. appl. membrane proteins that are .rmly embedded in the membrane and dif.cult to remove. They are either transmembrane proteins or covalently linked to the membrane via lipids.
integrase n. enzyme that catalyses the inte­gration of one DNA molecule into another by recombination.
integrate n. a piece of DNA inserted into another DNA molecule.
integrated pest management combined use of biological, chemical and cultivation methods to keep pests at an acceptable level.
integration n. (1) in the nervous system, the coordination of inputs from many neu­rons in a single neuron which combines them to produce a new signal for onward transmission; (2) the insertion by recom­bination of phage, plasmid, viral or other double-stranded DNA into another DNA molecule (e.g. bacterial chromosome or eukaryotic cell chromosome) to form a new continuous DNA molecule.
integrifolious a. with entire leaves.
integrin n. any of a large family of dimeric transmembrane glycoproteins that act as cell adhesion molecules on animal cells. Cells adhere to other cells and to extra­cellular matrix by binding of integrins to receptor proteins on these surfaces.
integument n. (1) a covering, investing or coating structure or layer; (2) coat of ovule.
a. integumentary.
intein n. an internal part of a protein sequence that is excised after translation to form an independent protein, while the two remaining parts (the exteins) are spliced together to make another protein. Inteins have been found in pro­teins from bacteria and some lower eukaryotes.
intentional behaviour behaviour that involves a mental representation of a goal that guides the behaviour.
intention movement preparatory motions an animal goes through before a complete behavioural response, e.g. the snarl before the bite.
inter-pre.x derived from L. inter, between. interaction domain protein domain that recognizes particular motifs and other
domains on proteins and is involved in speci.c interactions with other proteins.
interactome n. the interactions between all the proteins expressed by a given genome.
interalveolar a. among alveoli, appl. cell islets.
interambulacral a. appl. area of echinoderm test between two ambulacral areas.
interatrial a. appl. groove and partition separating the two atria of the heart.
interatrial septum thin transparent sheet of nervous tissue from heart, containing neurons receiving input from the vagus nerve, and innervating the muscles of the atrium.
interauricular a. between auricles of heart.
interaxillary a. placed between the axils.
interband n. in polytene chromosomes, a lightly staining region of variable size alternating with densely staining regions (bands) along the length of the chromo­some, each chromosome having a charac­teristic and invariant banding pattern.
interbrachial a. between arms or rays (e.g. of echinoderms).
interbranchial a. appl. partitions between successive gill slits.
interbreed v. to cross different varieties, species or genera of plants and animals.
interbreeding a. appl. a population whose members can breed successfully with each other.
intercalary a. (1) inserted between others;
(2)
appl. growth elsewhere than at growing point; (3) (bot.) appl. meristematic layers between masses of permanent tissues;

(4)
(zool.) appl. veins between main veins of insect wings; (5) appl. discs: transverse wavy bands formed by boundaries of sar­comeres in heart muscle. alt. intercalated.
intercalation n. insertion between two other structures, as of certain mutagens such as acridine dyes, which insert between the base pairs in DNA.
intercarpal a. among or between carpal bones, appl. joints.
intercarpellary a. between the carpels.
intercellular a. among or between cells,
appl. spaces, material.
intercellular adhesion molecules (ICAMs) family of cell-surface molecules of the immunoglobulin superfamily that bind to leukocyte integrins.
intercentral a. between two centra.
intercentrum n. a 2nd central ring in some types of vertebrae.
interchondral a. appl. articulations and ligaments between costal cartilages.
interchromosomal a. (1) between chromo­somes, appl. recombination; (2) appl. .brils that play a part in the beginning of cell-wall formation in plants.
interclavicle n. bone between clavicles of shoulder girdle in most reptiles.
interclavicular a. between the clavicles.
intercostal a. (1) between the ribs, appl. art­eries, veins, glands, muscles; (2) between the veins of a leaf.
intercostobranchial a. appl. lateral branch of 2nd intercostal nerve which supplies upper arm.
intercrine chemokine q.v.
intercropping n. growing two types of crop on the same land, usually where one bene.ts the other, as legumes and cereals.
intercross n. the crossing of heterozygotes of the F1 generation amongst themselves.
interdeferential a. between the vasa deferentia.
interdemic selection situation when entire breeding groups (demes) are the basic unit of natural selection.
interdigitating reticular cell dendritic cell
q.v.
interfascicular a. (1) appl. parenchyma separating vascular bundles from each other in stems of some conifers and dico­tyledons; (2) situated between the vascular bundles, appl. cambium.
interfemoral a. between the thighs.

interference n. (1) in virology, the case where the presence of one type of virus in a cell prevents concurrent infection with, or multiplication of, another virus; (2) in genetic recombination, the effect that the presence of one crossover reduces the probability of another occurring in its vicinity.
interference colours colours produced by optical interference between re.ections from different layers of the surface.
interferons n.plu. family of small proteins produced by vertebrate cells in response to viral infection and as cytokines during an immune response. Interferons a and ß (IFN-a and IFN-ß) are induced by viral infection and have antiviral effects as a result of their inhibition of translation and stimulation of the breakdown of viral nucleic acid within the infected cell. Inter­feron . (IFN-.) is secreted by activated T lymphocytes during an immune response and is an vital growth factor for lymphocytes and other immune system cells.
interfertile a. able to interbreed.
interganglionic a. connecting two ganglia, as nerve cords.
intergemmal a. between taste buds, appl. nerve .bres.
intergeneric a. between genera, appl. hybridization.
intergenic a. situated between genes.
interglacial a. appl. or pert. the ages be­tween glacial ages, particularly of the Pleistocene.
interlamellar a. (1) appl. vertical bars of tissue joining gill lamellae of molluscs;
(2) appl. compartments of lung book in scorpions and spiders; (3) appl. spaces between lamellae or gills of agarics.
interleukin n. any of a diverse group of cytokines produced by activated macro­phages and lymphocytes during an immune response. They act on lymphocytes and other leukocytes to stimulate their prolif­eration and differentiation. Abbreviated IL-1, IL-2, IL-3, etc.
interleukin-2 (IL-2) one of the .rst cytokines produced by activated T cells, and which acts in an autocrine manner to stimulate the proliferation and differentia­tion of the T cells that produce it.
interlittoral a. shallow marine zone to a depth of around 20 m.
interlobar a. (1) between lobes; (2) appl. sulci and .ssures dividing cerebral hemi­spheres into lobes.
intermalar a. situated between the cheek bones. intermaxilla n. (1) bone between maxillae;
(2) premaxilla q.v.
intermediary a. appl. nerve cells receiving impulses from afferent cells and transmit­ting them to efferent cells.
intermediary metabolism metabolic pathways by which the basic molecular building blocks in a cell (e.g. mono­saccharides, amino acids, nucleotides) are interconverted and incorporated into larger molecules.
intermediate n. in a chemical reaction, a chemical species that forms transiently along the reaction path from starting material to product.
intermediate compartment cis Golgi network q.v.
intermediate .lament (IF) one of the three main components of the eukaryotic cell cytoskeleton. A tough and durable type of .lament that forms a network in eukaryotic cells subject to stress. Examples are the keratin .laments of skin cells, the neuro-.laments of nerve cell axons, the vimentin .laments of connective tissue cells, and the lamin .laments of the nuclear lamina. Intermediate .laments are constructed of subunits of various types of intermediate .lament proteins.
intermediate host organism in which a parasite lives for part of its life-cycle but in which it does not become sexually mature.
intermediate junction type of adhesive junction between epithelial cells which runs in a band around the circumference of the upper part of the cell, just below the tight junction. alt. belt desmosome, adhaerens junction, zonula adhaerens.
intermediate memory type of memory that lasts longer than small-term memory but not as long as long-term memory.
intermedin melanocyte-stimulating hor­mone q.v.
intermedium n. a small bone of carpus and tarsus.
intermembrane space space between the two membranes surrounding organelles such as mitochondria and chloroplasts, and between the inner and outer membranes of bacterial envelopes.
intermolecular a. between molecules, appl.
e.g. hydrogen bonds, distances. intermuscular a. between or among muscle .bres.
internal a. (1) located on inner side; (2) nearer middle axis; (3) located or produced within.
internal fertilization fertilization of eggs by sperm that takes place inside the body of the female, as e.g. in insects, mammals and birds. cf. external fertilization.
internal phloem primary phloem found internal to primary xylem.
internal respiration the biochemical, intracellular reactions of respiration.
internarial a. between the nostrils, appl. septum.
International Unit (IU) (1) unit used to measure vitamin content of foods; (2) unit of enzyme activity, 1 IU de.ned as the amount of enzyme that will catalyse the conversion of 1 micromole of substrate in 1 minute.
interneuron n. small neuron in grey matter of central nervous system, interposed between afferent and efferent neurons in spinal cord, e.g. between sensory and motor neurons in a re.ex arc, and forming connections between neural pathways generally.
internode n. the part of a plant stem between two leaf origins (nodes) or two joints.
internuncial a. intercommunicating, as paths of transmission or nerve .bres.
interoceptor n. sense organ or receptor that detects stimuli originating inside the body. cf. exteroceptor.
interocular a. between the eyes.
interoptic a. between the optic lobes of brain.
interorbital a. between the orbits of the eye, appl. sinus.
interosseous a. between bones, appl. arteries, ligaments, muscles, membranes, nerves.
interparietal n. in many vertebrates, a bone arising between parietals and supraoccipital.
interpeduncular a. appl. fossa between cerebral peduncles, and to ganglion.
interphase n. (1) the period between one mitosis and the next in a eukaryotic cell. It is the period during the cell cycle dur­ing which growth occurs, the chromosomes are in the uncondensed state, transcription occurs, and during which the DNA is rep­licated. see Fig. 7 (p. 105); (2) the period sometimes occurring between the 1st and 2nd meiotic division.
interpositional growth of cells, by inter­position between neighbouring cells with­out loss of contact.
interradius n. a radius between the four primary radii of a radially symmetrical animal.
interrenal a. between the kidneys, appl. veins.
interrenal body a gland situated between kidneys of elasmobranch .shes, represent­ing the adrenal cortex of higher vertebrates.
interrupted a. (1) with continuity broken;
(2) irregular; (3) asymmetrical. interrupted gene see split gene. interruptedly pinnate pinnate with pairs
of small lea.ets occurring between larger ones, appl. leaves.
interscapular a. (1) between the shoulder blades, appl. e.g. feathers; (2) appl. brown stout: the so-called hibernating gland, found between the shoulder blades of some rodents.
interscutal a. between scuta or scutes. intersegmental a. (1) between segments;
(2) between spinal segments, appl. axons, septa. interseminal a. between seeds or ovules, appl. scales in certain gymnosperms. interseptal a. appl. spaces between septa or partitions.
intersex n. (1) organism with characteristics intermediate between a typical male and a typical female of its species; (2) organism .rst developing as a male or female, then as an individual of the opposite sex.
interspeci.c a. between distinct species, appl. crosses, as mule, hinny, cattalo, tigon.
interspeci.c competition competition between the members of different species for the same resource. cf. intraspeci.c competition.
interspersed repeated DNA repeated DNA sequences occurring as individual copies interspersed throughout the genome, rather than as blocks of tandemly repeated sequences.
interspinal a. occurring between spinal processes or between spines, appl. bones, ligaments, muscles.
intersterility n. incapacity to interbreed.
intersternal a. (1) between the sterna; (2) appl. ligaments connecting manubrium and body of sternum.
interstitial a. occurring in interstices or spaces; (1) appl. .ora and fauna living between sand grains or soil particles; (2) appl. cells, see interstitial cells; (3) appl. .uid, see extracellular .uid.
interstitial cells (1) connective tissue cells in the vertebrate testis. They lie between the tubules and produce testosterone; (2) small cells lying in the interstices between other cells in coelenterates and which give rise to various cell types. alt. I-cells.
interstitial cell-stimulating hormone (ICSH) luteinizing hormone q.v.
intertemporal n. paired membrane bone, part of sphenoid complex.
intertentacular organ structure in certain bryozoans through which eggs are released.
intertidal a. appl. shore organisms living between high- and low-water marks. see Fig. 34 (p. 594).
intertrochanteric a. between trochanters, appl. crest, line.
intertrochlear a. appl. an ulnar ridge .tting into a groove of the humerus.
intertubular a. (1) between tubules, esp. kidney tubules, appl. capillaries; (2) be­tween seminiferous tubules.
intervarietal a. appl. crosses between two distinct varieties of a species.
intervening sequence intron q.v.
interventricular a. (1) between the ventr­icles of heart or brain; (2) appl. foramen: foramen of Monro, the passage between the third and lateral ventricles in brain.
intervertebral a. occurring between the vertebrae, appl. e.g. discs, .brocartilages, veins.
intervillous a. (1) occurring between villi;
(2) appl. spaces in placenta .lled with maternal blood. interxylary a. between xylem strands, appl. phloem.
intestine n. that part of the alimentary canal from the pyloric end of the stomach to the anus. In humans it comprises the duodenum, jejunum, ileum, caecum, colon and rectum in that order. a. intestinal.
intima n. layer of tissue forming the inner­most lining of a part or organ. alt. tunica intima.
intine n. the cellulosic inner layer of wall of pollen grain.
intolerant a. incapable of living in a par­ticular set of conditions.
intra-pre.x derived from L. intra, within,
and signifying within, inside. cf. inter-. intrabulbar a. within a taste bud. intracapsular a. contained within a capsule.
intracardiac endocardiac q.v. intracellular a. within a cell. intracellular motility movement that is
generated within a cell, e.g. cytoplasmic streaming, muscle contraction, intracellular transport.
intracellular signalling pathways the biochemical pathways that carry signals from cell-surface receptors to their intracellular destination. They are made up of sets of interacting proteins and some also involve small molecules such as cyclic AMP and inositol trisphosphate.
intracellular transport the directed transport of material and movement of organelles within the cell. Material is transported between certain organelles, and to the plasma membrane for secre­tion, in the form of small membrane-bounded vesicles. Large particles are also went through the cytoplasm.
intrachromosomal a. appl. duplication or rearrangement occurring within a chromosome.
intracistronic complementation the ability of two mutations in one gene (a cistron) to produce a normal phenotype (to complement each other) when present in the same cell.
intraclonal a. within a clone, appl. differentiation.
intracortical a. within the cortex, appl. nerve tracts linking different parts of the cerebral cortex.
intrademic selection selection within a local breeding group.
intradermal a. within the dermis of skin.
intraepithelial a. occurring within epithe­lium, appl. glands.
intrafascicular a. within a vascular bundle.
intrafoliaceous a. appl. stipules encircling stem and forming a sheath.
intrafusal a. (1) appl. muscle .bres within muscle spindle; (2) appl. nerve endings in tendons.
intragemmal a. within a taste bud.
intrageneric a. among members of the same genus.
intragenic a. within a gene, appl. e.g. mutation, recombination, spacer.
intragenic suppression see back mutation.
intrajugular n. projection in middle of jugular notch of occipital bone.
intralamellar a. within a lamella, appl. trama of gill-bearing fungi.
intramembrane particles protein particles seen studding the interior faces of freeze-broke membranes.
intramembranous a. within a membrane, appl. bone development. intramolecular a. occurring or existing within a molecule.
intramolecular respiration formation of carbon dioxide and organic acid by nor­mally aerobic organisms if deprived of oxygen.
intranarial a. inside the nostrils. intranuclear n. within the nucleus. intraparietal a. (1) enclosed within an
organ; (2) within the parietal lobe of the
brain. intrapleural a. within the thoracic cavity. intrasexual a. appl. selection amongst com­
peting individuals of the same sex. intraspeci.c a. (1) within a species, appl.
e.g. variation; (2) appl. competition be­tween the members of the same species for the same resource, e.g. mates, food, territory.
intrastelar a. within the central vascular tissue (stele) of a stem or root. intrathecal a. within the meninges of the
spinal cord. intrauterine a. within the uterus. intravaginal a. (1) within vagina; (2) con­
tained within a sheath, as grass leaves or
branches. intravascular a. within blood vessels. intraventricular a. within a ventricle, appl.
caudate nucleus of corpus striatum, seen
within a ventricle of brain. intravesical a. within the bladder. intravitelline a. within the yolk of an egg
or ovum.
intrazonal a. (1) within a zone; (2) appl. locally limited soils, differing from preval­ent or normal soils of the region or zone.
intrinsic a. (1) inherent; (2) inward; (3) appl. inner muscles of a part or organ; (4) appl. rate of natural increase in a population having a balanced age distribution; (5) appl. sensation of brightness due to the differential response of retinal cells to dif­ferent wavelengths; (6) appl. membrane proteins: proteins which span the whole membrane. cf. extrinsic.
intrinsic factor glycoprotein secreted by stomach, binding cobalamin (vitamin B12) in the intestine and carrying it into the blood. Its de.ciency causes pernicious anaemia due to impaired cobalamin ab­sorption. alt. gastric intrinsic factor.
intrinsic isolating mechanism any ge­netic mechanism preventing interbreeding.
intrinsic pathway series of reactions in blood leading to clot formation, triggered by contact with an abnormal surface such as the walls of a glass vessel, the .nal stages being the same as in the extrinsic pathway.
intrinsic rate of increase the fraction by which a population is growing at each instant of time, symbolized by r.
introduced a. appl. plants and animals not native to the country and thought to have been brought in by humans.
introgression n. the gradual diffusion of genes from the gene pool of one species into another when there is some hybridiza­tion between the two species as a result of incomplete genetic isolation.
intromission n. insertion of the erect penis into the vagina during copulatory behaviour.
intromittent a. adapted for insertion, appl. male copulatory organs.
intron n. a non-coding nucleotide sequence, one or more of which interrupt the coding sequence of most eukaryotic genes. Many genes have substantial numbers of long intron sequences that take up most of the total length of the gene. Introns are tran­scribed into RNA as part of the primary RNA transcript, and subsequently removed by RNA splicing to produce a functional mRNA or other RNA. They are extremely rare in prokaryotic DNA. Some introns are self-splicing, i.e. they can remove them­selves from RNA without the aid of any protein enzymes. Self-splicing introns con­sist of two main classes, group I and group II, in which the detailed biochemistry of the splicing reaction is different. alt. inter­vening sequence. cf. exon. see also RNA splicing.
intron–exon boundary the boundary between an intron and its adjacent exon in a gene or primary RNA transcript, each intron having a 3′ and a 5′ boundary.
The boundaries are the sites of RNA splicing.
introrse a. (1) turned inwards or towards the axis; (2) of anthers, opening towards the centre of the .ower.
introvert a. that which can be drawn in­wards, e.g. anterior region of some polyps or of certain annelid worms.
intumescence n. (1) the process of swell­ing up; (2) a swollen or tumid condition.
intussusception n. growth in surface extent or volume by intercalation of new material among that already present. cf. accretion, apposition.
inulase inulinase q.v.
inulin n. linear polysaccharide made up of fructose units, a storage carbohydrate in the roots, rhizomes and tubers of many Compositae.
inulinase n. enzyme hydrolysing inulin to fructose. EC 3.2.1.7. alt. inulase.
invagination n. (1) the sinking in of a wall of a hollow structure such as a vessel or a blastula, which draws the exterior layer into the interior, forming a cavity; (2) involution or turning inside out of a tube; (3) drawing into a sheath. v. invaginate.
invariant chain polypeptide that binds to peptide-binding site in a MHC class II molecule in the endoplasmic reticulum, preventing premature binding of antigen peptides.
inversion n. (1) a turning of a part inward, or inside out, or upside down; (2) (genet.) chromosomal rearrangement in which a sequence of genes appears in the reverse of its normal order on the chromosome, due to the repositioning of a segment of chromosome; (3) (biochem.) hydrolysis of sucrose to glucose and fructose.
invert sugar sucrose q.v.
invertase n. enzyme found in plants, fungi and bacteria that hydrolyses terminal non-reducing ß-d-fructofuranoside residues in ß-d-fructofuranosides (e.g. sucrose). EC 3.2.1.26, r.n. ß-d-fructofuranosidase.
invertebrates n.plu. general term for all animals without backbones, i.e. all groups except the vertebrates.
inverted repeats in nucleic acids, two adjacent or nearby copies of an identical sequence, one being in reverse orientation.
inverted U-shaped curve a dose–response curve in the form of an inverted U. It indi­cates a response in which increasing doses of drug elicit increasing responses at .rst, but response declines at higher doses.
investing bone membrane bone q.v.
investment n. the outer covering of a part, organ, animal or plant.
in vitro appl. biological processes and reac­tions occurring in (i) cells or tissues grown in culture or (ii) cell extracts or synthetic mixtures of cell components.
in vitro fertilization (IVF) the fertilization of an ovum outside the mother’s body, generally followed by replacement of the fertilized egg into the mother or into a pseudopregnant “foster mother” where it develops normally.
in vitro translation or transcription translation or transcription carried out in isolated extracts of cells, or de.ned sys­tems consisting of the appropriate enzymes and accessory proteins.
in vivo occurring in a living organism.
involucrate a. bearing involucres.
involucre n. (1) circle of bracts at the base of a compact .owerhead; (2) leaves sur­rounding the groups of antheridia and archegonia in mosses and liverworts. a. involucral.
involucrum n. (1) the notum of metathorax in Orthoptera; (2) layer of bone formed around dead bone in some diseased conditions.
involuntary a. not under the control of the will, appl. e.g. movements.
involuntary muscle smooth muscle q.v.
involute a. (1) having edges rolled inwards at each side, appl. leaves; (2) tightly coiled, appl. shells.
involution n. (1) reduction of enlarged, modi.ed or deformed conditions to nor­mal; (2) decrease in size, or other structural or functional changes, as in ancient age; (3) a rolling inwards, as of edges of leaves;
(4) movement of exterior cells to interior, as in gastrulation; (5) a. resting, appl. spores or stage in life-cycle.
iodine (I) n. halogen element which forms a black shiny volatile solid in the elemental state (I2), and is present in small amounts in seawater and concentrated in seaweed. It is an essential micronutrient for humans and other vertebrates as it is a constituent of some thyroid hormones. The radioiso­tope 131I (half-life 8.6 days) is used as a tracer to diagnose and treat thyroid gland disorders.
iodophilic a. (1) staining darkly with iodine solution; (2) appl. bacteria that stain blue with iodine.
iodopsin n. photosensitive protein pigment with rhodopsin-type protein component, in the cones of the retina. alt. visual violet.
iodothyroglobulin n. compound of iodine and thyroglobulin, extractable from the thyroid gland.
ion n. atom or molecule that has bought an electrical charge by gaining or losing one or more electrons.
ion channel membrane protein that forms an aqueous pore in the membrane, allow­ing the passive .ow of ions across the membrane. Ion channels may be perman­ently open or may be gated, i.e. open only in response to a speci.c stimulus. Different ion channels convey different ions, being divided broadly into those conveying anions and those that carry cations. see Fig. 26 (p. 386).
ion exchange the adsorption of ions of one type onto a resin, or clay particles in soil, in exchange for others which are lost into solution.
ion-exchange chromatography separa­tion of molecules such as proteins on the basis of their net charge by differential binding to a column of carboxylated poly­mer, positively charged molecules binding to the column.
ionic bond, ion pair electrostatic bond q.v.
ionic detergent detergent in which the polar end of the molecule is charged
(e.g. sodium dodecyl sulphate). It is a strong detergent that can denature (unfold) proteins.
ionic coupling the direct passive .ow of ions from the cytoplasm of one animal cell to an adjacent cell through gap junctions.
ionizing radiation small wavelength high-energy radiation such as gamma-rays and quick-moving particles such as alpha- and beta-particles emitted by radioisotopes. They cause the formation of ions in tis­sues, thus contributing to DNA and tissue hurt.
ionophore n. small hydrophobic molecule that dissolves in lipid bilayers and increases the permeability of the membrane to ions.
ionophoresis n. movement of ions under the in.uence of an electric current. a. ionophoretic.
ionotropic a. appl. neurotransmitter receptor that contains an ion channel which is opened when the neurotransmitter binds to the receptor. cf. metabotropic.
ion pump protein that actively transports an ion across a biological membrane against a concentration gradient.
IP3 inositol trisphosphate q.v.
ipsilateral a. pert. or situated on the same side. cf. contralateral.
IPSP inhibitory postsynaptic potential q.v.
IPTG isopropylthiogalactoside q.v.
IR infrared radiation q.v.
IRE iron-response element q.v.
iridal, iridial a. pert. the iris of the eye.
Iridales n. order of herbaceous monocots and including the families Corsiaceae, Iridaceae (iris) and others.
iridocytes, iridophores n.plu. (1) guanine-containing granules, bodies or plates of which the re.ecting, silvery or iridescent tissue of skin of .sh and reptiles is com­posed; (2) iridescent cells in integument of some cephalopods.
Iridoviridae n. family of enveloped, double-stranded DNA viruses that infect insects (e.g. insect iridescent viruses) and vertebrates.
iris n. thin circular contractile disc with a central aperture, the pupil, lying in front of the lens in the vertebrate eye.
iris cells pigment cells surrounding cone and retinula of an ommatidium of a com­pound eye.
IRM innate releasing mechanism q.v.
iron (Fe) n. metallic element that is an es­sential micronutrient for living organisms. Fe atoms are part of prosthetic groups, such as haem, in many enzymes and other proteins. In humans, a major requirement for iron is for haemoglobin synthesis.
iron–copper centre active site in the res­piratory enzyme cytochrome oxidase at which molecular oxygen is held by an iron and a copper atom while being converted to water.
iron-oxidizing bacteria diverse group of aerobic bacteria and archaea that can use ferrous iron (Fe2+) as their sole source of energy, oxidizing it to ferric iron (Fe3+). Most also oxidize sulphur and are obli­gately acidophilic.
iron-response element (IRE) control se­quence in DNA that binds an iron-binding gene regulatory protein, and by which a gene is regulated by iron.

iron–sulphur (FeS) protein any of a group of proteins containing a prosthetic group of non-haem iron complexed with sulphur atoms (iron–sulphur centres). They are components of photosynthetic electron-transport chains. There are various types of centres: FeS (one Fe complexed with four S atoms), Fe2S2 and Fe4S4. alt. non-haem iron protein.
irreciprocal a. (1) not reversible; (2) one-way.
irregular a. appl. .owers showing bilateral rather than radial symmetry. alt. zygomorphic.
irritability n. capacity to receive external stimuli and respond to them.
irrorate a. (1) covered as if by minute droplets; (2) with minute colour markings, as the wings of some butter.ies.
IS1, IS2, etc. insertion sequence q.v.
isandrous a. having similar stamens, their number equalling the number of sections of the corolla.
isanthous a. having uniform or regular .owers.
ischaemia n. tissue hurt and localized death due to lack of oxygen. a. ischaemic.
ischiadic a. pert. or in region of hip.
ischiatic sciatic q.v.
ischiopodite n. proximal joint of walk­ing legs of certain crustaceans, or of maxillipeds.
ischiopubic a. appl. gap between ischium and pubis.
ischiopubis n. a fused ischium and pubis.
ischiorectal a. pert. ischium and rectum, appl. fossa and muscles.
ischium n. the ventral and posterior bone of each half of pelvic girdle of vertebrates except .shes, often fused to pubis.
iscom immune stimulatory complexes, of antigens complexed with that composed of lipids.
IS element insertion sequence q.v.
isidia n.plu. coral-like soredia on surface of some lichens. sing. isidium.
island biogeography the study of the .ora and fauna of islands with a view to under­standing the nature and evolution of biodiversity in an isolated environment.
islets of Langerhans groups of cells scat­tered throughout the pancreas that secrete glucagon (from a or A cells) or insulin (from ß or B cells).
isoaccepting a. appl. tRNAs which pick up the same amino acid and are charged by the same aminoacyl-tRNA synthetase.
isoagglutination n. (1) agglutination of erythrocytes by blood from another mem­ber of the same species or of the same blood group; (2) agglutination of sperm by agglutinins of ova of the same species.
isoagglutinin n. agglutinin of ova which reacts with sperm of the same species.
isoalleles n.plu. alleles that are identical in their yucky phenotypic effects but which can be distinguished at the DNA or pro­tein product level.
isoantigen alloantigen q.v.
isobilateral a. appl. a form of bilateral sym­metry where a structure is divisible in two planes at right angles.
isocarpous a. having carpels and perianth divisions equal in number.
isocercal a. with vertebral column ending in median line of caudal .n.
isochromatic, isochromous a. (1) uni­formly coloured; (2) equally coloured.
isochromosome n. abnormal chromosome with two genetically identical arms.
isochronous a. (1) having equal duration;
(2) occurring at the same rate. isocitrate see isocitric acid. isocitrate dehydrogenase either of
two enzymes. One (EC 1.1.1.41, NAD-requiring) is located in mitochondria and catalyses formation of a-ketoglutarate from isocitrate, a regulatory step in the tricarboxylic acid cycle. The other (EC 1.1.1.42, NADP-requiring) is located in the cytoplasm.
isocitric acid, isocitrate six-carbon inter­mediate of the tricarboxylic acid cycle, formed from citrate via cis-aconitate in reactions catalysed by the enzyme aconitase.
isocortex n. the six outermost cell layers of the cerebral cortex.
isocytic a. with all cells equal.
isodactylous a. with all digits of equal size.
isodemic a. (1) with, or pert., populations composed of an equal number of indi­viduals; (2) appl. lines on a map which pass through points representing equal population density.
isodiametric a. (1) having equal diameters, appl. cells or other structures; (2) appl. rounded or polyhedral cells.
isodont homodont q.v.
isodynamic a. (1) of equal strength; (2) providing the same amount of energy, appl. foods.
isoelectric focusing the separation of proteins on the basis of charge, by their differential migration on electrophoresis in a pH gradient, proteins of different charge each stopping their migration at their iso­electric point.
isoelectric point (IEP) the pH at which an amphoteric molecule, such as a protein, carries no net charge, being a de.nite value for each protein.
isoenzyme n. any one of a group of enzyme variants of slightly different sequence, encoded by different genes, that catalyse the same reaction in the same pathway but differ in properties such as optimum pH or rate of reaction. They can be distinguished by isoelectric point or electrophoretic mobility. Different tissues often express different isoenzymes. alt. isozyme.
Isoetales n. order of Lycopsida having linear leaves, and a “corm” with complex secondary thickening, and including the quillworts.
isoforms n.plu. (1) the different forms of a polymorphic protein; (2) two or more proteins or RNAs that are produced from the same gene by differential transcription and/or differential RNA splicing.
isogametangiogamy n. the union of similar gametangia.
isogamete n. one of a pair of gametes that are morphologically similar.
isogamy n. the fusion of gametes that are morphologically similar, i.e. of equal size and similar structure. a. isogamous.
isogeneic syngeneic q.v.
isogenes n.plu. lines on a map which con­nect points where the same gene frequency is found.
isogenetic a. (1) arising from the same or a similar origin; (2) of the same genotype.
isogenic homozygous q.v.
isogenomic a. containing similar sets of chromosomes, appl. nuclei.
isogenous a. of the same origin.
isognathous a. having both jaws alike.
isogonal a. forming equal angles, appl. branching.
isograft n. tissue graft taken from another individual of the same genotype as the recipient.
isohaline a. appl. water having the same level of salinity.
isokont isomastigote q.v.
isolate n. (1) a breeding group limited by isolation; (2) the .rst pure culture of a microorganism derived e.g. from soil or tissues.
isolateral a. having equal sides, appl. leaves with palisade tissue on both sides.
isolating mechanisms mechanisms that prevent breeding between two populations of the same species, and eventually lead to speciation. The most vital is geo­graphical isolation, but there may also be genetic, anatomical and behavioural barriers to successful interbreeding.
isolation n. prevention of mating between breeding groups owing to spatial, topo­graphical, ecological, morphological, phy­siological, genetic, behavioural or other factors.
isolecithal a. appl. eggs with yolk distrib­uted nearly equally throughout.
isoleucine (Ile, I) n. amino acid with a non­polar side chain, stereoisomer of leucine, constituent of proteins, and essential in diet of humans and other animals.
isomastigote a. having .agella or cilia of the same length.
isomer n. any one of a set of molecules hav­ing the same kind and number of atoms but differing in arrangement of the atoms and in physical and (sometimes) chemical prop­erties. a. isomeric. n. isomerism. see also anomer, enantiomer, epimer, optical iso­mer, racemate, tautomer, stereoisomer.
isomerases n.plu. enzymes that catalyse rearrangements of atoms within molecules.
Examples are epimerases, mutases, race­
mases, tautomerases. EC class 5.
isomere n. a homologous structure or part.
isomeric a. appl. an isomer of a molecule.
isomerism n. the existence of isomers of a molecule.
isomerization n. the conversion of one isomer into another.
isomerous a. (1) having equal numbers of different parts; (2) appl. .owers with equal numbers of parts in each whorl.
isometric a. (1) of equal measure or growth rate; (2) appl. contraction of muscle under tension without change in length.
isometry n. growth of a part at the same rate as the standard or as the whole.
isomorphic, isomorphous a. (1) super.­cially alike; (2) appl. alternation of haploid and diploid phases in morphologically similar generations.
isomorphism n. apparent similarity of indi­viduals of different race or species.
isonym n. a new name, e.g. of species, based upon oldest name or basinym.
iso-osmotic a. appl. two solutions of the same osmotic concentration. see isotonic.
6N-isopentenyladenine n. naturally occur­ring cytokinin in plants and a constituent of some transfer RNAs.
isopetalous a. having similar petals.
isophagous a. feeding on one or allied species, appl. fungi.
isophane n. a line connecting all places within a region at which a biological phe­nomenon, e.g. .owering of a plant, occurs at the same time.
isophene n. a contour line delimiting an area corresponding to a given frequency of a variant form.
isophyllous a. having uniform foliage leaves, on the same plant.
isophytoid n. an “individual” of a com­pound plant not differentiated from the rest.
isoplankt n. a line representing on a map the distribution of equal amounts of plankton, or of particular species.
isoploid a. with an even number of chromo­some sets in somatic cells.
Isopoda, isopods n., n.plu. group of marine, freshwater and terrestrial malacostracan crustaceans, including the woodlice and water slaters, having a dorsoventrally .attened body and no carapace.
isopodous a. having the legs alike and equal.
isopogonous a. having the two sides of feather equal and similar.
isopolyploid a. appl. polyploid with an even number of chromosome sets, e.g. tetraploid, hexaploid.

isoprene n. .ve-carbon unsaturated aliphatic hydrocarbon that is a subunit of many lipids.
isoprenoid n. any of a large and varied group of lipids built up of .ve-carbon isoprene units. They include carotenoids, terpenes, natural rubber and the side chains of e.g. chlorophyll and vitamin K.
isopropylthiogalactoside (IPTG) a non-metabolized inducer of the lac operon in
E. coli.
Isoptera n. order of social insects compris­ing the termites and their relatives, which live in large organized colonies contain­ing reproductive forms (the queen and the king) and non-reproductive wingless soldiers and workers, all offspring of the king and queen. Termites have a gut .ora that enables them to digest wood, and they can be serious pests, devouring wooden buildings, trees and paper.
isosporous homosporous q.v.
isostemonous a. having stamens equal in number to that of sepals or petals.
isotelic a. (1) exhibiting, or tending to pro­duce, the same effect; (2) homoplastic q.v.
isotomy n. forking repeatedly in a regular manner.
isotonic a. (1) of equal tension; (2) of equal osmotic pressure, appl. solutions; (3) appl. contraction of muscle with change in length. cf. isometric contraction.
isotonicity n. normal tension under pres­sure or stimulus.
isotope n. a form of a chemical element having the same atomic number (number of protons) and identical chemical prop­erties as another, but differing in atomic mass as a result of a different number of neutrons in the atomic nucleus. a. isotopic.
isotopic fractionation the differential incorporation of particular isotopes of
e.g. carbon (12C), sulphur (32S), or other elements into biological material. see also carbon isotope ratio.
isotropic, isotropous a. (1) singly refract­ing in polarized light, appl. the light stripes of striated muscle under the microscope;
(2)
symmetrical around longitudinal axis;

(3)
not in.uenced in any one direction more than another, appl. growth rate; (4) without predetermined axes, as some ova.
n. isotropy.
isotypes n.plu. the different heavy-chain types (a, d, e, ., µ) and light-chain types (., .) of immunoglobulins. They are the result of sequence differences in the constant regions of the heavy and light chains. The isotype of the heavy chain determines the functional class of the antibody. see immunoglobulin A, im­munoglobulin D, immunoglobulin E, immunoglobulin G, immunoglobulin M.
isotype switching in B lymphocytes, a switch from the production of µ heavy chains to heavy chains of other isotypes that occurs during the progression of an immune response, with the consequent switch from production of IgM to IgG, IgA or IgE. It involves a permanent DNA rearrangement. alt. class switching.
iso-utility curves in the study of animal behaviour and ecology, curves joining all points of equal utility or bene.t, and which show how different behaviours (e.g. feeding behaviours) can result in the same quant­itative bene.t. alt. indifference curves.
isoxanthopterin n. colourless pterin in the wings of cabbage butter.ies and in eyes and bodies of other insects. alt. leucop­terin B.
isozoic a. inhabited by similar animals.
isozyme isoenzyme q.v.
isthmus n. narrow structure connecting two larger parts.
iter n. a passage or canal, as those of middle ear and in brain.
iteration n. repetition, as of similar trends in successive branches of a taxonomic group.
iteroparity n. production of offspring by an organism in successive groups. a. itero­parous. cf. semelparity.
ITM intermediate memory q.v.
IU International Unit q.v.
IVF in vitro fertilization q.v.
ivory n. dentine of teeth, usually that of
tusks, as of elephant and narwhal. IVS intervening sequence. see intron.
J

J joule q.v.
Jaccard index, Jaccard coef.cient of similarity measure of the similarity (see coef.cient of community) between two plant communities based on their species composition. It can be calculated from the formula J = c/(a + b – c) where a = no. of species at site A, b = no. of species at site B, and c = no. of species common to each site. J will lie between 0 and 1. The greater the value of J, the more similar the sites are in their species composition. cf. Gleason’s index, Kulezinski index, Morisita’s similarity index, Simpson’s index of .oristic resemblance, Sørensen similarity index.
jacket cell cell in antheridium that gives rise to the wall.
Jacobson’s cartilage vomeronasal carti­lage supporting Jacobson’s organ.
Jacobson’s organ a diverticulum of the olfactory organ in many vertebrates, often developing into an epithelium-lined sac opening into the mouth.
jactitation n. scattering of seeds by a censer mechanism.
jaculatory a. (1) darting out; (2) capable of being emitted.
jaculatory duct part of vas deferencs which can be protruded, in many animals.
jaculiferous a. bearing dart-like spines.
Janus kinases (JAKs) family of cyto­plasmic protein tyrosine kinases associ­ated with the receptors for many cytokines, and which activate the STAT family of transcription factors.
Java man fossil hominid found in Java and originally called Pithecanthropus erectus, now called Homo erectus, and dating from mid-Pleistocene.
jaw-foot maxilliped q.v.
jaws n.plu. (1) in vertebrates, a skeletal struc­ture, the upper and lower jaws, forming part of the mouth, bearing the teeth or horny tooth-plates, the upper jaw articu­lating with the braincase, the lower jaw movable, articulating with the upper jaw to open and shut the mouth; (2) structures of similar function and mechanism in invertebrates.
J chain joining chain, a small polypeptide holding individual immunoglobulin mole­cules together in IgM pentamers and in oligomers of IgA.
jecoral a. of or pert. the liver.
jejunum n. part of small intestine between duodenum and ileum in mammals. It has large villi and is the main absorptive region.
jelly.sh common name for the Scyphozoa
q.v.
jelly fungi common name for a group of basidiomycete fungi whose fruiting bodies are typically of a jelly-like consistency,
e.g. the funnel-shaped Tremella and the ear-shaped Auricularia.
jelly of Wharton gelatinous connective tissue surrounding the vessels of the um­bilical cord.
Jerne plaque assay assay for B cells that produce antibodies speci.c for a cell-surface antigen, based on complement-mediated lysis.
J gene segment any one of the “joining” gene segments found in all immunoglobu­lin and T-cell receptor loci. They are small DNA sequences one of which is spliced to a V gene segment (or to a V + D sequence) to produce a DNA sequence coding for the variable region of the polypeptide chains. alt. J segment, J region, which may also designate the corresponding region of the polypeptide chain. see also D gene seg­ment, gene rearrangement, variable region, V gene segment.
Johnston’s organ sensory organ in 2nd segment of insect antennae, concerned with balance or with sensing sound or mechan­ical vibrations.
joining chain J chain q.v.
joining gene segment J gene segment q.v.
joint n. the articulation between two neigh­bouring parts, such as the knee joint between the tibia and femur of a leg.
jordanon n. a right-breeding unit below the species level, with small variability, such as a race, subspecies or variety. alt. microspecies.
joule (J) n. the derived unit of energy or work in the SI system, which is used instead of the calorie. One joule of work is done whenever a force of 1 N (equi­valent to 1 kg m s–2) is sustained over 1 m in that force’s direction. Therefore, 1 J is equivalent to 1 kg m2 s –2. 1 J = 0.23892 calories.
J region region of an immunoglobulin or T-cell receptor polypeptide chain that is encoded by a J gene segment.
J-shaped growth curve type of growth curve exhibited by populations that grow exponentially and then crash to very low numbers, typical of organisms with many generations per year, e.g. planktonic algae. cf. S-shaped growth curve.
Juan Fernandez region region of the Neotropical phytogeographical kingdom comprising the Juan Fernandez islands off the west coast of Chile.
jubate a. with a mane-like growth.
jugal n. a component of the cheekbone in vertebrate skull.
jugate a. having pairs of lea.ets.
Juglandales n. order of dicot trees, often aromatic, with pinnate leaves and compris­ing two families Juglandaceae (walnut) and Rhoiptelaceae.
jugular a. (1) pert. neck or throat; (2) appl. vein, the main veins carrying blood from the head in vertebrates; (3) appl. ventral .ns beneath and in front of pectoral .ns.
jugum n. (1) (bot.) a pair of opposite lea.ets or leaves; (2) (zool.) small lobe on poster­ior of forewing of some moths; (3) ridge or depression connecting two structures.
jumping genes colloquial term for trans­posable genetic elements. see also con­trolling element, insertion sequence, transposition, transposon.
Juncales n. order of herbaceous monocots with long, narrow, channelled or grasslike leaves, and comprising the families Juncaceae (rush) and Thurniaceae.
junction see cell junction, gap junction, septate junction.
junctional complex in epithelial cells, the region of attachment between neighbouring cells. see desmosome, hemidesmosome, tight junction.
junctional diversity the diversity in immunoglobulins and T-cell receptors that is due to variation made at the junctions between gene segments during the gene rearrangement process.
junk DNA colloquial term for any DNA in the genome that appears to have no coding, regulatory or structural function. It is used in particular for the large amount of DNA in the genomes of plants and animals which is composed of repetitive non-coding sequences.
Jurassic a. pert. or appl. geological period lasting from ca. 213 to 144 million years ago, after the Triassic and before the Cretaceous.
juvenal, juvenile a. youthful, appl. plumage replacing nestling down of 1st plumage.
juvenile n. young bird or other animal, before it has bought full adult plumage or form.
juvenile diabetes insulin-dependent dia­betes mellitus q.v.
juvenile hormone insect hormone that maintains larval state and prevents prem­ature metamorphosis into an adult form.
juxta- pre.x derived from Latin juxta, close to.
juxta-articular a. near a joint or articulation.
juxtaglomerular cells cells surrounding the arteriole feeding the network of blood capillaries around the dilated end of a kid­ney tubule, and which secrete renin.
juxtamedullary a. near medulla, appl. inner part of zona reticularis of adrenal glands.
juxtanuclear a. beside the nucleus, appl. bodies: basophil deposits in cytoplasm of vitamin D-de.cient parathyroid cells.
K

k see kappa.
K (1) symbol for the chemical element potassium q.v.; (2) lysine q.v.
K (1) symbol for the carrying capacity (q.v.) of the environment; (2) equilibrium con­stant q.v.
Kd dissociation constant q.v.
Km Michaelis constant. see Michaelis– Menten kinetics.
kainate n. structural analogue of glutamate, used to de.ne a class of glutamate recep­tors in central nervous system.
kairomone n. chemical messenger or pheromone emitted by one species which has an effect on a member of another species, sometimes to the detriment of the transmitter. An example would be a chemical that attracts a male to a female and also attracts a predator.
kallidin lysylbradykinin q.v.
kallikrein n. plasma kallikrein (EC 3.4.21.34) is a serine peptidase that cleaves kininogen to produce kinin (bradykinin). Tissue kallikrein (EC 3.2.21.35) cleaves kininogen to produce lysylbradykinin. alt. kininogenase.
kanamycin kinase enzyme conferring resistance to the antibiotics kanamycin, neomycin and related compounds. It phosphorylates them, rendering them inactive. EC 2.7.1.95. alt. neomycin phosphotransferase.
kanamycins n.plu. antibiotics produced by Streptomyces spp. that interfere with bacterial protein synthesis.
kappa (k) n. one of the two types of light chain found in mammalian antibodies.
Kartagener’s syndrome recessive genetic defect in humans leading to non-functional cilia on e.g. respiratory epithelium. Situs inversus also sometimes occurs.
karyaster n. star-shaped group of chromo­somes.
karyo-pre.x derived from Gk karyon, nut (= nucleus), signifying pert. nucleus or chromosomes.
karyoclasis n. breaking down of cell nu­cleus. a. karyoclastic. karyogamy n. fusion of the nuclei of two gametes after cytoplasmic fusion. karyokinesis n. nuclear division. see mitosis. karyology n. study of the nucleus and the chromosomes. karyolysis n. disintegration of cell nucleus.
a. karyolytic. karyomixis n. mingling or union of nuclear
material of two gametes. karyon n. the cell nucleus. plu. karya. karyopherin nuclear transport receptor q.v. karyoplast n. an isolated nucleus, with a
small amount of cytoplasm adhering. karyorhexis, karyoschisis n. fragmenta­tion of cell nucleus. karyosome n. structure in nucleus of some
protozoa, analogous to nucleolus. karyota n.plu. nucleated cells. karyotype n. photographic representation
of the chromosome complement of a cell, with individual mitotic chromosomes ar­ranged in pairs in order of size.
Kaspar Hauser experiment experiment in which an animal is reared in com­plete isolation from members of its own species.
kasugamycin n. antibiotic that blocks ini­
tiation of translation in bacteria. katagenesis n. retrogressive evolution. katakinetic a. appl. processes leading to
discharge of energy. katal n. derived SI unit of enzyme activity, de.ned as the amount of enzyme that
will catalyse the conversion of 1 mole of substrate in 1 second. 1 nanokatal = 0.06 IU.
kataphoric a. appl. passive action.
kataplexy cataplexis q.v.
katharobic a. living in clean waters, appl. protists.
katharometer n. thermal conductivity detector device for measuring the com­position of sample vapours emerging from gas–liquid chromatography.
kb, kbp kilobase or kilobase pairs = 1000 bases or base pairs of DNA.
kcal kilocalorie q.v.
K cell see killer cells.
K+
channel potassium channel q.v.
kD, kDa, kdal kilodalton q.v.
KDEL C-terminal amino-acid sequence that directs the retention of proteins in the endoplasmic reticulum.
keel n. (1) narrow ridge; (2) ridge on sternum of .ighted birds and bats to which wing muscles are attached; (3) boat-shaped structure formed by two anterior petals in .owers of the pea family.
kelp n. common name for seaweeds of the Laminariales, marine multicellular brown algae with a large broad-bladed thallus attached to the substratum by a tough stalk and holdfast.
kenenchyma n. tissue devoid of its living contents, e.g. cork.
Kennedy’s disease X-linked spinal and bulbar muscular atrophy, a genetic disease characterized by progressive muscle weak­ness and atrophy. It is caused by ampli.ca­tion of trinucleotide repeats in the gene for the androgen receptor.
kenosis n. (1) process of voiding, or con­dition of having voided; (2) exhaustion;
(3) inanition.
kentragon n. larval stage following the cypris stage of parasitic cirripeds (bar­nacles), which penetrates into the body of host.
keraphyllous a. appl. layer of a hoof be­tween horny and sensitive parts.
kerasin n. cerebroside from brain yielding on hydrolysis a fatty acid (lignoceric acid), galactose and sphingosine.
keratan n. polysaccharide of galactose and N-acetylglucosamine which is part of a proteoglycan from cartilage and other connective tissues, where it occurs as keratan sulphate.
keratin n. .brous protein rich in cysteine, constituent of intermediate .laments (kera­tin .laments). It is the chief material in horn, hair, nails and the upper .aky layer of skin.
keratinization n. intracellular deposition of keratin to form an inert horny material,
e.g. nails, claws, horns, outer layers of skin. alt. corni.cation.
keratinizing a. becoming horny due to intra­cellular deposition of keratin, appl. cells of epidermis (skin cells, hair shaft cells, cells of .ngernails and toenails, epidermal cells at base of horns). alt. cornifying.
keratinocyte n. epidermal skin cell that synthesizes keratin.
keratinolytic a. breaking down keratin, appl. enzymes.
keratinophilic a. growing on horn or other keratinized substrate, appl. certain fungi.
keratinous a. pert., containing, or formed by, keratin.
keratocyte n. epithelial cell, containing keratin .laments, of the epidermis of .sh and frogs. They can migrate rapidly to close wounds.
keratogenous a. horn-producing.
keratohyalin n. substance formed in the middle layer of vertebrate epidermis (skin), preceding full keratinization.
keratoid a. resembling horns.
keratose a. having horny .bres in skeleton, as certain sponges.
kernel n. the inner part of a seed containing the embryo.
keroid keratoid q.v.
ketogenesis n. the production of ketone bodies, which occurs especially during starvation or fasting.
ketogenic a. appl. amino acids that give rise to ketone bodies during their oxida­tion to carbon dioxide and water, e.g. leucine, phenylalanine and tyrosine.
ketogenic hormone lipotropin q.v. a-ketoglutaric acid, a-ketoglutarate
.ve-carbon carboxylic acid intermediate of the tricarboxylic acid cycle, which is decarboxylated to yield succinyl-CoA, a reaction catalysed by the a-ketoglutarate dehydrogenase complex. It can also be aminated to form glutamate.
ketone body any of a group of compounds such as acetoacetate, d-3-hydroxybutyrate and acetone. They are formed in the liver from acetyl-CoA produced during fatty acid oxidation, especially during fasting or in conditions such as diabetes. They can be used by the brain as an alternative fuel to glucose.
ketose n. any monosaccharide containing a keto (C=O) group. cf. aldose.
ketosis n. change in the energy source for the brain from glucose to ketone bodies which occurs e.g. during starvation.
key n. (1) means of identifying objects or organisms by a series of questions with alternative answers, each answer leading on to another question or a positive identi­.cation; (2) winged nutlet hanging in clus­ters, as in ash. alt. samara.
keystone species species that have a key role in an ecosystem, affecting many other species, and whose removal leads to a series of extinctions within the system.
kidney n. one of a pair of organs in verte­brates concerned with excretion of nitro­genous waste as urine and regulation of water balance.
kidney duct tube through which urine is excreted to the exterior in .shes and amphibians.
kidney tubule one of numerous convoluted .ne tubules in cortical region of kidney that carry urine from glomeruli to the collect­ing ducts. In mammals and birds, water is reabsorbed from the urine during passage through the tubules.
killed vaccine vaccine made of whole but inactivated viruses or bacteria.
killer cell see cytotoxic T cell, natural killer cell.
killer inhibitory receptor (KIR) type of receptor on natural killer cells which prevents these cells attacking healthy uninfected cells.
kilobase (kb), kilobase pair (kbp) unit of length used for nucleic acids and poly­nucleotides, corresponding to 1000 bases or base pairs.
kilocalorie (kcal) n. 1000 calories.
kilodalton (kD, kDa, kdal) unit of mass equal to 1000 daltons, or 1000 units of mol­ecular mass, also sometimes abbreviated to K (e.g. a 30K protein), used chie.y for
proteins.
kilojoule (kJ) n. 1000 joules.
Ki-MSV Kirsten mouse sarcoma virus.
kinaesthesis n. perception of movement due to stimulation of muscles, tendons and joints.
kinaesthetic a. pert. sense of movement or muscular effort.
kinase n. any enzyme that transfers a phosphoryl (phosphate) group from ATP or other high-energy phosphates to an acceptor molecule. EC 2.7.1–4. alt. phosphokinase. see also protein kinase.
kindling n. long-term electrophysiological and morphological changes produced in neuronal tissue following repeated elec­trical stimulation, and which can e.g. result in a seizure after a mild stimulus many months afterwards.
kindred n. group of people related by mar­riage or ancestry.
kinesin n. protein with ATPase/GTPase activity that can go along microtubules and thus can act as a molecular motor for various types of movement within cells. see axonal transport.
kinesis n. (1) random movement; (2) an orientation movement in which the organ­ism swims at random until it reaches a better environment, the movement depend­ing on the intensity, not the direction, of the stimulus.
kinesth-see kinaesth-.
kinete n. motile zygote stage in the life-cycle of piroplasms.
kinethmoid n. small bone in .sh skull inter­mediate between premaxilla and cranium, involved in protrusion of premaxillae.
kinetic a. (1) pert. movement; (2) active;
(3) appl. energy employed in producing or changing motion.
kinetics n. the sequence of events involved in a movement or activity and the rates at which they occur. see also enzyme kinetics.
kinetid n. unit composed of one or more kinetosomes, and their cilia, and the rib­bons of microtubules and striped .bres (kinetodesmata) that surround them, in ciliate protozoa.
kinetin n. adenine derivative with cytokinin activity, probably not occurring naturally.
kinetium n. row of kinetosomes, in ciliate protozoa. plu. kinetia.
kinetoblast n. ciliated membrane of aquatic larvae, used for locomotion.
kinetochore n. densely staining .brillar region at the centromere of a chromosome, to which spindle microtubules attach dur­ing meiosis or mitosis. It is composed of specialized proteins that bind speci.cally to the centrosomal DNA and associated proteins.
kinetochore .bres, kinetochore microtubules microtubules of the mitotic or meiotic spindle which are attached to kinetochores of chromosomes at one end and to a pole of the spindle at the other.
kinetodesma n. striped .bre that is part of a kinetid in ciliate protozoans. plu. kinetodesmata.
kinetoplast n. large mitochondrion associ­ated with the base of the .agellum in some .agellate protozoans.
kinetoplastids n.plu. .agellate protozoans that contain a kinetoplast. This group in­cludes the parasitic trypanosomes.
kinetosome n. (1) basal body of cilium in ciliate and .agellate protozoans; (2) one of a group of granules occupying the polar plate region in sporogenesis in mosses.
kinetospore n. a motile spore.
king n. in social hymenopterans or termites, a male reproductive individual.
king crabs horseshoe crabs q.v.
kingdom n. (1) in taxonomy, the name given to a primary division of living organisms. Until quite recently, .ve kingdoms were generally recognized: Prokaryotae (bac­teria and other prokaryotes), Protoctista (or Protista) (simple eukaryotic organisms such as the protozoa and algae), Fungi, Plantae (multicellular green plants other than algae) and Animalia or Metazoa (multicellular animals). Most recently, the living world has been divided on the evidence of DNA sequence data into three primary superkingdoms or domains: Bacteria (q.v.), Archaea (q.v.) and Eukarya (q.v.). The Bacteria and Archaea correspond to the Prokaryotae, while the Eukarya com­prises all the remaining kingdoms. King­doms are divided into phyla, for animals, and divisions for plants. see Appendices 1–6; (2) biogeographical kingdom q.v.
kinin n. (1) bradykinin, lysylbradykinin (kallidin) q.v., (2) cytokinin q.v.
kininogen n. precursor protein from which a kinin is produced by post-translational cleavage by kallikrein. Also acts as a cofactor in the blood clotting pathway, helping to activate Factor XII.
kinoplasmasomes n.plu. phragmoplast .bres seen at periphery of cell plate in plant cell division.
Kinorhyncha, kinorhynchs n., n.plu. phylum of marine microscopic pseu­docoelomate invertebrate animals having bodies of jointed spiny segments and spiny heads.
kin selection the selection of genes in a population due to individuals favouring or disfavouring the survival of relatives, other than offspring, who possess the same genes by common descent.
kinship n. possession of a common ancestor in the not too distant past.
kinship, coef.cient of coef.cient of kin­ship q.v.
KIR killer inhibitory receptor q.v.
kirromycin n. antibiotic that inhibits elon­gation of the polypeptide chain during translation in bacteria by inhibiting the elongation factor EF-Tu.
kJ kilojoule q.v.
Kjeldahl analysis technique widely used to determine the total nitrogen content of a tissue.
K+
leak channel ion channel in plasma membrane of animal cells that allows potassium ions (K+) to leak passively out of the cell down their concentration gradi­ent. Together with the Na+-K+ ATPase, it is involved in generating and maintaining plasma membrane electrical potential.
Klenow fragment the fragment of DNA polymerase I from Escherichia coli that contains both the polymerase and 3′. 5′ exonuclease activity but lacks the 5′. 3′ exonuclease activity.
kleptobiosis see cleptobiosis.
kleptoparasitism n. type of parasitism in which the female searches out the prey or stored food of another female, usually of a different species, and takes it for her own offspring. alt. cleptoparasitism.
Klinefelter’s syndrome syndrome occur­ring in men having genetic constitution XXY, resulting in underdevelopment of male sexual organs and sterility.
klinokinesis n. movement in which an or­ganism continues to go in a straight line until it meets an unfavourable environment, when it turns, resulting in its remaining in a favourable environment, the frequency of turning depending on the intensity of the environmental stimulus.
klinotaxis n. taxis in which an organism orients itself in relation to a stimulus by moving its head or whole body from side to side symmetrically in moving towards the stimulus, and so compares the inten­sity of the stimulus on either side.
knee n. (1) joint between tibia and femur;
(2) root that emerges above water or ground in certain trees living in swampy land; (3) joint in stem of some grasses.
knee-jerk re.ex re.ex composed of a simple neural circuit of sensory neuron (stretch receptor), spinal interneuron, and motor neuron, that makes the lower leg swing up when tapped just below the knee.
knob n. (1) in cytogenetics, a darkly stain­ing chromomere that identi.es a particu­lar chromosome; (2) in parasitology, cells infected with certain strains of the malaria parasite Plasmodium falciparum, which produce knobs that attach to endothelial cells and block cerebral vessels, giving rise to cerebral malaria.
knock-in see gene knock-in.
knock-out see gene knock-out.
knot n. in wood, the base of a branch sur­rounded by concentric layers of new wood and hardened under pressure.
Koch’s postulates the criteria that need to be satis.ed to prove a particular micro­organism is the cause of a disease: (i) that the microbe be found in the body in all cases of the disease, (ii) that it be isolated from a disease case and grown in a series of pure cultures in vitro, and (iii) that it reproduce the disease on the inoculation of a pure culture into a susceptible animal.
koilin lining the grinding surface inside a bird’s gizzard.
Kornberg cycle glyoxylate cycle q.v.
Kornberg enzyme DNA polymerase in­
volved in DNA repair. Kranz anatomy type of leaf anatomy in plants with C4 photosynthesis, in which
photosynthetic outer mesophyll cells are arranged in a “wreath” around inner bundle sheath cells. This enables the intercellular transport of the C4 acids, which are the .rst products of photosynthesis, from the mesophyll cells to the bundle sheath cells in which they are decarboxylated to provide CO2 for the Calvin cycle. C3 metabolites are transported the other way to act as a substrate for the initial CO2 .xation.
krasnozem n. deep friable red loamy soil found in the subtropics and developed from base-rich parent materials.
Krebs cycle tricarboxylic acid cycle q.v.
Krebs–Henseleit cycle urea cycle q.v.
krill n. planktonic crustaceans, which are abundant in the oceans and form the prin­cipal food of the .lter-feeding whales.
kringle n. a small protein domain found in a number of different proteins.
K-selected species species selected for its superiority in a stable environment. Members of such species typically have a slow development, relatively large size, and produce only a small number of offspring at a time.
K selection selection favouring superiority in stable, predictable environments in which rapid population growth is unimportant. cf. r selection.
Kulezinski index index of similarity (see coef.cient of community) between two plant communities based upon their spe­cies composition. It is calculated by divid­ing the number of species in common by the sum of the total number of species in each community. cf. Gleason’s index, Jaccard index, Morisita’s similarity index, Simpson’s index of .oristic resemblance, Sørensen similarity index.
Kupffer cells star-shaped phagocytic cells (macrophages) found in liver sinusoids and which ingest defunct red blood cells.
kuru n. transmissible spongiform encephalopathy that used to affect people in Papua New Guinea and was caused by the consumption of infected human brains. see prion.
kwashiorkor n. de.ciency disease caused by an insuf.ciency of protein.
kynurenine n. metabolic product of trypto­phan, and a precursor of some ommato­chromes and other pigments in insects.
L

l wavelength (of light). see also lambda.
L leucine q.v.
L-, D-pre.xes denoting particular molecular con.gurations, de.ned according to con­vention, of certain optically active com­pounds esp. monosaccharides and amino acids, the l con.guration being a mirror image of the d. In living cells such mol­ecules usually occur in one or other of these con.gurations but not both (e.g. glucose as d-glucose, amino acids always in the l form in proteins).
labella n. pair of grooved lobes at end of labium in some dipteran insects, for mopping up liquid food.
labellate a. furnished with labella or small lips.
marked a. appl. a molecule made detect­able and traceable by incorporation of a radioactive element or by linkage to a detectable chemical tag—the mark.
labia n.plu. (1) lips; (2) lip-like structures;
(3) plu. of labium. labial a. pert. labium. labial palp lobe-like structure near mouth
of certain insects.
labia majora outer lips of vulva.
labia minora inner lips of vulva.
labiate a. (1) lip-like; (2) possessing lips or thickened margins; (3) n. any member of the dicot family Labiatae, which in­cludes the mints and balsams. They are characterized by typically square stems, opposite decussate leaves and .owers with a corolla divided into two lips.
labidophorous a. possessing pincer-like
organs. labiella n. mouthpart of millipedes. labile a. (1) readily undergoing change;
(2) unstable; (3) appl. genes that have a tendency to mutate.
lability n. in evolutionary theory, the ease and speed with which particular categories of traits evolve.
labiodental a. pert. lip and teeth, appl. the surface of tooth nearest to lip.
labium n. (1) a lip or lip-shaped structure;
(2) in insects the fused 2nd maxillae, form­ing the lower mouthpart; (3) inner margin of mouth of gastropod shell. plu. labia.
Laboulbeniomycetes n. group of highly specialized ascomycete fungi, parasitic on the outer surface of insects and arachnids. They have an ascogonium with a tri­chogyne and fertilization by spermatia.
labrum n. (1) anterior mouthpart of some arthropods; (2) outer margin of gastropod shell. a. labral.
labyrinth n. (1) the complex convoluted membranous and bony structures of the inner ear. see ear; (2) much folded acces­sory respiratory organ above gills in some .shes; (3) any of various other convoluted structures.
labyrinthodont a. having teeth with a com­plicated arrangement of dentine.
Labyrinthodontia, labyrinthodonts n., n.plu. subclass of extinct early amphi­bians, of the late Paleozoic and Mesozoic, with labyrinthodont teeth. They in­cluded temnospondyls, anthracosaurs and ichthyostegalians.
Labyrinthulomycota n. the net slime moulds or slime nets, a phylum of colo­nial non-photosynthetic eukaryotic micro­organisms that form colonies of cells that go and grow within a slime track that they secrete. Found mainly in estuarine and nearshore habitats associated with leaves, algae and organic debris. Two families are recognized: Labyrinthulaceae and Thraustochytriaceae. They have been classi.ed variously as fungi or protists and are now generally included in the Stramenopila. cf. Mycetozoa.
lac n. (1) resinous secretion of lac glands of certain insects. Some types are used to make shellac; (2) designation for the lactose operon of bacteria, which encodes enzymes involved in the uptake and meta­bolism of lactose to glucose.
laccate a. appearing as if varnished.
lacertiform a. shaped like a lizard.
Lacertilia, lacertids n., n.plu. suborder of reptiles containing the lizards (e.g. geckos, iguanas, agamas, skinks, bearded lizards, monitors). Most species are four-legged, some running on their hind-legs, but some
(e.g. slow-worms) are legless. They include insectivorous, herbivorous and carnivorous species, and are adapted to a wide range of habitats, including very dry regions.
lacewings see Neuroptera.
lacinia n. (1) (bot.) segment of .nely cut leaf or petal; (2) slender projection from a thallus; (3) (zool.) extension of posterior part of proglottis; (4) inner branch of maxilla in insects.
laciniate a. (1) irregularly cut, as of petals;
(2) fringed. laciniform a. fringe-like. laciniolate a. minutely incised or fringed. lacinula n. (1) a small lacinia; (2) the
in.exed sharp point of a petal. plu.
lacinulae. lacinulate a. having lacinulae. lacrimal a. (1) secreting or pert. tears;
(2) pert. or situated near the tear gland, appl. e.g. artery, duct, nerve. lacrimal bone bone in skull near the tear gland. lacrimal gland gland in the corner of the
eye which secretes tears. alt. tear gland.
lacrimiform a. tear-shaped, appl. spores.
lacrimonasal a. pert. lacrimal and nasal bones and duct.
lacrimose a. bearing tear-shaped append­ages, appl. gills of certain fungi.
lacrioid a. tear-shaped.
lactalbumin n. an albumin protein present in milk.
b-lactam antibiotic any of a large group of antibiotics, including the penicillins and cephalosporins, that contain a ß-lactam group.
b-lactamase n. enzyme secreted by penicillin-resistant bacteria, which destroys penicillin’s antibiotic activity, hydrolys­ing the ß-lactam ring. EC 3.5.2.6. alt. penicillinase.
lactase n. enzyme that hydrolyses terminal non-reducing ß-d-galactose residues in ß-d-galactosides (e.g. lactose) to glucose and galactose. EC 3.2.1.23, r.n. ß­galactosidase.
lactate see lactic acid.
lactate dehydrogenase enzyme catalys­ing reduction of pyruvate by NADH to lactic acid in animal tissues and some bacteria. EC 1.1.1.27.
lactation n. (1) secretion of milk in mam­mary glands; (2) period during which milk is secreted.
lacteals n.plu. (1) lymphatic vessels of the small intestine; (2) (bot.) ducts that carry latex.
lacteous a. milky in appearance or texture.
lactescent a. producing milk or latex.
lactic a. pert. milk.
lactic acid, lactate three-carbon carboxylic acid, usually formed by the reduction of pyruvate. It is formed in animal cells, esp. muscle, when insuf.cient oxygen is supplied for the full oxidation of sugars. It is also produced during fermen­tation of sugars by certain bacteria, esp. lactobacilli.
lactic acid bacteria lactobacilli q.v.
lactifer n. any latex-containing cell, series of cells or duct.
lactiferous a. (1) forming or carrying milk;
(2) carrying latex. lacti.c a. milk-producing. lactobacillus n. member of the bacterial
genus Lactobacillus, Gram-positive bac­teria characteristically producing lactic acid as an end-product of anaerobic respira­tion, and responsible for souring milk. plu. lactobacilli.
lactoferrin n. an iron-binding protein in animals.
lactogenesis n. initiation of milk secretion.
lactogenic a. (1) pert. or stimulating secre­
tion of milk; (2) appl. hormone: prolactin q.v.; (3) appl. interval between parturition and ovulation, or between parturition and menstruation.
lactoglobulin n. milk protein soluble in ammonium sulphate but insoluble in water alone.

lactoperoxidase n. peroxidase (q.v.), an antimicrobial enzyme in milk and saliva.
lactoprotein n. any of the proteins in milk.
lactose n. disaccharide composed of glucose and galactose, abundant in milk.
lactose intolerance inability to digest lac­tose, leading to abdominal symptoms when large amounts are ingested, e.g. in milk. It is shown by many adults as a result of decreased production of the enzyme lactase after weaning. It may also be a genetically determined condition, present from birth, which is due to a de.ciency in the gene for lactase.
lactosis lactation q.v.
lacuna n. a space or cavity. plu. lacunae.
lacunar a. having, resembling, or pert. lacunae.
lacunate a. possessing or forming lacunae.
lacunose a. having many cavities.
lacunosorugose a. having deep furrows or pits, as some seeds and fruits.
lacunula n. minute cavity or air space.
lacustrine a. pert., or living in or beside, lakes.
lacZ the bacterial gene for ß-galactosidase, which is often used as a marker in a fusion gene to detect expression of an attached promoter in e.g. particular cell types, as expression of ß-galactosidase can be de­tected histochemically.
laeotropic a. inclined, turned or coiled to the left.
laevigate levigate q.v.
laevo-rotatory a. appl. optically active molecules that rotate a beam of plane polarized light in an anticlockwise (–) direction.
lagging strand, lagging chain in DNA replication in vivo, the new DNA strand that is synthesized discontinuously at the replication fork. cf. leading strand.
lageniform a. shaped like a .question.
Lagomorpha, lagomorphs n., n.plu. the rabbits, hares and pikas. An order of herbivorous mammals known from the Eocene, with skulls and dentition similar to rodents, but with a second pair of in­cisors, and with hindlimbs modi.ed for leaping.
lagopodous a. having hairy or feathered feet.
lag phase the .rst phase of growth of a bacterial culture, in which there is no appreciable increase in cell numbers.
LAI leaf area index q.v.
LAK lymphokine-activated killer cells q.v.
laking haemolysis q.v.
Lamarckism n. a theory of evolution chie.y formulated by the French scientist J.B. de Lamarck in the 18th century, which embodied the principle, now known to be mistaken, that somatic characteristics bought by an organism during its life­time can be inherited.
lambda (l) n. (1) DNA bacteriophage that infects Escherichia coli and whose genetic structure and function have been minutely dissected. Lambda is a temper­ate bacteriophage that may persist as a prophage integrated into the bacterial DNA or multiply within the bacterial cell, eventually destroying it. It is used as a vector in recombinant DNA work; (2) (anat.) the junction of the lambdoid and sagittal sutures of skull; (3) (immunol.) one of the two types of light chain found in immunoglobulins.
lambda particles (1) cytoplasmic inclu­sions in the ciliate Paramecium; (2) par­ticles of phage lambda.
lambdoid a. (1) appl. phages genetically and morphologically resembling phage lambda; (2) lambda-shaped, appl. cranial suture joining occipital and parietal bones.
lamella n. (1) any thin or plate-like struc­ture; (2) a gill of a mushroom or toadstool;
(3) a layer of cells. plu. lamellae. lamellar, lamellate a. composed of thin plates.
lamellasome n. layered membranous structure in cyanobacteria which bears the biochemical apparatus of photosynthesis.
lamellated corpuscles Pacinian bodies
q.v.
lamellibranch(iate) a. (1) having plate-like gills on each side; (2) with bilaterally com­pressed symmetrical body, like a bivalve.
lamellibranchs n.plu. the Lamellibranchia, a large subclass of bivalve molluscs, in­cluding e.g. clams, cockles, mussels.
lamellicorn a. having segments of antennae expanded into .attened plates.
lamelliferous a. having small plates or scales.
lamelliform, lamelloid a. plate-like.
lamellipodium n. thin sheet-like pseu­dopodial extension temporarily place for­ward by animal cells such as .broblasts or lymphocytes when moving over a surface. plu. lamellipodia.
lamellirostral a. having inner edges of bill bearing lamella-like ridges.
lamellose a. (1) containing lamellae; (2) having a lamellar structure.
Lamiales n. order of dicot herbs, shrubs and trees including Verbenaceae (verbena), Lamiaceae (e.g. mint) and others.
lamin n. any of a small family of proteins that form intermediate .laments in the nuclear lamina.
lamina n. (1) thin layer, plate or scale;
(2) layer of cell bodies in cerebral cortex and other brain structures; (3) blade of leaf or petal; (4) .attened part of a thallus.
lamina basalis Bruch’s membrane q.v.
lamina cribrosa region of sclera at site of attachment of optic nerve and with per­forations for axons of retinal ganglion cells.
lamina fusca inner layer of sclera, adjoin­ing lamina suprachoroidea.
lamina propria layer of loose connective tissue in the mucosa of gut and other tubular tracts, which houses the bases of glands and contains blood and lymph vessels and mucosa-associated lymphoid tissues. It lies immediately under the epi­thelium and basement membrane.
laminar a. (1) consisting of plates or thin layers; (2) (bot.) appl. placentation of ovule, attachment over the surface of carpel.
laminarian a. appl. zone between low tide line to about 30 m depth, i.e. the zone typically inhabited by Laminaria seaweeds.
laminarin n. any of various carbohydrates which are the main food reserves in brown algae and are stored in solution. They consist mainly of glucose units but some contain mannitol.
lamina suprachoroidea delicate tissue layer between choroid and sclera of eye.
laminated a. composed of thin plates, appl. plant cuticle.
lamina terminalis thin layer of grey matter forming anterior boundary of 3rd ventricle of brain.
lamination n. (1) the formation of thin plates or layers; (2) arrangement in layers, as of the nerve cell bodies of cerebral cortex.
lamina vasculosa outer layer of choroid in retina beneath suprachoroid membrane.
laminiform a. (1) like a thin layer or layers;
(2) like a leaf blade; (3) laminar q.v. laminin n. glycoprotein of the extracellular matrix. laminiplantar a. having scales of meta­tarsus meeting behind in a smooth ridge.
lampbrush chromosome type of bivalent chromosome formed in many vertebrate oocyte nuclei during extended meiosis. The chromosomes are in an extended state, with loops of chromatin folded out from each chromatid at the chromomeres to give an appearance rather like a bottle-brush (or the brush that was used to clean oil lamps). The chromatin loops are being transcribed, as can be seen in the electron microscope.
lampreys n.plu. common name for primitive .sh-like freshwater and marine chordates of the order Petromyzoniformes, in the class Agnatha, which as adults have suck­ing and rasping mouthparts.
lamp shells common name for the Brachiopoda q.v.
lanate a. covered with small woolly hairs.
lancelets common name for the Cephalo­cordata q.v.
lanceolate a. slightly broad or tapering at base, and tapering to a point at tip, appl. leaves.
lance-oval a. having a shape intermediate between lanceolate and ovate, appl. leaves.
lancet n. one of the paired parts ventral to stylet of sting in bees and wasps.
Langerhans’ cell immature dendritic cell found in the skin and parts of gastrointest­inal tract. It is involved in the uptake of antigen, which it then transports to sec­ondary lymphoid organs to initiate an immune response.
Langerhans’, follicles of see follicles of Langerhans. Langerhans’, islets of see islets of Langerhans. laniary a. adapted for tearing, appl. canine tooth. laniferous, lanigerous a. wool-bearing or .eecy.
lantern n. (1) Aristotle’s lantern q.v.; (2) a light-emitting organ, as of lantern .shes.
lanuginose, lanuginous a. covered in down.
lanugo n. the downy covering on a foetus which starts to be shed before birth.
lapidicolous a. appl. animals that live under stones.
lappaceous a. (1) like a burr; (2) prickly.
lappet n. (1) any of various hanging, lobe-like structures; (2) wattle of a bird.
LAR leaf area ratio q.v.
large intestine the caecum, colon and appendix in some vertebrates, sometimes used for the colon only.
lariat n. tailed circle composed of intron RNA that is formed when an intron is spliced out of primary transcript RNA.
larmier dacryocyst q.v.
larva n. independently living, post-embryonic stage of an animal that is markedly differ­ent in form from the adult and which under­goes metamorphosis into the adult form,
e.g. caterpillar, grub, tadpole. plu. larvae.
a. larval.
Larvacea n. class of tunicates (urochordates) which retain the larval “tadpole” form throughout their lives.
larvicide n. an agent that kills insect larvae.
larviform a. shaped like a larva.
larviparous a. giving birth to offspring at
the larval stage. larvivorous a. larva-eating. larvule n. a young larva. laryngeal a. pert. or near the larynx, appl.
e.g. artery, vein, nerve.
laryngeal prominence in primates, a subcutaneous projection of the thyroid cartilage in front of the throat, causing a ridge on the ventral surface of the neck, and more pronounced in males. alt. Adam’s apple.
larynges plu. of larynx.
laryngopharynx n. part of pharynx between soft palate and oesophagus.
laryngotracheal a. (1) pert. larynx and trachea; (2) appl. chamber into which lungs open in amphibians.
larynx n. in mammals, the organ in throat that produces sound, the voice box. It is the upper end of the windpipe stiffened by cartilage and with two membranes (vocal chords) each extending half-way across the windpipe leaving a narrow slit between them. Sound is produced when air is driven through the slit, setting the vocal chords vibrating. a. laryngeal. plu. larynges.
laser capture microdissection technique for isolating a very small area, even a single cell, from a tissue. The tissue is coated with a thin plastic .lm and a laser is then used to fuse the required area to the plastic, cut round it and remove it.
lash .agellum .agellum in which the main .lament ends in a thinner part, the lash.
lasso n. a contractile .lamentous noose formed by certain soil fungi and used to trap nematodes.
lasso cell colloblast q.v.
last universal common ancestor (LUCA) the hypothetical microorganism from which all present-day organisms are descended.
late a. (1) appl. period from the start of virus nucleic acid replication within a cell to the release of infectious bacteriophage or virus; (2) appl. bacteriophage or viral genes expressed at later stages of infection.
latebricole a. living in holes.
latency n. property of enzymes in a cell extract which fail to show maximum act­ivity unless treated with detergents. Such enzymes are membrane-bound enzymes.
latent a. (1) lying dormant but capable of development under certain circumstances, appl. buds, resting stages; (2) appl. char­acteristics that will become apparent under certain conditions; (3) appl. virus infection in which the virus remains quiescent for long periods of time, symptoms appearing as the virus resumes multiplication.
latent bodies the resting stage of certain .agellate blood parasites.
latent period reaction time q.v.
laterad a. (1) towards the side; (2) away from the axis.
lateral a. pert., or situated at, the side.
lateral bud bud arising in the axil of a leaf at the node of a stem, and which will de­velop into a side shoot.
lateral diffusion usually refers to the abil­ity of membrane proteins and membrane lipids to diffuse in the plane of the lipid bilayer.
lateral ganglia ganglia of the autonomic nervous system. In mammals they lie in two chains alongside the aorta and are linked to each other by nerve .bres. cf. collateral ganglia.
lateral gene transfer horizontal gene transfer q.v.
lateral geniculate nucleus organized region of cell bodies in each cerebral hemisphere, at which optic nerve .bres terminate, and from which impulses are relayed to the visual area of the cerebral cortex.
lateralia n.plu. the lateral plates of barnacles.
lateral inhibition (1) mechanism in com­pound eyes which enhances contrast at boundaries between lighter and darker parts of the visual .eld; (2) (dev. biol.) devel­opmental mechanism ensuring that repeat­ing structures such as insect sensillae or the individual units (ommatidia) of a compound eye are evenly spaced. The developing cell or structure inhibits the differentiation of similar elements within a given distance away.
lateralis organ neuromast q.v.
lateral line longitudinal line on each side of the body in .shes. It marks the position of cutaneous sensory cells of the acoustico­lateralis system concerned with the per­ception of movement and sound waves in water, the cells on the lateral line being known collectively as the lateral line system.
lateral line organ neuromast q.v.
lateral meristems dividing tissues in plants that are concerned with the production of secondary tissues rather than with the primary apical growth of the plant. They include the vascular cambium (produc­ing the vascular bundles) and the cork cambium (producing the outer layer of cork or bark).
lateral plate mesoderm mesoderm that gives rise to the splanchnic and somatic mesoderm in vertebrates, from which inter­nal organs and blood system will develop. cf. somites.
lateral roots roots which branch off the primary root, and which themselves may give rise to further lateral roots.
lateral ventricle large .uid-.lled cavity in centre of cerebral hemisphere of brain.
laterigrade a. walking sideways, like a crab.
laterinerved a. with lateral veins, appl. leaves.
laterite a. appl. tropical red soils con­taining alumina and iron oxides and small silica owing to leaching under hot moist conditions.
laterobronchi n.plu. secondary bronchi aris­ing from the mesobronchus in birds.
laterosensory a. appl. lateral-line system in .shes. see lateral line.
laterosphenoid n. bone on the mid-line of the reptilian skull, behind the orbit and above the palate.
late wood wood formed in the later part of the growing season. It shows as a dense ring of small cells in the annual ring. alt. summer wood.
latex n. thick milky or clear juice or emul­sion of diverse composition present in plants such as rubber trees, spurges, and in certain agaric fungi.
lathyrism n. disease of animals character­ized by fragile collagen. It is caused by eating seeds of certain Lathyrus species that contain ß-aminopropionitrile, which inhibits essential post-translational modi­.cation of collagen.
laticifer lactifer q.v.
laticiferous a. conveying latex, appl. cells, tissues.
latifoliate a. with broad leaves.
Latin name see binomial nomenclature.
latiplantar a. having hinder tarsal surface rounded.
latirostral a. broad-beaked.
latiseptate a. having a broad septum in the siliqua.
latitudinal furrow a cleavage furrow run­ning round the dividing fertilized egg above and parallel to the equatorial furrow.
latosol n. leached red or yellow tropical soil.
Laurales n. order of dicot trees, shrubs and climbers, with aromatic ethereal oils in the cells, and including the families Calycanthaceae (calycanthus), Lauraceae (laurels), and others.
Laurasia n. former northern land mass that consisted of present-day North America, Europe and Northern Asia before they were separated by continental drift. cf. Gondwanaland.
laurilignosa n. type of subtropical forest and bush composed of laurel.
laurinoxylon n. fossil wood.
law of equal segregation see segrega­tion of alleles.
law of independent assortment see
independent assortment.
law of the minimum rule stating that the factor for which an organism or species has the narrowest range of tolerance or adaptability limits its existence.
lax a. loosely clustered, appl. panicle of .owers.
layer n. horizontal stratum in a plant com­munity. The following are usually distin­guished: the tree layer or canopy, the shrub layer, comprising the shrubby understorey, the herb layer of grass and herbaceous plants, and the ground (moss) layer, com­prising the ground surface and lichens and mosses.
LC lethal concentration q.v.
LC50 concentration of any toxic chemical that kills 50% of the organisms in a test population per unit time.
LCA family of lectins isolated from lentils, Lens culinaris.
L chain light chain q.v.
LCM lymphocytic choriomeningitis virus q.v.
LCR locus control region q.v.
LD50 a measure of infectivity for viruses or of toxicity of chemicals, the dose at which 50% of test animals die.
LDL low-density lipoprotein q.v.
LDL receptor receptor on non-hepatic cells which is speci.c for low-density lipopro­tein (LDL). It is vital in uptake of cholesterol and its clearance from the circulation.
leaching n. (1) process by which chemicals in the upper layers of the soil are dissolved and carried down into lower layers; (2) microbial leaching, extraction of metals from their ores by microbial action.
lead (Pb) n. metallic element, a “heavy metal” which is toxic to many organisms and is an environmental pollutant.
leader n. (1) topmost growing shoot or main branch of tree; (2) leader sequence q.v.
leader peptide signal peptide q.v.
leader sequence (1) region in mRNA (and DNA) preceding the coding region, which is transcribed but is not translated, alt. untranslated leader, 5′ UTR; (2) translated leader q.v.; (3) signal sequence q.v.
leading strand, leading chain in DNA replication in vivo, the new DNA strand that is synthesized continuously at the rep­lication fork. cf. lagging strand.
leaf n. an expanded .attened or needle-like outgrowth from plant stem, usually green and the main photosynthetic organ of most plants. plu. leaves.
leaf area index (LAI) of a given area of vegetation, the total area of photosynthetic leaf surface divided by the area of soil covered.
leaf area ratio (LAR) the ratio of the photosynthetic surface area of a leaf to its dry weight.
leaf blade the thin .at part of a leaf.
leaf buttress lateral prominence on shoot axis caused by an underlying leaf primordium.
leaf cushions prominent persistent leaf bases on stems of some trees, e.g. palms, and furnishing diagnostic characters in some fossil plants.
leaf divergence the .xed proportion of the circumference of the stem by which each leaf is separated from the next.
leaf gap region of ground tissue interrupt­ing the pattern of the stele, resulting from the divergence of vascular tissue (the leaf trace) away from stele to leaf.
leaf hair trichome q.v.
leaf insects common name for some mem­bers of the Phasmida whose bodies mimic leaves in form.
lea.et n. (1) a small leaf; (2) individual unit of a compound leaf; (3) (mol. biol.) one layer of a lipid bilayer.
leaf mosaic the arrangement of leaves on a plant which results in minimum overlap and maximum exposure to sunlight.
leaf scar the trace, usually covered with a corky layer, left on stem after leaf has fallen.
leaf sheath extension of leaf base sheath­ing the stem, as in grasses.
leaf stalk petiole q.v.
leaf trace vascular tissue extending from stele of stem into base of leaf.
leaky a. appl. mutations with some residual function. see also hypomorph.
learning n. any process in an animal in which its behaviour becomes consistently modi.ed as a result of experience, and including conditioning, habituation and imprinting. see also long-term potentiation, memory.
learning set in animal behaviour, the apparent learning of a general rule for solving a set of problems.
leberidocyte n. cell containing glycogen, present in blood of arachnids at moulting.
lechriodont a. with vomerine and pterygoid teeth in a row nearly transverse.
lecithin phosphatidylcholine q.v.
lecithoprotein n. lipoprotein in which lecithin (phosphatidylcholine) is the lipid component.
lecithotrophic a. feeding on stored yolk, as in some sea urchin larvae.
lecithovitellin n. lipoprotein composed of lecithin (phosphatidylcholine) and the yolk protein vitellin.
lectin n. any of a group of plant proteins that can agglutinate animal cells in vitro by binding to speci.c sugar residues in membrane glycoproteins. Some lectins also have mitogenic activity. Their natural role in plants is likely to be in cell–cell adhe­sion, cell recognition and defence against infection. see also C-type lectin.
lectotype n. specimen chosen from syntypes to designate type species.
leech common name for a member of the Hirudinea q.v.
left–right asymmetry the normal asym­metrical arrangement of the internal organs of the human body.
leghaemoglobin n. red oxygen-binding protein pigment, resembling haemoglobin, found in the nitrogen-synthesizing root nodules of leguminous plants.
Legionnaire’s disease respiratory disease caused by the bacterium Legionella pneumophila. alt. legionellosis.
legume n. (1) a pod, a type of fruit derived from a single carpel and which splits down both sides at maturity, characteristic of the pea family; (2) any member of the Leguminosae, e.g. peas, beans, clovers, vetches, gorse, broom.
legumin n. protein present in seeds of legu­minous plants.
Leguminosae n. large family of dicotyledonous plants, commonly called legumes or leguminous plants, and includ­ing trees, shrubs, herbs and climbers. They have typical sweet-pea shaped .owers and fruit in the form of pods. They include peas, beans, clovers, vetches. see also Fabales.
leguminous a. (1) pert. Leguminosae; (2) pert., or consisting of, peas, beans or other legumes.
leimocolous a. inhabiting damp meadows.
leiosporous a. with smooth spores.
leiotrichous a. having straight hair.
Leishmania genus of parasitic protozoa, infecting humans and other mammals, with sand.ies as the intermediate host and vector. L. donovani causes the chronic and often fatal tropical disease visceral leishmaniasis, L. tropica causes cutaneous leishmaniasis or tropical sore.
leishmanial a. appl. small stout forms of trypanosome lacking a free .agellum.
Leitneriales n. order of resinous dicot shrubs comprising the family Leitneriaceae with the single genus Leitneria.
lek n. special arena removed from nesting and feeding grounds, used for communal courtship show (lekking) preceding mating in some birds (e.g. ruffs and black grouse). The term is sometimes applied to similar areas used by other animals for communal displays.

lekking n. highly ritualized sexual show by birds such as black grouse, which takes place on a particular show ground, the lek, and which precedes mating.
lemma n. lower of the two bracts enclosing a .oret (individual .ower) in grasses.
lemniscus n. band of white matter in midbrain and medulla oblongata.
lens n. (1) transparent structure in the eye through which light is focused onto the retina, the crystalline lens of the vertebrate eye being formed from prism-shaped, refractile dead cells .lled with the protein crystallin; (2) modi.ed part of the cor­nea in front of each element of a compound eye; (3) modi.ed cells of luminescent organ in certain .shes.
lens .bres elongated, lifeless, crystallin-.lled cells making up the lens of the eye.
lens placode local thickening of the ectoderm opposite the optic vesicle, which invaginates to form the lens pit, which then closes to become the lens vesicle, devel­oping into the lens. alt. lens rudiment.
lentic a. (1) appl. standing water; (2) appl. organisms living in swamp, pond, lake or any other standing water.
lenticel n. pore in periderm of trees and shrubs, allowing the passage of air to internal tissues.
lenticula n. (1) a spore case in certain fungi;
(2)
lenticel q.v.; (3) lentigo or freckle. lenticular a. (1) shaped like a double convex

lens; (2) pert. lenticels. lenticulate a. (1) meeting in a sharp point;

(2)
depressed, circular and often ribbed. lentiform a. lentil-shaped. lentiform glands lymphoid glands situated
between pyloric glands.
lentiform nucleus the putamen and globus pallidus. see basal ganglia.
lentigerous a. having a lens.
lentiginose, lentiginous a. freckled, speckled, or bearing many small dots.
lentiviruses n.plu. subfamily of non-oncogenic slow-acting retroviruses, which cause chronic infections that only be­come manifest years after infection. They include HIV (human immunode.ciency virus).
lepidic a. (1) consisting of scales; (2) pert. scales.
lepidodendroid a. having scale-like leaf scars.
Lepidodendron a genus of fossil tree-fern with small leaves producing scale-like leaf scars.
lepidoid a. resembling a scale or scales.
lepidomorium n. small scale or unit of composite scale, with bony base and con­ical crown of dentine, containing a pulp cavity and sometimes covered with enamel.
a. lepidomorial. lepidophyte n. fossil fern. Lepidoptera, lepidopterans n., n.plu.
order of insects commonly known as moths and butter.ies. Their bodies and wings are covered by small scales, often brightly and variously coloured, forming characteristic patterns. They undergo complete meta­morphosis, the larval (caterpillar) stage giving rise to a pupa in which meta­morphosis occurs, with development of adult structures such as the two pairs of wings, the legs and compound eyes. Adult Lepidoptera feed largely on nectar, through a hollow proboscis. a. lepidopterous.
Lepidosauria, lepidosaurs n., n.plu. subclass of reptiles comprising the liz­ards, snakes and amphisbaenians, and the tuatara, with a diapsid skull, and with limbs and limb girdles unspecialized, reduced or absent.
lepidosis n. character and arrangement of scales on an animal.
lepidote a. covered with minute scales.
lepidotrichia n.plu. bony unjointed rays at the edge of .ns in teleost .shes.
lepospondylous a. having hour-glass­shaped vertebrae.
leprosy n. chronic disease caused by the bacterium Mycobacterium leprae, leading to extensive tissue hurt, deformity and disability.
leptin n. protein hormone secreted by stout cells and which acts at leptin receptors in the brain to lessen hunger and discourage eating. It is thought to be a means of monitoring the amount of body stout. When the amount of stout is low, leptin is not produced.
lepto-pre.x derived from Gk leptos, slender.
leptocaul a. having a slender primary stem.
leptocentric a. appl. concentric vascular bundle with phloem at centre.
leptocephaloid a. resembling or having the shape of eel larvae.
leptocephalus n. translucent larva of cer­tain eels, before the elver stage.
leptocercal a. with a long slender tapering tail, appl. some .shes.
leptodactylous a. having slender .ngers.
leptodermatous a. thin-skinned.
leptoid n. living food-conducting cell joined with others to form a simple conducting tissue in stems of some mosses.
leptoma n. thin area in the wall of a gymno­sperm pollen grain, through which the pollen tube emerges.
leptome n. (1) sieve elements and paren­chyma of phloem; (2) similar conducting elements in bryophytes.
leptomonad a. appl. long slender form of trypanosome with a free .agellum.
leptonema n. .ne thread-like chromosome that appears at leptotene stage of meiosis.
leptophyllous a. (1) with slender leaves;
(2) having a small leaf area, under 25 mm3. leptosome a. tall and slender.
leptospirosis n. disease commonly charac­terized by jaundice and nephritis, caused by pathogenic species of the spirochaete bacterium Leptospira. Humans usually contract the disease from infected pets or from wild rodents. alt. Weil’s disease.
leptosporangiate a. appl. ferns in which the sporangia develop from a single initial cell, which .rst produces a stalk and then a capsule. cf. eusporangiate.
leptosporangium n. stalked sporangium, in ferns. plu. leptosporangia.
leptotene n. early stage in the prophase of meiosis in which the chromatin is start­ning to become compacted and the chromo­somes show up under the microscope as .ne threads.
leptotrombicula n. larval form of a trombicula, a mite transmitting scrub typhus (tsutsugamushi disease), a ricket­tsial disease.
leptoxylem n. rudimentary wood tissue.
leptus n. the six-legged larva of mites.
Lesch–Nyhan syndrome inherited disease characterized by an nearly complete de.ciency of the enzyme hypoxanthine-guanine ribosyltransferase and by symp­toms of self-mutilation, mental de.ciency and spasticity.
lesion n. area of tissue destruction.
lesser omentum a fold of peritoneum which connects the stomach and liver and supports the hepatic vessels.
lestobiosis n. the relation in which colonies of small species of insect nest in the walls of the nests of larger species and enter their chambers to prey on the brood or rob food stores.
lethal a. (1) causing death; (2) of a parasite, fatal or deadly in relation to a particular host; (3) appl. mutations or alleles which when present in an embryo cause its death at an early stage.
lethal concentration (LC) where death is the criterion of toxicity, the results of toxicity tests are expressed as a number (LC50, LC70) which indicates the percent­age of test organisms killed at a particular concentration over a given exposure time,
e.g. the 48-hour LC70 is the concentration of a toxic material that kills 70% of the test organisms in 48 hours.
lethal dose (LD) dose of a toxic chemical or of a pathogen that kills all the animals in a test sample within a certain time. cf. median lethal dose.
lethality n. (1) the capacity to cause death;
(2) ratio of fatal cases to the total number of cases affected by a disease or other harmful agent.
lethal synthesis the synthesis in vivo of a metabolic poison from a substance that is not itself toxic.
leuc-see also leuk­
leucine (Leu, L) n. a-amino isocaproic acid, an amino acid with a non-polar hydro­carbon side chain, a constituent of protein and essential in human and animal diet.
leucine-rich repeat (LRR) protein motif present in the extracellular domains of many plant transmembrane serine/ threonine kinases.
leucine zipper DNA-binding protein struc­ture in which two a-helices containing leucine are held together by formation of a coiled-coil in one part, while the two separate arms grip the DNA. It is found in many proteins involved in gene regulation.
leucism n. presence of white plumage or fur in animals with pigmented eyes and skin. cf. albinism.
leuco-pre.x derived from Gk leukos, white.
leucoanthocyanidins n.plu. a group of colourless .avonoids.
leucocarpous a. with white fruit.
leucocidin leukocidin q.v.
leucocyte alt. spelling of leukocyte.
leucoplast(id) n. colourless plastid, such as the starch-containing amyloplast, in cells of plant epidermis and internal tissues.
leucopterin n. white pterin pigment of cabbage white butter.ies (Pieris) and other Lepidoptera and wasps, and which can be reduced to xanthopterin. see also isoxanthopterin.
leucosin n. storage polysaccharide forming whitish granules in some yellow-brown algae.
leukaemia n. malignant disorder of white blood cells in which precursors proliferate and fail to differentiate.
leukaemogenesis n. the generation of a leukaemia. leukemia alt. spelling of leukaemia.
leukin n. basic polypeptide extracted from leukocytes and active against Gram-positive bacteria.
leuko-see also leuco­
leukoblast myeloblast q.v.
leukocidin n. protein toxin that lyses white blood cells, produced by bacteria such as Staphylococcus aureus and Streptococcus pyogenes.
leukocytes n.plu. the colourless cells of the blood, commonly known as white blood cells. They consist of the basophils, eosinophils, neutrophils, lymphocytes and monocytes. All derive from a common progenitor in bone marrow and are in­volved in protecting the body from infec­tion. Basophils, eosinophils and neutrophils are known collectively as granulocytes or polymorphonuclear leukocytes. Monocytes become macrophages when they enter tissues.
leukocytolysis n. breakdown or disintegra­tion of white blood cells. alt. leukolysis.
leukocytosis leukosis q.v.
leukopenia n. reduction in the number of circulating white blood cells, characteristic of many diseases.
leukopoiesis n. generation of white blood cells.
leukosis n. an increase in the numbers of circulating white blood cells. alt. leukocytosis.
leukotrienes n.plu. class of compounds derived from arachidonic acid and released from mast cells in local in.ammatory reactions, formerly known as slow-reacting substance of anaphylaxis (SRS-A). They cause smooth muscle contraction (leukotriene E4) and attract polymorpho­nuclear leukocytes and eosinophils to sites of injury or infection (leukotriene B4).
levan n. any polysaccharide made up of fructose units.
levator n. a muscle serving to raise an organ or part.
levigate a. made smooth, polished.
ley n. temporary agricultural grassland, which is sown and used as a crop.
Leydig cells cells in interstitial tissue of testis which secrete testosterone.
Leydig’s organs minute organs on antennae of arthropods, possibly chemoreceptors.
L-form n. stage of certain mycoplasmas in which they will pass through the normal bacterial .lter and which consists of spe­cialized reproductive bodies, produced in extreme conditions.
LH luteinizing hormone q.v.
LHC light-harvesting complex q.v.
LHCP light-harvesting chlorophyll-protein. see photosynthetic unit.
LHRH luteinizing hormone-releasing hor­mone q.v.
liana n. any woody climbing plant of trop­ical and semitropical forests.
Lias n. marine and estuarine deposits of the Jurassic period, containing remains of fossil cycads, insects, ammonites and saurians. a. Liassic.
libriform a. appl. woody .bres with thick walls and simple pits.
Librium trade name for one of the com­monly used benzodiazepines q.v.
lice plu. of louse.
lichen n. a composite organism formed from the symbiotic association of certain basidiomycete or ascomycete fungi and a green alga or a cyanobacterium. This forms a simple thallus, found e.g. encrusting rocks and tree trunks.
lichen acids diverse secondary metabolites produced by lichen fungi and which con­tribute to weathering of rocks on which the lichens grow.
lichenase n. enzyme that breaks down lichenin to glucose, and so digests lichens. It is found in the gut of reindeer and cari­bou and some gastropods. EC 3.2.1.6, r.n. endo-1,3(4)-ß-d-glucanase.
lichenicole, lichenicolous a. living or growing on lichens.
lichenin n. glucose polysaccharide pre­sent in the walls of lichen fungi, and also in some seeds, esp. oats, and which is hydrolysed by the enzyme lichenase to glucose.
lichenization n. (1) production of a lichen by alga and fungus; (2) spreading or coat­ing of lichens over a substrate; (3) effect of lichens on their substrates.
lichenoid a. resembling a lichen.
lichenology n. the study of lichens.
lichenometry n. estimation of lichen growth to measure minimum time elapsed since lichens .rst developed on the substrate in
question.
lichen starch lichenin q.v.
Lieberkühn’s crypts crypts of Lieberkühn
q.v.
Liebig’s law (1) the nutrient least plentiful in proportion to the requirements of plants limits their growth; (2) law of the min­imum q.v.
lienal a. pert. spleen.
lienculus n. an accessory spleen.
lienogastric a. pert. spleen and stomach, appl. artery supplying parts of spleen and parts of stomach and pancreas.
life n. living organisms can be distinguished from other complex physicochemical sys­tems by their storage and transmission of molecular information in the form of nucleic acids, their possession of enzyme catalysts, their energy relations with the environment and their internal energy conversion processes (e.g. photosynthesis, respiration and other enzyme-catalysed metabolic activities), their ability to grow and reproduce, and their ability to respond to stimuli (irritability). Viruses, which sat­isfy only some of these criteria, are also generally considered as part of the living world. see also origin of life.
life-cycle the various stages an individual organism passes through from its origin to maturity and reproduction.
life-cycle assessment type of analysis used in environmental cost–bene.t ana­lysis, which can be implemented in various ways. It essentially calculates the environ­mental impact of a product or process throughout its lifetime from creation to waste, taking into account also any hazards and improvements.
life expectancy see average life expectancy.
life form the typical adult form of a species.
life tables demographic data required to
calculate, e.g., the intrinsic rate of increase of a population. They comprise the survi­vorship schedule, which gives the number of individuals surviving to each particular age, and the fertility schedule, which gives the average number of female offspring that will be produced by a single female at each particular age. From these the net re­productive rate, R0, which is the average number of female offspring produced by each female during her lifetime, can be calculated. The intrinsic rate of increase, r, of the population can be computed from survivorship and fertility schedules using the Euler–Lotka equation.
ligament n. (1) strong .brous band of tissue connecting two or more movable bones or cartilages; (2) band of elastic tissue forming the hinge of a bivalve shell.
ligand n. any molecule that binds speci.c­ally to another molecule. Examples are a hormone binding to its receptor, an inhi­bitor binding to an enzyme, oxygen binding to haemoglobin, an antigen binding to an antibody.
ligand-gated appl. ion channels in cell membranes which are stimulated to open or close by the binding of a particular molecule (the ligand).
ligase n. any of a class of enzymes that catalyse the joining together of two mol­ecules coupled with the breakdown of a pyrophosphate bond in ATP or a similar nucleoside triphosphate. They include the synthetases, carboxylases, and the DNA and RNA ligases. EC 6.
ligation n. of DNA, the joining of two molecules of DNA end to end by the enzyme DNA ligase to form a continuous DNA. The enzyme catalyses the forma­tion of a phosphodiester bond in the sugar– phosphate backbone.
light chain (L chain) (1) the smaller of the two types of polypeptide chain in an immunoglobulin molecule. In each mol­ecule there are two identical light chains;
(2) in myosin, any of four polypeptide chains attached to the globular heads of the molecule.
light-harvesting complex (LHC) complex of chlorophyll (and other pigments) and protein that collects light energy and passes it to the photosynthetic reaction centre. In green plants and green algae there are two light-harvesting complexes, one asso­ciated with photosystem I and one with photosystem II.
light microscopy type of microscopy that uses visible light and optical lenses to make the magni.ed image, the instru­ment used being known as a light micro­scope or optical microscope. alt. optical microscopy. cf. electron microscopy.
light reaction in photosynthesis, reactions occurring in the thylakoid membranes of chloroplasts, in which light energy drives the synthesis of NADP and ATP. cf. dark reaction.
ligneous a. woody, or resembling wood in structure.
lignescent a. developing the character of woody tissue.
lignicole, lignicolous a. growing or living on or in wood.
ligni.cation n. (1) wood formation; (2) the thickening of plant cell walls by deposition of lignin, which occurs in both primary and secondary walls.
lignin n. hard material found in walls of cells of xylem and sclerenchyma .bres in plants and which stiffens the cell wall. It is a very variable cross-linked polymer of phenylpropane units such as coniferyl alco­hol (guaiacyl), sinapyl alcohol (syringyl) or hydroxycinnamyl alcohol.
lignivorous a. eating wood, appl. various insects.
lignocellulose n. lignin and cellulose com­bined, a constituent of woody tissue.
lignolytic a. lignin-degrading.
lignosa n. vegetation made up of woody plants.
ligula a. band of white nerve .bres in dorsal wall of 4th ventricle of brain.
ligula, ligule n. (1) thin .attened outgrowth at junction of leaf blade and leaf sheath or petiole; (2) small scale on upper surface of leaf base in some club mosses and quillworts; (3) a strap-shaped corolla.
ligular a. tongue-shaped.
ligulate a. (1) having or pert. ligules;
(2)
having a strap-shaped corolla, like a ray .oret of .owers of the Compositae;

(3)
appl. .owerhead of strap-shaped .orets. liguliferous a. having ligulate .owers only. Ligustrales Oleales q.v. Liliales n. order of monocot plants, growing
from rhizomes or bulbs, mostly herbace­ous, and including the families Liliaceae (lily), Agavaceae (agave), Alliaceae (onion), Amaryllidaceae (daffodil), Dioscoreaceae (yam) and others. alt. Lilii.orae.
Liliopsida n. in some plant classi.cations the name for the class containing the monocotyledons.
limacel(le) n. concealed vestigial shell of slugs.
limaciform a. (1) slug-shaped; (2) like a slug.
limacine a. pert. slugs.
limb n. (1) arm, leg or wing in vertebrates;
(2) expanded part of calyx or corolla, the base of which is tubular; (3) a branch of a tree.
limbate a. (1) with a border; (2) bordered and having a differently coloured edge.
limb bud small protuberance on side of vertebrate embryo from which an arm, leg or wing develops.
limbic a. bordering.
limbic system the brain regions involved in emotions, learning and memory, con­sisting of (in each hemisphere) olfactory bulb, cingulate cortex, thalamus, hip­pocampus, fornix, mamillary body, and amygdala.
limbus n. (1) any border if distinctly marked off by colour or structure; (2) transitional zone between cornea and sclera.
limicolous a. living in mud.
liminal a. pert. a threshold, appl. minimal stimulus or quantitative difference in stimulus that is perceptible. cf. subliminal.
limit dextrins branched oligosaccharide fragments formed as one of the .nal prod­ucts of starch hydrolysis by a-amylase.
limiting factor any single factor (e.g. nutrient, temperature, space) that limits e.g. a biochemical process, the growth of an organism, or its abundance or distribution.
limiting membrane layer of connective tissue behind the vertebrate retina.
limivorous a. mud-eating, appl. certain aquatic animals.
limnetic a. (1) living in, or pert., marshes or lakes; (2) living in open water; (3) appl. zone of deep water between surface and compensation depth (depth at which photo­synthesis cannot be supported owing to insuf.cient light).
limnium n. a lake community.
limnobiology n. the study of life in stand­ing waters, i.e. ponds, marshes, lakes.
limnobios n. freshwater plants and animals collectively.
limnobiotic a. living in freshwater marshes.
limnology n. the study of the biological
and other aspects of standing waters.
limnophilous a. living in freshwater marshes.
limnophyte n. a pond plant.
limnoplankton n. the .oating microscopic life in freshwater lakes, ponds and marshes.
Limulus a genus of horseshoe crab q.v.
Limulus assay highly sensitive assay for bacterial endotoxin, used in the prepara­tion of pharmaceuticals, which employs extracts of amoebocytes of the horseshoe crab, Limulus. Endotoxin forms a precipit­ate with the extract.
lincomycin n. antibiotic produced by act­inomycetes and which inhibits bacterial protein synthesis.
LINE long interspersed element, intermediate-repeat retrotransposon-like DNA sequence found in mammalian genomes, exempli.ed by the mammalian L1 elements. LINEs are 6–7 kb long and are present in many thousands of copies (~600,000 copies in the human genome). see also SINE.
linea n. a line-like structure or mark.
lineage n. organisms allied by common descent. see also cell lineage.
lineage group group of species allied by descent from a common ancestor.
linear a. appl. leaves, the long narrow leaves which are characteristic of monocotyledons.
linear-ensate a. between linear and sword-shaped, appl. leaves.
linear growth type of growth in which the amount increases by the same amount over each set period (e.g. a year). cf. exponen­tial growth.
linear-lanceolate a. between linear and lanceolate in shape, appl. leaves.
linear-oblong a. between linear and oblong in shape, appl. leaves.
lineolate a. marked by .ne lines or striae.
line transect the recording of types and numbers of plants along a measured line.
Lineweaver–Burk plot a way of plotting the course of an enzymatic reaction in which reciprocals of the velocity of the reaction and of the substrate concentration are plotted against each other to give a straight line.
lingua n. a tongue or tongue-like structure.
lingual a. pert. tongue, appl. e.g. artery, nerve, vein.
linguiform a. tongue-shaped.
lingula n. small tongue-like projection of bone or other tissue.
lingulate a. shaped like a small broad tongue. cf. ligulate.
linin n. (1) protein of .ax seed; (2) a bitter purgative substance obtained from purg­ing .ax.
linkage n.(genet.) the case when a set of alleles of different genes from one of the parents are inherited together, in opposi­tion to Mendel’s law of independent assort­ment. It is usually due to the genes being close together on the same chromosome and thus no recombination occurring be­tween them.
linkage disequilibrium condition in which certain alleles at two linked loci are non-randomly associated with each other. This is either because of very close physical proximity which virtually precludes recom­bination between the two loci, or because the allele combination is under some form of selective pressure.
linkage group the genes carried on any one chromosome.
linkage map genetic map q.v.
linker n. small stretch of synthetic DNA, usually containing a restriction site, which may be used to connect two different DNA molecules to form a recombinant DNA. It allows the DNA of interest to be easily recovered by treatment with the appropri­ate restriction enzyme after cloning.
linking number number of turns of one strand of a closed circular DNA about the other, L = T (degree of twisting of the double helix) + W (extent of supertwisting).
Linnean a. pert. or designating the system of binomial nomenclature and classi.cation established by the 18th century Swedish biologist Carl von Linné or Linnaeus.
linneon n. a taxonomic species distinguished on purely morphological grounds, esp. one of the large species described by Linnaeus or other early naturalists.
linoleic acid common 24-carbon unsatur­ated fatty acid, essential for growth in mammals and necessary in the diet.
linolenic acid common 20-carbon unsatur­ated fatty acid, necessary for growth in mammals but not essential in the diet as it can be synthesized from linoleic acid.
lipase n. (1) any of a group of widely distributed enzymes (produced esp. by pancreas) hydrolysing triacylglycerols to diacylglycerols plus a fatty acid anion. EC 3.1.1.3, r.n. triacylglycerol lipase, and EC 3.1.1.34, r.n. lipoprotein lipase; (2) some­times used for any enzyme that breaks down fats.
lipid n. any of a diverse class of compounds found in all living cells, insoluble in water but soluble in organic solvents such as ether, acetone and chloroform. Lipids include fats, oils, triacylglycerols, fatty acids, glycolipids, phospholipids and ster­oids, some lipids being essential com­ponents of biological membranes, others acting as energy stores and fuel molecules for cells.
lipid anchor any of various types of lipid that are covalently linked post­translationally to some proteins and serve to attach them to the cytoplasmic side of the plasma membrane.
lipidation n. the covalent attachment of a fatty acid to a protein.
lipid bilayer double layer of molecules formed by phospholipids in an aqueous environment. Each molecule is oriented with the hydrophilic group on the outside and the hydrophobic group to the interior of the layer. This is the basic structure of biological membranes in most organisms. alt. bilayer, bimolecular sheet. cf. lipid monolayer. see Fig. 26 (p. 386).
lipid bodies lipid storage structures found in oil-rich plant seeds, composed of a large droplet of triacylglycerol surrounded by a single-layered membrane. alt. oil bodies.
lipid droplet large droplet of (usually) triacylglycerols found in cells specialized for stout storage.
lipid-exchange protein lipid-transfer pro­tein q.v.
lipid monolayer (1) type of membrane structure present in the Archaea, in which diglycerol tetraethers span the membrane as single molecules with a hydrophilic glycerol moiety on each side of the mem­brane; (2) membrane formed from a single layer of phospholipid or fatty acid, found
e.g. around stout droplets in cells.
lipid raft an area of plasma membrane with a lipid composition distinct from that of the surrounding membrane, enriched in sphingolipids, cholesterol and membrane proteins.
lipid-transfer protein, lipid-transport protein cytosolic protein that carries membrane lipids from their site of syn­thesis at the endoplasmic reticulum to chloroplast and mitochondrial membranes.
lipoamide n. enzyme cofactor derived from lipoic acid, and which is an acyl group carrier esp. vital in carbohydrate metabolism.
lipoate see lipoic acid.
lipochroic a. with pigmented oil droplets.
lipochrome n. a stout-soluble pigment.
lipocyte n. cell specialized for lipid produc­tion and storage. alt. (in animals) adipocyte, stout cell.
lipofection n. transfer of material into a cell by enclosing it in liposomes, which fuse with the cell membrane.
lipofuscin granules residual orange .uorescent bodies seen in ageing cells, derived from lysosomes. alt. age pigments.
lipogenesis n. synthesis of fatty acids and other lipids.
lipogenous a. stout-producing.
lipoglycan n. long-chain heteropolysac­charide covalently linked to a membrane lipid. Found in the cell membrane of many mycoplasmas.
lipoic acid, lipoate n. 1,2-dithiolane-3­valeric acid, a compound composed of a fatty acid (valeric acid) and disulphide. It is required for carbohydrate metabolism as a precursor for the enzyme cofactor lipoamide.
lipoid (1) a. resembling a fatty substance;
(2) n. a substance that is not a right lipid but resembles one in certain properties and is extracted in organic solvents. Examples are sterols and steroids.
lipolysis n. (1) enzymatic breakdown of fats, as during digestion; (2) breakdown of triacylglycerols in adipose cells during mobilization of food reserves.
lipolytic a. (1) pert. lipolysis; (2) appl. enzymes capable of breaking down stout;
(3) appl. hormone: lipotropin q.v. lipopalingenesis n. loss of a developmental stage or stages during evolution.
lipopexia n. deposition and storage of fats in tissues.
lipopolysaccharide (LPS) n. usually refers to the complex compound of lipid and polysaccharide that is the main con­stituent of the outer membrane of Gram-negative bacteria. It produces fever and other symptoms in humans and other mammals, and by stimulating the systemic overproduction of the cytokine TNF-a it contributes to the septic shock that sometimes follows a blood infection. alt. endotoxin.
lipopolysaccharide layer outer membrane of Gram-negative bacteria.
lipoprotein n. complex of lipid and protein.
liposome n. (1) arti.cially constructed sphere of lipid bilayer enclosing an aque­ous compartment. Liposomes are used in experimental biology to study properties of biological membranes and clinically as a possible means of delivering drugs to cells more ef.ciently, alt. lipid vesicle;
(2) fatty droplet in cytoplasm, esp. of an egg cell.
lipoteichoic acid any of a class of glycerol-containing teichoic acids covalently bound to membrane lipids in the cell walls of Gram-positive bacteria.
lipotropic a. concerned with the mobiliza­tion of storage lipids and their breakdown into fatty acids and triacylglycerols.
lipotropin (LPH) n. in mammals, either of two peptide hormones produced by the anterior pituitary gland which stimulate lipolysis.
lipovitellin n. lipoprotein in amphibian egg yolk.
lipoxenous a. leaving the host before devel­opment is complete, appl. parasites.
lipoxidase lipoxygenase q.v.
lipoxin n. member of a class of molecules produced from arachidonic acid in cells in response to injury or during in.am­mation. They can cause contraction of smooth muscle, vasodilation, chemotaxis, hyper.ltration in the kidney, and inhibi­tion of natural killer cell activity.
lipoxygenases n.plu. enzymes that cata­lyse the addition of a molecule of oxygen to the double bonds of certain unsaturated fatty acids and their derivatives. They are involved inter alia in the synthesis of some eicosanoid chemical mediators
(e.g. leukotrienes and lipoxins).
liquor folliculi .uid surrounding the ovum in a Graa.an follicle.
lirella n. type of long apothecium in some lichens.
Lissamphibia n. in some classi.cations a subclass of amphibians containing all extant species and divided into three orders: Salientia (Anura), Urodela and Apoda.
lissencephalous a. having few or no con­volutions on the surface of the brain.
listeriosis n. disease caused by the bac­terium Listeria monocytogenes, which is an opportunistic pathogen and can cause disease if ingested in contaminated food.
lithite n. (1) calcareous secretion in ear;
(2) statolith q.v. lithocarp n. a fossil fruit. lithocyst n. (1) minute sac or groove
containing statoliths, found in many in­vertebrates; (2) enlarged cells of plant epidermis, in which cystoliths are formed.
lithocyte n. large cell in hydrozoan statocyst (gravity detector) containing a concretion of calcium salts, whose movement under gravity is detected by an adjacent sensory cell.
lithodomous a. living in holes or clefts in rock.
lithogenous a. rock-forming or rock-building, as the reef-building corals.
lithophagous a. (1) stone-eating, as some birds; (2) rock-burrowing, as some mol­luscs and sea urchins.
lithophyll n. fossil leaf or leaf impression.
lithophyte n. plant growing on rocky ground.
lithosere n. plant succession originating on rock surfaces.
lithosol n. soil that develops at high alti­tudes on resistant parent materials that withstand weathering. It is a humus-rich, shallow, stony soil.
lithosphere n. (1) the Earth’s crust; (2) the non-living, non-organic part of the en­vironment, such as rocks or the mineral fraction of soil.
lithotomous a. stone-dull, as certain molluscs. lithotroph chemolithotroph q.v. a. lithotrophic. litter n. (1) (bot.) partly decomposed plant residues on the surface of soil; (2) (zool.)
offspring produced at a single multiple birth.

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