KAMUS BIOLOGY 3

APOCRINE GLAND. Type of gland in which only the apical part of the cell from which the secretion is released breaks down during secretion, e.g.mammarygland. COmpareHOLOCRINEGLAND,MEROCRINE

GLAND.

APODA (GYMNOPHIONA). Caecilians. Order of limbless burrowing amphibians

with small eyes and, sometimes, a few scales buried in the

dermis of the skin, and a pair of tentacle-like structures in grooves

above the maxillae.

APOENZYME. The protein component of a holoenzyme (enzyme-cofactor complex) when the COF A c TOR is removed. It is catalytically inactive by itself.

APOGAMY. See APoMIxIs.

AROGEOTROPIC. Growth of roots \force of gravity (i.e. into the air). away from the earth and from the

APOMICT. Plant produced by APOMIXIS.

APOMIXIS. Most common in botanical contexts. (1) AGAMOSPERMY, reproduction which has the superficial appearance of ordinary sexual reproduction (amphimixis) but occurs. without fertilization and/or meiosis. Affords the advantages of the seed -habit (dispersal, and survival through’unfavourable conditions) without risks in achieving pollination. Often genetically equivalent to asexual reproduction. See PARTHENOGENESIS. (2) Vegetative apomixis; ASEXUAL methods of propagation such as by rhizomes, stolons, runners and bulbils.

APOMORPHOUS. In evolution, of a character derived as a noveltv from pre-existing (plesiomorphous) character. The two form a homologous pair of characters, termed an evolutionary transformation seriesin CLADISTICS.SeeSYkAPOMORPHY.

APOPLAST movemenfof substances in the cell walls is termed apoplastic movement or trans,” porIt. Compare SvMPLAsT.

APOSEMATIC, Colour, sound, behaviour or .other quality /advertising noxious or otherwise potentially harmful qualities of an animal. See MIMICRY.

APOSPORY. SEE APOMIXIS, PARTHENOGENESIS.

APOTHECIUM. Cup- or saucer-shaped fruit body of. certain Ascomycotina and lichens; lined with a hymenium of asci and para physes. Sessile or stalked, often brightly coloured; varying from a few mm to more than 40 cm across.

APPENDAGE. A functional projection from an animal surface; termed paired appendages if bilaterally symmetrical. Two such pairs (e.g. limbs, fins) generally occur in gnathostome vertebrates. Primitively

one pair per segment in arthropods (walking legs, mouthparts, antennae) and polychaetes (parapodia).

APPENDIX, VERMIFORM. Small diverticulum of human caecum, of many other primates, and of rodents, containing lymphoid tissue. Not a vestigial structure, contrary to common belief.

APPETITIVE BEHAVIOUR. Behaviour (e.g. locomotory activity) variable with circumstances, increasing the chances of an animal satisfying some need (e.g. for food, nesting material) usually through a more

stereotyped see ACT, such as eating. To this extent it -is goal-oriented.

APPOSITION. (Bot.) Growth in thickness of cell walls by successive deposition of material, layer upon layer. Compare ItiTUSSUSCtiPTION.

APTEROUS. Wingless; either of insects which are polymorphic for winged and wingless forms, e.g. aphids, many social insects; or of insects which have discarded wings, as do some ants and termites; or

of primitively wingless (apterygotan) insects.

APTERYGOTA (AMETABOLA). Subclass of primitively wingless insects. Comprises orders Thysanura (bristletails, silverfish), Collembola (springtails), Protura and Diplura. Probably a polyphyletic assemblage. Some abdominal segments in members of all four orders have small paired lat era1 appendages, another primitive characteristic. Metamorphosis slight or absent. See PTERYGOTA.

AQUEOUS HUMOUR. Fluid filling the space between cornea and VITREOUS HUMOUR of vertebrate EYE. The iris and lens lie in it. Much like cerebrospinal fluid in composition. Continuously secreted by ciliary

body, and drained by -canal of Schlemm into blood. Much . less viscous than vitreous humour. Links circulatory system to lens and cornea, neither having blood vessels for optical reasons; also maintains intraocular pressure .

ARACHNIDA. Class of chelicerate arthropods. Most living forms terrestrial, using lung books (scorpions), lung books and tracheae (spiders), tracheae alone (e.g. pseudoscorpions, larger mites), or just the body surface (smaller mites) for gaseous exchange. Usually there is TAGMOSIS into a prosoma of eight adult segments anteriorly and an opisthosoma of 13 segments posteriorly. No head/thorax distinction. Prosoma lacks antennae and mandibles; first pair of appendages clawed and prehensile chelicerae; second pair (pedipalps) may be prehensile and sensory, copulatory or stridulatory devices. Remaining four pairs of prosoma1 appendages are legs. Bases (gnathobases) of

ARACHI’WID MEMBRANE second atid subsequent pairs of-appendages are often modified for crushing and ‘chewing’ (in absence of true jaws). Includes orders: Atari (mites and ticks), Araneae (spiders), Seorpiones (scorpions), Pseudoscorpiones (false scorpions), Palpigrada (palpigrades), Solifugae (solfugids) and Opiliones (harvestmen). Xiphosura (king crabs), and the predatory and extinct Eurypterida are usuaily placed as ssubclassesofthe M E R O S T O M A T A .

ARACHNOID MEMBRANE. One of the MENINGES around vertebrate spinal cord and brain.

ARANEAE (ARANEII)A)..O~~~~ of ARACHNIDA. Spiders. Abdomen (opisthosoma) almost always v)li$hout any trace of segmentation and joined to prosoma (cbphalothorax) by ‘waist’; silk produced from two to four spinning glands (spinnerets); pedipalps in Yale modified as intromittant organs for copulation; ends of chelicerae modified as poisonous rangs.

ARCHAEAN (ARCHAEOZOIC). Geological division preceding PROTOZOIC; earlier than about 2600 Myr BP

ARCHAEBACTERIA. Ancient lineage of bacteria distinct from other bacteria (eubacteria) and from eukarybtes, Many live in hot acidic conditims (Le. they are thermophilic and acidophilic), growing best

at temperatures approaching 100°C. Formerly in two groups, either aerobic (Sulfolobales) br anaerobic (Thermoproteales), facultative anaerobic forms are now known. Many unusual biochemical character-

. istics including possession of a novel 16S-like ribosonial RNA component in the small ribosome subunit, which with their peculiar membrane composition indicates that there may be a deep divide among prokaryotes between archaebacteri? and eubacteria. Halophiles, methanogens and sulphur-dependent thermophiles occur.

ARCHAEOPTERYX. Most ancient recognized fossil bird (late Jurassic, 150-145 Myr BP). Exhibits mixture of reptilian and bird-like chara& ters, having feathered wings and tail (impressions clear in limestone)

atid furcula (fused clavicles and.interclavicles); but with teeth, bony tail, and claws on three digits of fore-limbs.. Only known representative of Subclass Archaeornithes of Class AVES.

ARCHE~ZONIOPHORE. In some liverworts, a stalk bearing archegonia.

ARCHEGONNJM. ‘Female’ sex organ of liverworts, mosses, ferns and related plants, and of most gymnophytes. Multicellular, with neck composed of one or more tiers of cells, and swollen base (venter)

containing egg-cell.

ARCHENTERON. Cavity within early embryo (at gastrula stage) ofmany animals, communicating with exterior by BLASTOPORE.Formed by invagination of mesoderm and endoderm cells at gastrulation;becomes the gut cavity. /’

ARci4ESPoRIUM. Cells or cell from which spores e,g. in deGv1eloping pollen sac, fern sporangium.a r e ultimately derived,

ARCHOSAURS; ‘Ruling reptiles’; the Subclass Archosauria. -Originatingwith thecodonts in the Triassic, it includes the bipedal carnivorousdinosaurs (sauriscians) and the bird-like dinosaurs (ornithiscians).Crocodiles and alligators are living representatives. Birds are descendants.See DINOSAUR. -

ARGINAS~. Enzyme catalysing hydrolysis of arginine’ to ornithine andurea in urea cycle (see uRE.4); in mammals occurs in liver cell ‘cytosol.

ARIL. Accessory seed covering, often formed~‘from an outgrowth” at -the base of the OVULE (e.g. yew); often brightly coloured, aidingdispersal by attracting animals that eat it and carry seed away from _

ARISTA. see AWN.

ARMS RAKE. Term sometimes used to express the dialectical changesin selection pressure that occur when regular, often unavoidable,conflicts of interest between two or more ‘ways of life’ .favour ,anadaptation for one party which creates a fresh ‘challenge’ for theother to respond by adapting to. Such conflicts are common: predator/prey; parasite/host; parent/offspring and ‘male/female. It hasbeen argued that selection will be the stronger where one party hasmore to lose by ‘not evolving’ and minimizing the probability oflosing the conflict. Consequence to a prey organism of losing apredator/prey conflict ig probably more serious than to a predator onany occasion. Much depends on how likely such conflict encountersare as to whether selection will favour whatever “costs’ may beinvolved in evolving a ploy to avoid or win the conflict. Conflicts arebest generalized as conflicts of ‘ways of life’, or strategies, rather-thanas conflicts between individuals, per se. Some conflicts of interestmay resolve in favour of one of the parties through inability of thegenetic system to ‘represent’ the other in the a’rms race. See

ALTRUISM, COEVOLUTION.

AROUSAL. General causal term (and factor) invoked- to account fort&e fact that animals are variably alert and responsive to potentialstimuli. There may be a general $leepjng/waking’ difference; but it isless clear that there is a continuum of levels of awareness or responsivenessduring either of these st,ates. The phenomenology ofarousal may be correlated with neural activity in the RETI cu LA R F OR M A TI o N of the medulla, hypothalamus and cortex of the verte-I brate brain. Physiological processes which facilitate certain behavioursinclude hormone release and endogenous rhythms. Both. may then be said to be arousal mechanisms, or to affect motivation.

ARP HENOTOKY. S~~MALEHAPLOIDY.

ARTERY. Any relatively large blood vessel carrying blobd (not necessarily oxygenated) from the heart towards the tissues. Vertebrate arterieshave thick elastic walls of smooth muscle and connective tissue(larger ones have capillaries in them), damping blood pressurechanges. Their innermost layer is endothelium, as with all vertebrate blood vessels. They divide repeatedly to form arterioles.

ARTHROPODA. The largest phylum iri the animal kingdom in terms of both number of taxa and (protozoans probably notwithstanding) biomass. Bilaterally symmetrical and metamerically segmented coelomates, with appendages on some or all segments (somites). A chitinous cuticle provides the exoskeleton, flexible to provide joints. Haemocoele is the main body cavity (coelom reduced). They lack true nephridia and cilia (onychophorans have the latter); with annelidlikecentral nervous system and one or more pairs of coelomoducts acting as gonoducts or excretory ducts. Taxonomy varies. Thirteen classes are widely recognized, including: Pnychophora (peripatids), Myriapoda (centipedes and millipedes), Insecta (insects), Trilobita(trilobites, extinct), Merostomata (king, or horseshoe crabs Bnd extincteurypterids), Arachnida (scorpions, spiders, harvestmen, solfugids, mites and ticks), Crustacea (crabs, prawns and shrimps, water-fleas). The extent and patterning of TAGMOSIS reflects the locomotory method, while appendages have proved marvelously adaptable and account in large measure fo: the success of the group. There appear to be ,three major evolutionary lineages: the Onychophora-Myriapoda-&secta group, the Merostomata- Arachnida-Trilobita group, and then Crustacea. The phylum may be regarded as a GRAPE, a polyphyletic origin not yet discounted.

ARTICULAR CARTILAGE., Cartilage providing the articulating surfaces of vertebrate joints.

ARTIFICIAL INSEMINATION. Artificial injection of semen into female reproductive tract. Much used in animal breeding.

ARTIFICIAL KEY. Any, JDENTIFICATION KEY not baked tipdli‘evblutionary relationships but rather upon any convenient distinguishing characters. See NATURAL CLASSIFICATION.

ARTIFICIAL SELECTION. Directional selection imposed by humans, deliberately or otherwise, upon wild or domesticated organisms. Crop plants originated in many cases from such deliberate crosses, sometimes involving one or more polyploid stocks. Procedures employed in harvesting these crops commonly involve unintentional (but still artificial) selection upon plants growing with the crops, favduring weed properties (see WEEDS). The phenomenon was well known to DAR w IN and examples of conscious human selection provided an analogy through which his readers could grasp the theory of NATURALSELECTION. RNA CAPPtNG 500 and in others (retroviruses) it is reverse-transcribed into DNA. tRNA molecules fold back upon themselves by complementary base-pairing to form double-stranded ‘stems’ and single-stranded ‘loops’. A loop at one end bears a specific nucleotide triplet (the anti&don) while the 3’-end of the molecule carries a t RNA-specific amino acid - both essential for protein synthesis to proceed by means df a GENETIC CODE. Ribosomal RNA subunits associate with protein molecules to form RIBOSOMES. All tRNA and Rrna molecule types are encoded by DN.A (see GENE), and there are many more of these molecules per cell than there are of mRNA. Some RNA molecules have catalytic activity (see RNA PROCESSING, ORIGINOF LIFE,RIBOZYMES). release into the cytosol. This involves attach ment of a cap of 7- mkthylguanosine triphosphate to their Y-end; Ribosomes recognize this cai and commence translation at the A u G codon nearest to the cap, finishing at the first stop codon, ensuring that translation is Usually UIOIlOCktrOUiC. See CODON, RNA'PROCESSING, PROTEIN SYN~THESIS. RNA POLYMEIIASES. Enzymes producing RNA from ’ ribonucleoside &phosphates. Unlike DNA polymerases they do not require a polynucleotide primer. Three’ types occur in eukaiyotic cells, polymeraie I making&rge ribosomal R N As, polymerase II transcebing structural genes (introns and exons), palymerase III making smah R NAs such as tR-NAs and,“rRNAs. See RN+ PAOCESS,ING:R#A PROCESWNG. mRN A transcription . within- the nucleus produces R NAs of various sizes (heterogeneotis RNA, hnRNA) which are modified (processed) before passage to the cytosol for translation on ribosomes. The 5’-end of the molecule is first capped (see &N A ‘c A P PIN G) and then rhas a long poly-A M P sequence. bound to the 3’- end, which may facihtate the rest of ‘processing and passage’ to the cytosol. Major feature of nuclear processing is ‘the excision from hnRN A of non-coding IN TRON sequences. This is achieved by cutting these sections out using a PHOSPHOD~ESTEFASE, and thens PL I GIN G the transcript. This “may rejoin. one encoding region (exon) to another that is not its official nearest neighbour in the hnRNA. Alternatively, an exon may get cut out. This provides flexibility ineventual protein production and is important to I)mphocytes ingenerating ANTIBODY DIVERSITY. Eukaryotic ribosomal RNA is processed in the nucleus prior to -assembly into ribosomes. Both RNA processing and ‘gene splicing’ are involved’ in the production of antibody diversity by different mature B-cell clones. RNase (RIBONUCLEASE). Any of several enzymes which hydrolyse RNA by breaking their phosphodiester bonds: ROQ CELL. Highly light-sensitive secondary receptor of vertebrate

ASEXUAL. Of reproduction (or organisms) lacking all the following: meiosis, gamete production, fertilization (leading to genome or nuclear union), transfer of genetic material between individuals, and PARTHENOGENESIS. Includes,orissynonymous,with, VEGETATIVE R E P R 0 D u c T I 0 N. Often employed (with parthenogenesis) as a meansof rapidly increasing progeny output during a favourable period these having practically uniform genotype); hence common in internal parasites (see POLYEMBRYONY). The basis of natural cloning (artificially imposed in the propagation of plants by cuttings). May alternate with sexual phase in LIFE CYCLE (see ALTERNATION OF G E N E R A T I 0 N s). Some organisms (e.g. Amoeba, trypanosomes) are obligately asexual,, and this raises questions about the evolutionaryand ecological significance o

A-SITE. (Of ribosome) binding site on ribosome for charged (amino-acyl, hence A for acyl) t RNA molecule in PROTEIN SYNTHESIS.See P-SITE.

ASSIMILATION. Absorption of simple substances by an organism (i.e. across cell membranes) and their conversion into more complicated molecules tihich then become its constituents.

ASSOCIATION. (Of plants) climax plant community dominated by particular species and named according to them; e.g. oak-beech association of deciduous forest. Sometimes applied to very small natural units of vegetation. See CONS0 CIA TION.

ASSORTATIVE MATING (A. BREEDING). Non-random mating, involving selection of breeding partner, usually based on some aspect of its phenotype. This ‘choice’ (consciousness not implied) may be performed by either sex, and may be positively assortative (choice like self in some respect) or negatively assortative (disassortative, choice unlike self in some respect). Likely to have consequences for degree of inbreeding and maintenance of POLYMORPHISM. Some INCOMPATIBILITY mechanisms in plants are analogies. See SEXUAL SELECTION. ,

ASTER. A system of cytoplasmic striations radiating from the centriole and consisting of MI CROT UBULES; often conspicuous during cleavage of egg, or during fusion of nuclei at fertilization. Also probably present in many other animal cells during division. Absent from higher plants.

ASTEROIDEA. Class of ECHINODERMATA. Starfishes. Star-shaped;arms, containing projections of gut, not sharply marked off from central part of body; mouth downwards; suckered TUBE FEET; spines and pedicillariae. Carnivorous (some notoriously on oysters orcorals). I

ASTHMA. See ALLERGIC REACTION.

ASTROCYTE. One type of GLIAL CELL of central nervous system. Star-shaped, with numerous processes, they provide mechanical support for transmitting cells by twining round them and attaching them to their blood vessels. Different types are found in white and greymatter of the CNS.

ATHEROSCLEROSIS. See SCLEROSIS.

ATLAS. First VERTEBRA,% modified for articulation with. skull. Modified further in amniotes, which have freer head movement, than in amphibians. Consists of simple bony ring, while a peg (odontoid process) of thenext vertebra (the AXIS) projects forward into the ring (through which the spinal cord also runs). This peg represents part of the atlas (its centrum) which has become detached and fused,to the axis. Nodding the-head takes place at the skull-atlas joint; rotation of head at atlas-axis joint.

ATP. -Adenosine triphosphate. Adenyl nucleotide diphosphate. The common ‘energy currency’ of all cells, whose hydrolysis accompanies and powers most cellular activity, be it mechanical, osmotic or chemical. Its two terminal phosphate groups have a more negative s T A N D A R D F R E E E N ERG Y of hydrolysis than phosphate compounds below it on the thermodynamic scale (e.g. sugar phosphates), and less negative than those higher (e.g. phosphocreatine, phosphoenolpyruvate), -but this varies with intracellular concentrations of ATP, ADP and free phosphate as well as pH. A HIGH-ENERGY PHOSPHATE (see -Fig. 33), it tends to lose its terminal phosphate to substances / lower on the scale, provided an appropriate enzyme is present, and its ‘mid-position on the scale enables it to serve as a common intermediate in the bulk of enzyme-mediated phosphate-group transfers in cells. Its relationship with AD P and A M P may be summarized: ATP + H, 0 G= AMP + PPi (pyrophosphate) - 10 kcal mol-l ATP + AMP-ADP + ADP . A TP = AD P + Pi (inorganic phosphate) - 7.3 kcal mol-l The energy ValLleSZireSTANDARD FR.EEENER G Y changes a .t pH 7, standard temperature and pressure, at 25°C. Cells normally contain about ten times as much A T P as AD P and

AMP, but when metabolically active the drop in the ATP/(A DP+

AM P) ratio results in acceleration of GLY COLY SI s and aerobic respiration (see RESPIRATION), the signal being detected by ALLOSTE RIC enzymes in these pathways whose modulators (see ENZYME) are AT P, AD P or AM P. AT P is not a reservoir of chemical energy in the cell but rather a transmitter or carrier of it. The bulk of ATP in eukaryotic cells is provided by mitochondria, where these are present. Some extra-mitochondrial ATP is produced anaerobically in the cytosol, and chloroplasts produce it but do not export it. ATP hydrolysis is used to transfer energy when work is done in cells. \

ATPase activity is found in MYosrt$&.g. MUSCLE CONTRA”CEIN (e.g. ciIiary/flagellar beating). Membrane ion pumps (e.g. sodium and calcium pumps) and macromolecular syntheses of all kinds involve ATPase activity. Ultimately the energy source for ATP formation in the biosphere is solar energy trapped by autotrophs in photosynthesis - plus some lithotrophy. All heterotrophs depend upon respiratory oxidation of these organic compounds to power their own

ATP synthesis. ATP is, like the other common nucleoside triphosphates in cells (CTP, GTP, TT P, U TP), a substrate in nucleic acid synthesis, and its hydrolysis provides the energy needed to build the resulting AMP mondmer into the growing polynucleotide chain. These other triphosphates may participate in some other energy transfers; but ATP has by far the major role. See AMP, ADP, PHOSPHAGEN, BACTERIORHODOPSIN.

AT Pase. Enzyme bringing about either (i) orthophosphate (Pi) cleavage of ATP yielding A DP and inorganic phosphate, or (ii) . pyrophosphate (2Pi) cleavage of AT P to yield A M P and pyrophosphate. The latter provides a greater decrease in free energy and is involved tihere a ‘boost’ is needed for an enzyme reaction. ATPase activity is found in myosin and dynein molecules, chloroplast thylakoids and inner mitochondrial’ membranes (as A TP synthetase). SeeMITOCHONDRION, CHLOROPLAST, BACTERIORHODOPSIN. ’

ATRIUM. (1) Chamber, closed except for a small pore, surrounding gill slits of Amphioxus and urochordates. (2) A type of heart chamber of vertebrate chordates synonymous with ‘auricle’; receives blood from major vein and passes it to ventricle. Walls not as muscular as those of ventricle. Fishes have single atrium, but tetrapods, breathing mainly or entirely by lungs, have two: one (the left) receives oxygenated blood from lungs, the other (the right) receives deoxygenated blood from the body. Much of the blood flow though the atria is passive (see HEART c YCLE). Non-chordates may have anatria component of the heart, e.g. some polychaete worms and most molluscs, in which the term ‘auricle’ is sometimes preferred. (3) A space or cavity iATROPHY. Diminution in size of a structure, or in the amount of tissue of part of the body. Generally involves destruction of cells, and may be under genetic and hormonal control, as is frequently the case in metamorphosis. May also result from starvation. Compare HYPERTROPHY.

ATROPINE. ALKALOID inhibiting action of acetylcholine and parasympatheticomimeticdrugs; its application may double heart rate in man, and generally blocks the effects of MUSCARINE on effecters of the vagus nerve.

ATTENUATION. (Of pathogenic microorganisms) loss of virulence. May&;be,achieved by heat treatment. See VA c CINE. A~DI$IRY (OTC) CAPSULE. Part of vertebrate skull, enclosing au&tory ' , organ.

AUDITORYNERVE. See VESTIBI~LOCOCHLEARNERYE.

AUDITORY ORGAN. Sense organ detecting pressure waves in air(‘sound’), in vertebrates represented by the cochlea of the inner -ear,but the term often intended to include VESTIB~LAR APPARATUS detecting positional and vectorial information as well. See L ATERA L LINE SYSTEM.

‘AUERB#CHWPL~XUS. That part of the autonomic nervous system in vertebrates (mostly from the vagus nerve) lying between the two main muscle layers of the gut and controlling its peristaltic movements. ~

AURICLE. (Zool.) ’ (1) Often used synonymously with atria1 heart Chamber r (see ATRIUM). (2) External ear of vertebrates. (Bot.) Smalf\ ear- or claw-like appendage occurring one on each side at the bases of leaf-blades in certain plants.

AU~RAUAN -REGION. ZOOGEOGRAPHICAL REGION consisting ’ mainly of Australia, New Guinea and the Celebes; demarcated from south-eastAsiaby WALL.ACE'S LINE.

AUSTRALOPITHECINE. Member of genus Australopithecus, no- exl tin&; of the primate family Hominidae (see HOMINID). It appears to have been a long extant genus (roughly from 4-l Myr BP), with perhaps as many as five African species. A. ufarensis (4.0-3.0 Myr BP) appears to have been a conservative species near the common ancestry of later forms; A. aethiopicus and A. africanus were later contemporaries ’ (2.1-3 Myr BP) and possibly -sister species with A. afarensis as common ancestor; A. ro&stus and ;Q. boisei were later still (approx 1.2-2Myr B P) and shared several derived features (synapomorphies). aethiojiicw, A. robustus and A. boisei were all ‘robust’ australopithecines, with heavy skulls and facial features; A. africanus had more ‘gracile’ features and was possibly,ancestral to HOMO. Cranial capacities of typical australopithecines were from 400-600 cm3 (modern man averages 1400 cm3), and the animals had bipedal posture a,nd gait. More fossil material and analysis are needed for definitive assessment of genus relationships. All the fossil material comes from Africa.

AUTECOLOGY. Ecology of individual species, as opposed to communities (synecology).

AUTOANTIGEN. Molecular component of organism, normally regarded as ‘self’ by its immune system, but here recognized as ‘non-self’ and eliCiting,an AUTOIMMUNE REACTION.

AUTOCATALYST. Any molecule catalysing its own production. The more produced, the more catalyst there is for further production. Most likely, some such process was involved in the origin of prebiological systems which, once enclosed in a membrane, might be called ‘living’. The current process whereby nucleic acid codes for enzymes that decode and replicate it is, in a strong sense, autocatalytic. SeeORIGIN OF LIFE.

AUTOCHT~ONOUS. (1) Of soil microorganisms whose metabolism is relatively unaffected by increase in organic content of soil. See ZYMOGENOUS. (2) The earliest inhabitant or product of a region (aboriginal). In this sense contrasted with allochthonous (not native to a region).

, AUTOCLAVE. Widely-used equipment for heat-sterilization. Commonly air is either pumped out prior to introduction of steam, or simply replaced by steam as the apparatus is heated under pressure. Material being sterilized is usually heated at 121°C and 138-172 kNm pressure for 12 minutes, which destroys vegetative “bacteria, all bacterialspores and viruses; but these figures will vary with the size and nature of material.

AUTOECIOUS. Of rust fungi (Basidiomycotina) having different spore forms of the life cycle all produced on one host species, as in mint l-USt.COmpal-eHETEROECIOUS.F AUTOGAMY. Fusion of nuclei derived from the same zygote but from different meioses. Includes all forms of self-fertilization. See AUTOMIXIS.

AUTOGRAFT. Tissue grafted back onto the original donor. See ALLOGRA FT,ISOGRAFT,XENOGRAFT.

AUTOIMMUNE REACTION. Response by an organism’s immune system to molecules normally regarded as ‘self’ but which act as antigens. Quite often the thyroid gland, adrenal cortex or joints become damaged by the*action there of antibodies or sensitized lymphocytes ,%'eB-CELL,T-CELL.

AUTOLYSIS. (1) Self-dissolution that tissues undergo after death of their cells, or during metamorphosis or atrophy. Involves LY soso M E activity within cell. (2) Pro karyotic self-digestion, dependent upon enzymes of cell envelope.

AUTOMIXIS. Fusion of nuclei derived both from the same meiosis. See AUTOG AMY, from the same zygote and PARTHENOGENESIS.

AUTONOMIC NERVOUS SYSTEM (ANSI. Term sometimes referring to the entire vertebrate visceral nervous system, but more often restricted to the efferent- (motor) part of it (the visceral motor system), supplying smooth muscles and glands. Neither anatomically nor physiologically

AUTOSTYLIC JAW SUSPENSION autonomous from&e central nervous system. Sometimes termed the’ ‘involuntary’1 nervous system; but here again its effects (in humans) can largely be- brought under conscious control with training. For convenience the ANS can be subdivided into two components: the parasympathetic and sympathetic systems Parasympathetic nerve fibres are CHOLINERGIC and in mammals are found as motor components of CRANIAL NERVES III, VII, IX and X, as well as of three spinal nerves in sacral segments 2-4. Most of its effects are brought about by its distribution in the vagus (CN X), serving the .gut (see AUERBACH'S PLEXUS), liver and heart \ among “ot her organs Fibres of the sympathetic system originate within the s&nil cord _ departs from the cord and turns ventrally in a short white ramus (ramicommunicantes) to enter ?a sympathetic ganglion, a chain of whichlies on either side of the mid-line, In the sympathetic ganglia many of the preganglionic fibres relay with postganglionic fibres that innervate target organs (e.g. mesenteries and gut); others pass straight through as splanchnic nerves and meet in plexi (collectively termed the –solar plexus) beneath the lumbar vertebrae. “From here postganglionic fibres innervate much of the gut, liver, kidneys and adrenals. Postganglionic fibres usually” liberate catecholamines, particularly noradrenaline. Preganglionic parasympathetic, fibres are relatively long and usually synapse in a ganglion on .or near the effector, postganglionic fibres being short., In general ANS preganglionic fibres are myelinated, postganglionic unmyelinated and usually (there are exceptions) where the sympathetic stimulates, the parasympathetic inhibits, and vice versa; but organs are not always innervated by both. Both are, however, under central control, notably via the hypothalamus. Together they afford homeostatic nervous control of the internal organs, often reflexly:

AUTOPHAGY. Process whereby some secondary LYSOSOMES come to*c*o‘tn: tain *‘tand digest organelles of the cell in which they occur. 4 ‘ii , Yi ~-3

A~TOPOLYPLOID. In classicalcases, a POLYPLO~D (commonly a tetraploid) in which all the chromosomes are derived from the samespecies, frequently the same individual. Compare A L LO P o LY P'L 6 I D. '

AUTORADIOGRAPHY. Method using the energy of radioactive particles taken up by cells, tissues, etc., from an artificially enriched medium and localized inside them, to expose a plate sensitive to the emissions, thus indicating where radioactive atoms lie. Much used in tracing -pathways within cells. See LABELLING.

AUTOSOME. Chromosome that is not a SEX CHROMOSOME. . <>

AUTOSTYLIC JAW SUSPENSION. ‘The method of upper jatv suspension of modern chimaeras and lungfishes; presumed to have been- that

RUTOTOMY i tmployed’by earliest jawed fishes, in which the hyomandibular bone has no role in the suspension. The upper jaw (palatoquadrate) attaches directly to the cranium. See HYOSTYLIC, andAMPHISTYLICJAWSUSPENSION.

AUTOTOMY. Self-amputation of part of the body. Some lizards can break off part of the tail when seized by a predator, muscular action snapping a vertebra. Both here and in many polychaete worms which can shed damaged parts of the body, REGENERATION restores the

autotomized part.

AUXINS. Group of plant GROWTH SUBSTANCES, produced by many regions of active cell division and enlargement (e.g. growing tips of stems and young leaves), that regulate many aspects of plant growth. Promote growth by increasing rate of cell elongation, apparently by activating a proton pump in the plasmalemma, pumping H+ out of the cell, acidifying the cell T~+vall and- loosening bonds within it, so promoting cell expansion through turgor. Auxins also affect GENE EXPRESS I ON, increasing production of cell wall material in the longer term (apparently independently of the proton pump). Transported basipetally in shoots at a rate of about 1 cm/hour, they act synergistically with G [BBER ELLIN in stem cell elongation, and with CY TOKININ in control of buds behind the apical bud (apical dominance).Effects of auxins on cell growth include curvature responses, such as GEOTROPISM and PHOTOTROPISM. Auxins may also have mitogenic effects, as in initiation of cambial activity in association with cytokinins, and in adventitious root formation in cuttings. They are also implicated in flower initiation, sex determination, fruit growth, and delays in leaf fall and fruit drop.

AWL Naturally-occurring auxins include indole-3-acetic acid (I A A) and indole-3-acetonitrile (IAN). IAA has been isolated from such diverse sources as corn endosperm, fungi, bacteria, human saliva and, the richest natural source, human urine. In addition to naturally occurring auxins, many substances with plant growth regulatory activity (synthetic au&s) have been produced. Some are used on a very large scale for regulating growth of agriculturally and horticulturally important plants, as in inhibition of sprouting in potato tubers, prevention of fruit drop in orchards, achievement df synchronous flowering (and hence fruiting) in pineapple; also parthenocarpic fruit production, as in tomato, .avoiding risks of poor pollination. At increased, though still relatively low, concentyation, auxins inhibit growth, sometipes resulting in death. Some synthetic auxins have diSferentia1 toxicity in different plants:. toxicity of 2,4-dichlorophenoxyacetic acid (2,4-D) to dicotyledonous and non-toxicity to monocotyledonous, plants is perhaps best known, being exploited successfully incontrol of W EE D s in cereal crops and lawns.n some invertebrates (e.g. platyhelminths, some moHuscs) known as the genital atrium, ,which houses the penis and/or opening of the vagina, and into which these may open.

AUXOTRO~H. Mutant strain of bacterium, fungus or alga requiring nutritional supplement to the MINIMAL MEDIUM upon which the 1 wild-type strain can grow. See PROTOTROPH.

AVES. Birds. A class of vertebrates. Feathered ARCHOSAURS whose earliest known fossil, Archaeopteryx, was of upper Jurassic date and the sole known representative of the Subclass Archaeornithes. All other known birds (including fossils) belong to the Subclass Neornithes. The two superorders with living representatives are the Palaeognathae (ratites) and Neognathae (20 major orders; about half the 2900 living species, including songbirds, belonging to the Order Passeriformes, or ‘perching’ birds). Distinctive features include: FEATHERS; furcula (WISHBONE); forelimbs developed as wings. Bipedal and homeothermic, laying cleidoic eggs and (excluding Archaeopteryx and two other fossil genera) lacking teeth, but with the skin of the jaw margins cornified to form a beak (bill), whose diversity of form is in large part responsible for the Cretaceous, and subsequent, adaptive radiation of the group.

AWN (ARISTA). Stiff, bristle-like appendage occurring frequently on the flowering glumes of grasses and cereals.

AXENIC. (Of cultures of organisms) a pure culture.

AXIAL SYSTEM. In secondary xylem and secondary phloem, collective term for those cells originating from fusiform cambial initials. Long axes of these cells are orientated parallel with the main axis of the root or stem.

AXIL. (Of a leaf) the angle between its upper side and the stem which it i s borne; the normal position for lateral (axillary) buds.

AXILLARY. Term used to describe buds, etc., occurring in the AXI L of a leaf. d - ;

AXIS. (1) Embryonic axis of animals. There are generally three such: antero-posterior, dorso-ventral and medio-lateral, established very early in development, and sometimes by the POLARITY of the egg. The genetics of early development is under active study. See HO~\;OEOTIC, COMPARTMENT. (2) Second amniote VERTEBRA, modified for supportjng the head. See ATLAS.

AXON. The long process which grows out $from the cell bodies of some neurones towards a specificstarget with which it connects and carries impulses away from the cell body. See NEURONE, NERVE FIBRE.

A,~ONEME. complex microtubular core Of @LI.tiM and‘flageli-um-

\ AXOPOI& P s E UI) 0 POD I u M of. some sarcomastigophoran protozoans inwhich there. is a thin ^skeletal rod of siliceous material upon which s streaming of the cytoplasm occurs. They may bend to enclose prey items. +

KAMUS 2

AMACRINE CELL cell wall of some diatoms (Bacillariophyta) from the central axis to the margin, and opening to the inside of the cell wall.
AMACRINE CELL One of three classes of neurone in mid-layer of vertebrate retina. Conducts signals laterally without firing action potentials. ,
AMBER MUTATION. One of three mRNA CODONS not recognized by transfer RN& commonly present in cells, and bringing about normal polypeptide chain termination. Its triplet base sequence is UAG. Any mutation producing this sequence within a reading frame results in termination of the TRANSLATION process and release of incomplete polypeptide. ikfissense or stop mutation. See OC HRE and OPALMUTATIONS,GENETICCODE.
AMENSALISM. Interaction in which one animal is harmed and the SYMBIOSIS.
AMES TEST. Test assessing mutagenic potential of chemicals. Strains of the bacterium Salmo nella typhimurium having qualities such as permeability to chemicals, inability to repair DNA damage, or ability to convert D N.A damage into heritable mutations, are made ,+UXOTROPHIC
for histidine. After mixing with potential mutagen prior to plating, increase in normal (PROT~TROPHIC) colonies indicates mutagenicity.
AMETABOLA. Primitively wingless insects (APTERYGOTA).
AMINO ACID. Amphoteric organic compounds of general structual formula H--N-C-CR,OH (where R may be one of 20 atomic groupings) occurring freely within organisms, and polymerized to form dipeptides oligopeptides and polypeptides. Amino acids differ in their R-groups / .and the amino acid sequence in a protein molecule determines not only itscharge sequence butsalso its configuration in solution. Rela- p tive molecular masses of the‘common forms vary from 75 (glycine) to G 204 (tryptophan). Only three commonly contain sulphurous Rgroups: methionine, cysteine and cystine (formed from two oxidized cysteines, providing ‘sulphur bridges’). During P R 0 T E I N s Y N T H ES I s the carboxy- and amino-terminal ends of adjacent amino acids condense to form peptide ,bonds, leaving only the N-terminal and Cterminal ends of the protein and some. R-groups ionizable. About 20 2 1 AMNIOTIC EGG amino acid radicals occur commonly in proteins, encoded by the GENETIC CODE. Their modification after attachment to a transfer RNA molecule may result in rare non-encoded amino acids occurring in proteins. Some amino acids (e.g. ornithine) never occur in proteins. Most naturally-occurring-~ amino acids except proline have‘ a free carboxyl and a free amino group on the R-carbon atom (alpha amino acids). Essential amino acids are required by an organism from its environment, due to inability to synthesize them from precursors (see VITAMINS, which they are not); there are about lOsuch for humans. AMITCIS. See ABERRANT CHROMOSOME BEHAVIOR
AMMOCOETE. Filter-feeding larva of lamprey, capable of attaining lengths of over 10 cm if conditions for metamorphosis do not prevail.
AMMONIFICATION. Decomposition of amino acids and other nitrogenous organic compounds; results in production of ammonia (NH,) and ammonium ions (NH,+), Bacteria involved are ammonifving bacteria. See NITROGEN CYCLE.AMMONITES. >Group of extinct cephalopod’ molluscs (Subclass Ammonoidea, Order Ammonitida) dominating the’,Mesozoic cephalopod fauna. Had coiled shells, with protoconch (calcareous chamber) at origin of the shell spiral. Of great stratigraphic value.
AMMONOTELIC. (Of animals) whose principal nitrogeqous excretory material is ammonia. Characterizes aquatic, especially freshwater, see UREOTELIC?URICOTELIC. '
AMNION. Fluid-filled sac in which AMNIOTE embryo develops. An EXTRA-EMBRYONIC MEMBRANE (Fig. 16) formed in reptiles, birds and some mammals by extraembryonic ectoderm and mesoderm growing up and over embryo, the (amniotic) folds overarching and fusing to form the amnion surrounding the embryo, and the CHORION surrounding the amnion , ALLANTOIS and YOLK SAC The amnion usually expands to meet the chorion. In humans and many other mam-
,mals the amnion originates by rolling up of some of the cells of the INNER C&LL MASS during GASTRULATZON. Amniotic fluid (amounting to about one dm3 at birth in ~humans) is circulated in placental mammals by foetal swallowing, enabling wastes ,to pass to the placenta for removal. Provides a buff&ring cushion against mechanical damage, helps stabilize temperature and dilate the cervix during .birth. In amniocentesis, amniotic fluid containing cells from the
foetus is withdrawn surgically for signs of abnormal development.
AMNIOTE. Reptile, bird or mammal. Distinguished from anamniotes by presence OfEXTRA EMBRYONIC MEMBRANESin development.
AMNIOTIC EGG. Egg type characteristic of reptiles, birds and PROTOTHERI A (much modified in placental mammals). Shell leathery or calcified; ALBUMEN and yolk typically present. EXTRAEMBRYONIC AMOEBA MEMBRANES occur within it during development. See CLEIDOIC
EGG.
AMOEBA. Genus of sarcodine protozoans. Single-celled animals of irregular and protean shape, moving and feeding by use of PSEUDOPODIA. Some slime ^mould cells are also loosely termed ‘amoebae’, , while any CELL LOCOMOTION resembling an amoeba’s is termed ‘amoeboid’.
AMOEBOCYTE. Cell (haemocyte) capable of active amoeboid locomotion found in blood and other body fluids of invertebrates; in sponges, an amoeboid cell type implicated in mobilization of food from the feeding CHO AN o c Y TES and its conveyancing to non-feeding cells in absence of true vascular system AMOEBOID. Describing cells resembling those of the genus AMOEBA.
AMP. Adenosine monophosphate. Nucleotidc component of DNA and RNA (in deoxyribosyl and ribosyl forms respectively), and hydrolytic product of AD P and AT P. Converted to cyclic A M P (CA M P) by ADENYLATE CYCLASE, iUtWXllUk%r concentrations Of AMP rising
rapidly in response to extracellular (esp. hormonal) signals and falling rapidly due to activity of intracellular phosphodiesterase. Its level dictates rates of many biochemical pathways, depending upon Celltype. See CASCADE,SECOND MESSENGER,G-PROTEIN, GTP.
AMPHIBIA. Class of tetrapod vertebrate, its first fossil representatives being Devonian ichthyostegids and its probable ancestors rhipidistian crossopterygian fishes. A POLYPHYLETIC origin has not been ruled out. Many early forms had scaly skins, almost entirely lost in the one modern Subclass (Lissamphibia) of three orders: Apoda, legless caecilians; Urodela, salamanders and newts; Anura, toads and frogs. Compared with their mainly aquatic ancestors, the more terrestrialized amphibians have: vertebrae with larger, more articulating
neural arches and larger intercentra (see VERTEBRAL COLUMN); greater freedom of the PECTORAL GIRDLE from the skull, allowing some lateral head movement; PELVI c GIRDLE composed of three paired bones (pubis, ischium and ilium) with some fusion to form the
rigid PUBI c SY MPHY SI s; eardrums (homology with part of the spiracular gill pouch of fish) and a single middle ear ossicle, the columella, homologous with the hyomandibular bone of fish. Fertilization is internal or external (but intromittant organs are lacking). Most return to water to lay anamniote eggs, although some are viviparous. ’ The skin is glandular for gaseous exchange. Modern forms specialized and not representative of the Carboniferous amphibian radiation.
AMPHICRIBRAL. (Bot.) Type pf vascular arrangement where phloem surrounds the xylem.
AMPHIDIPLOID. See ALLOTETRAPLOID.
AMPHIMIXIS. Normal sexual reproduction, involving meiosis and AMYLOPLAST fusion of :haploid nuclei, usually borne by gametes. See AU T o M IX IS, APOMIXIS,PARTHkNOGENESIS.
AMMPHINEURA..Minionro Class off MOLUSCA, iincluding the chitons.. Marine, mostly on rock surfaces; head reduced and lacking eyes and tentacles; mantle all round head and foot; commonly eight calcareous shell plates over visceral hump; nervous system primitive, lacking definite ganglia.
AMPHIOXUS . Lancelets (Subphyllum CEPHALOCHORDATA).. Widely distributed marine filter-feeding burrowers up to 5 cm long. Two genera (Branchiostoma, Asymmetron). Giant larva resulting from prolonged pelagic life once given separate genus (Amphioxides) and develops premature gonads, providing support for the evolutionary origin of vertebrates by P R 0 GE N ES Is.
AMPHIPHLOCIC.. Type of asvcausclaulra rarangemen t where phloem is on both sides of the xylem. Compare AMPHICRIBRAL.
AMPHIPODA. Ordrere of Crustacea Sub class Mallaccostraca. Lack carapace; body laterally flattened. Marine and freshwater forms; about. 3600 species. Very important detritus feeders and scavengers. Includes gammarids.
AMPHISTYLIC. Method of upper jaw suspension in a few sharks, in which there is support for the jaw both from the hyomandibular and thebraincase. See AUTOSTYLIC, HYOSTYLIC. . .
AMPULLA. (Ofinerear) see VESTIBULAR APPARATUS AMYDALA (AMYGOALOID BODIES or NUCLEI). Basal ganglia of the subcortical region of the most ancient part of the vertebrate CER,EBRA HEM ISPHEREs, gathering olfactory and visceral + information.They appear to. be involved in the generation of emotions. Removal.vin humans increases sexual activity.
AMYLASES (DIASTASES). Group of enzymes hydrolysing starches or glycogen variously to dextrins, maltose and/or glucose; a-amylase (in saliva and pancreatic juice) yields maltose and glucose; /3-amylase (in malt) yields maltoie alone. Present in germinating cereal seeds (see ALEU RONE LAYER), where only a-anylase can digest intact starch grains, and produced by some microorganisms.
AMYCAOMPEYCCTOIPNE.CTIN.HigHhliyg hlbyr abnrcahnecdh edp oploylsyascacchcahraidried ec ocommppoonnenent To off tthhee plant storage carbohydrate STAR c H. Consists of homopolymer of linked glucose units, with cl[l,6]-linked branches every or so glucose radicals. Like G L Y c OGEN it gives a red-violet colour with iodine/K1 solutions. See AM'YLASES.
AMYLOPLAST. Colourless plastid (leucoplast) storing STARCH; e.g.found in cotyledons, endosperm and storage organs such as potato tubers.
AMYLOSE Straight-chain polysaccharide component of s T ARCH: Comprises a[l,4]-linked glucose units. Forms hydrated micelles in water, giving the impression of solubility. Gives a blue colour with iodine/K1 solutions. Hydrolysed by A M Y L A SE s to maltose and/or glucose.
ANABOLISM. Enzymatic synthesis (build-up) of more complex molecules from more simple ones. Anabolic processes include multi-stage photosynthesis, nucleic acid, protein and polysaccharide syntheses.ATP or an equivalent needs to be available and utilized for the reaction(s) to proceed. See CATABOLISM, GROWTH HORMONE, METABOLISM.
ANADROMOUS. Animals (e.g. lampreys, salmon) which must ascend rivers and streams from the sea in order to breed. See OSMOREGULATION.
ANAEROBIC. (Of organisms) ability to live anoxicaZZy i.e. in the absence of free (gaseous or dissolved) oxygen. (Of processes) occurring in the absence of such oxygen. Anaerobic respiration is the enzyme-mediated process by which cells (or organisms) liberate energy by oxidation of substances but without involving molecular oxygen. This involves less complete oxidation of substrates, with less energy released per g of substrate used, enabling anaerobes to exploit environments unavailable to obligate aerobes. Facultative anaerobes can switch metabolism from aerobic to anaerobic under anoxic conditions, as required of many internal parasites of animals, some yeasts and other microorganisms. GLYCOLYSIS is anaerobic but may require aerobic removal of its products to proceed. Relatively anoxic environments include animal intestines, rumens, gaps between teeth, sewage treatment plants, polluted water, pond mud, some estuarine sediments and infectedwoundssee OXYGEN DEBT,RESPIRATION.
ANAGENESIS. (1) Process by -which characters change during evolution , within Species, by NATURAL_SELECTION_OF.GENETIC Any non-branching speciation in which species originate along a single line of descent yet only one species represents the lineage after
. any speciation event (contrast c L A D o G E N ES IS). Gradual anagenetic speciation is not possible within the biological species concept, for reproductive isolation is never completed between ancestral and descendant species. c L A D I s T I c s excludes anagenetic speciation by
definition, but the term is retained in the context of characters. See SPECIES.
ANALOGOUS. A structure present in one evolutionary lineage is said to be analogous to a
structure, often performing a similar function, within the same or another evolutionary lineage if their phyletic and/or developmental origins were independent of one another; i.e. if there is Ho MO P L A SY. Tendrils of peas and vines and eyes of squids U N I T and vertebrates are pairs of analogous structures. See CONVERGENCE,HOMOLOGY,PARALLEL EVOLUTION,
ANAYNIOTE. (Of vertebrates) more primitive than the AMNIOTE grade.Includes agnathans, all fish, and amphibians.
ANANDROUS. (Of flowers) lacking stamens.
ANAPHASE. Stage of mitosis and meiosis during which either bivalents (meiosis Ior sister chromatids (mitosis, meiosis II) separate and move t0 Opposite poles OftheCell. SPINDLE.
ANAPHYLAXIS. A type of hvnersensitivity~ to antigen (allergen) in Ig E antibodies attach to mast cells and ’ in circulatory shock and asphyxia. See ALLERG
ANATROPOUS. (Of ovule) inverted through 180”, micropyle pointing towards placenta. Com pare ORTHOTROPOUS ,CAMPYLOTROPOUS.
ANDRODIOECIOUS. Having male and hermaphrodite flowers on separate plants. Compare ANDRoMoNoEcIous.
ANDROECIUM. A collective term referring to the stamens of a flower. Compare GYNOECIUM.
ANDROGEN. Term denoting any substance with male sex hormone activity in vertebrates, but typically steroids produced by vertebrate testis and to a much lesser extent by adrenal cortex. See TESTOST ERONE
ANDROMONOECIOUS. Having male and hermaphrodite flowers on the sameplant. Compare ANDRoDIoEcIous.
ANEMOPHILY. The pollination of flowers by the wind. Compare ENTOMOPHILY.
ANEUPLOID (HETEROPLOID). Of nuclei, cells or organisms having more or less than an integral multiple of the typical haploid chromosome number. See EUPLOID , MONOSOMY, TRISOMY, NULLISOMY.
ANGIOSPERM. Literally, a seed borne in a vessel’(carpe1); thus one of a group of plants (the flowering plants) whose seeds are borne within a mature ovary (fruit). See ANTHOPHYTA, which replaces Angiospermae.
ANGIOTENSINS. Angiotensin I is a decapeptide produced by action of kidney enzyme, renin, on the plasma protein angiotensinogen when blood pressure drops. It is in turn converted by a plasma enzyme in the lung to the octapeptide angiotensin 11, an extremely powerful tvasoconstrictor
which raises blood pressure and also results in sodium retention and potassium excretion by kidney. See OSMOREGULATION.

ANGSTROM UNIT (A). Unit of length, lo-lo metres (O.lnm); 1O-4 microns. Not an SI unit.

ANIMALIA. Animals. Kingdom containing those eukaryotes combining a lack of cell wall material with heterotrophic nutrition (although endosymbiotic photosynthetic *cells may occur in some tissues).Commonly divided into subkingdoms PROTOZOA, PARAZOA and. METAZOA. Classifications recognizing Kingdom Protista or Kingdom Protoctista would exclude protozoans from the Kingdom‘Animalia. In &hese systems, Kingdom Animalia includes all heterotrophic
eukaryotes lacking cell wall material and having a blastula stage in their development.
ANMAL POLE; Point on surface of an animal egg nearest to nucleus, or -extended to include adjacent region of cell. Often marks one end of a graded distribution of cytoplasmic substances. See POLARITY.
ANISOGAMY. Condition in which gametes which-fuse differ in size and or motility. In Bametes differ in both properties. Significantly, the sperm often contributes the sole centriole for the resulting
ZygOte. See FERTILIZATiOti,71SOGAMY, PARTHENOGENESIS.
ANNELYDA (ANNULATA). Soft-bodied, metamerically segmented coelomate worms with, typically, a closed blood system; excretion by N E P H R I D I A; a central nervous system of paired (joined) nerve cords ventral to the gut, and a brain comprising paired ganglia above the oeso phagus, linked by commissures to a pair below it. Cuticle collagenous; not chitinoas. Chitin present in CHAETAE, which may be quite long, bristle-like and associated laterally with fleshy parapodia
(e.g. ragworms,. Class Polychaeta) or shorter and not housed in parapodia (e.g. earthworms, Class Oligochaeta). Leeches (Class HIRUDINEA) have 34 segments, confused by surface annulations. CLITELLUM present in both oligochaetes and leeches. Septa between segments
often locally or entirely lost. The coelom acts as a hydrostatic skeleton against which longitudinal and circular muscle syncytia (and diagonal muscles in leeches) contract. Cephalization most pronounced in polychaetes (largely marine); eyes and mandibles often well developed but oligochaetes lack specialized head structures. Gametes leave’ via COLOMODUCTS Oligochaetes and leeches are typically hermaphrodite, polychaetes frequently dioecious.

ANNUAL. Plant completing its life cycle, from seed germination to seed production followed by death, within a single season. Compare BIENNIAL,EPHE.MERAL,PERENNIAL.DESERT,
SEE ANNUAL PING. Annual increment of secondary wood (xylem) in stems 2
and roots of woody plants of temperate climates. Because of sharp contrast in size between small wood elements formed in late summer and large elements formed in spring the limits of successive annual rings appear in a cross-section of stem as a series of concentric lines.

ANNULAR THICKENING. In protoxylem, internal thickening of a xylem vessel or tracheid wall, in rings at intervals along its length. Provides mechanical support, permitting longitudinal stretching as neighbouring cells grow.

ANNULUS. (1) Ring of tissue’ surrounding the stalk (stipe) of fruit bodies of certain Basidiomycotina (e.g. mushrooms); (2) line of ’ 1: specialized cells involved in opening moss capsules and fern sporangia to liberate spores. 1 i:

ANOESTRUS. Period between breeding seasons in mammals, when OESTROUS CYCLES areabsent. SEE OESTRUS.

ANOEPLURA. See si~mmcuLATA.

ANOXIA. Deficiency or absence of free (gaseous o\ dissolved) oxygen.

ANTAGONISM. Opposition of two or more processes or systems. (1) Of organisms, one interfering with or inhibiting growth or presence of another; (2) of drugs, hormones, etc., producing opposite physiological effects; (3) of muscles, producing opposite movements so that contraction of one must be accompanied by relaxation of the other. The normal way by which muscles regain their relaxed shape after contraction is by being extended by antagonistic muscle contraction.
SEE SYNERGISM.

ANTENNA. Paired, preoral, tactile and olfactory sense organs developing from. second or third embryonic somites of all arthropod classes other than Onychophora and Arachnida. Usually much jointed and mobile. In some crustaceans locomotory or for attachment, a pair of

ANTEN NULES (Often regarded as antennae) typically occurring on the segment anterior to that with antennae. ONYPHOPHORA have pair of cylindrical preantennae on first somite. See T E NT A c L E S .

ANTENNA COMPLEX. Clusters of several hundred chlorophyll molecules fixed to the thylakoid membranes of chloroplasts by proteins in such a way as to harvest light energy falling on them, and relaying it to a special chlorophyll molecule in an associated PHOTOSYSTEM. See PHOTOSYNTHESIS and Fig. 14b.

ANTENA COMPLE’X. Complex and segmentation loci in Drosophila which, when homozygously mutant, may result in conversion of antenna1 parts into leg structures. Intensely studied in contexts of MORPHOGENES‘IS, and POSITIONAL INFORMATION.Some loci in the complex ‘appear to be expressed only in specific embryonic COMPARTMENTS. See HOMOEOBOX.

ANTENNULE. Paired and most, anterior head appendages of crustaceans;uniramous, whereas antennae like most appendages in the class are biramous.

ANTHER. Terminal portion of a STAMEN, containing pollen in pollen sacs.

ANHERIDIOPHORE In some liverworts, a stalk that bears the antheridia.
ANTHERIDIUM. ‘Male’ sex organ (gametangium) of fungi, and of plants ,other than seed plants (e.g. algae, bryophytes, lycophytes, sphenophytes and pterophytes).
ANTHEROZOID. Synonym of SPERMATOZOID.
ANTHESIS. Flowering.
ANTHOCEROTOSIDA. Hornworts. Class of BRYOPHYTA. Small,widely distributed group, especially in tropical and warm temperate regions, growing in moist, shaded habitats. Plant a thin, lobed, dorsiventral T H A L L u S, anchored by rhizoids. Each cell usually has a single large chloroplast rather than the many small discoid ones found in cells of other bryophytes and vascular plants; and each chloroplast possesses a PYRENOID , all features suggesting algal affinities. Some (e.g. Anthocevos) contain Cyanobacteria (e.g. Nostoc spp.), supplying fixed nitrogen to their host plants.
ANTHOCYANINS. Group of water-soluble, flavonoid pigments (glycosides) occurring in solution in vacuoles in flowers, fruits, stems and leaves. Change colour, depending on acidity of solution. Responsible for most red, purple and blue colours of plants, especially in flowers; contribute to autumn (fall) colouring of leaves and tint of young shoots and buds in spring. Colours may be modified by other
pigments, e.g. yellow flavonoids. ,Flowering plants (formerly Angiospermae). Division of plant kingdom. Seed plants ,whose ovules are enclosed in a carpel, and with seeds borne within fruits. Vegetatively diverse; characterized by FLOWERS; pollination basically by insects, but other modes (e.g. ANEMOPHILY) have evolved in a number of lines. Gametophytes much reduced; male gametophyte, initiated- by pollen grain (microspore), comprising two non-motile gamete nuclei and a tube cell nucleus each associated with a little cytoplasm in the pollen tube; female gametophyte developing entirely within wall of megaspore which at maturity is a large cell containing eight nuclei, the EMBRYO SAC. Characteristic DOUBLE FERTILIZATION. Two classes: MonocotyZedonae (monocots, about 65 000 spp.), with flower parts usually in threes, leaf venation usually parallel, primary vascular bundles in the stem scattered, true secondary growth absent, and just a single cotyledon present; Dicotykdonae (dicots, about 170 000 spp.), with flower parts usually in fours or fives, leaf venation usually net-like, primary vascular bundles in the stem forming a ring, often with true secondary growth and vascular cambium, and two cotyledons present.

ANTHOZOA. SEE ACTINOZOA.

ANTHROPOID APES. Members of Family Pongidae (Order PRIMATES). Include orangutan, chimpanzee and gorilla. Common ancestor of pongids and hominids (‘men’) probably Miocene in age. Gibbons (Family Hylobatidae) are in same suborder (Anthropoidea) as ‘great apes’ (pongids) and occasionally included in the term ‘anthropoid ape’. Much ape anatomy stems from a brachiating mode of progression. Fundamentally quadrupedal; tendency to bipedal gait limited. Markedly prognathous, with diastemas. All are Old World forms. ANTHROPOIDEA. Suborder of PRIMATES. Three living superfamilies:
Ceboidea (New World monkeys); Cercopithecoidea (Old World monkeys); Hominoidea (gibbons, great apes and man). Eyes large and towards front of face; brain expansion associated with relative expansions of frontal, parietal and occipital bones of skull; -great manual dexterity. ”

ANTIAIJXINS. Chemicals which can prevent the action of AUXINS in plants, e.g. 2,6-dichlorophenoxyacetic acid; 2,3,5triiodobenzoic acid.

ANTIBIOTIC RESISTANCE ELEMENT. Genetic element, composed of DNA and often borne on a TRANSPOSON, conferring bacterial resistance to an antibiotic. Often with INSERTION SEQUENCES at either end, when capable of moving between PLASM I D, viral and bacterial DNA and selecting insertion sites, sometimes turning off expression of genes it inserts into or next to. Able to spread rapidly across species and other taxonomic boundaries, making design of new antibiotic drugs even more urgent. Many common pathogenic bacterial strains are now resistant to some of the best-known drugs. Non-homologous recombination between plasmids can give rise to multiple-resistance plasmids, bacterial plasmid Rl conferring resistance to chloramphenicol, kanamycin, streptomycin, sulphonamide and ampicillin. See PLASMID.

ANTIBIOTICS. Diverse group of generally low molecular mass organic compounds (in the category of non-essential ‘secondary metabolites’). Characteristically produced by spore-forming soil microorganisms during or just prior to sporulation they tend to. inhibit growth of potential competitors either reversibly (when bacteristatic), or irreversibly (bactericidal), generally by blocking one or more enzyme reactions in the affected cell. $treptomuin affects the svntheses of DNA, RN A and proteins, and alters the cell membrane and respiratory activity of sensitive cells; p prevents creycan chains of the peptidoglycanrof the bacterial cell walls, resulting in wall essorwall-deficientcells ;ACTINoMYCIN preVents TRANscRIPTIo , puromycins preventT~ANs~AT~oN; anthracyc Zinesblock DNA replicat -transcription. In most cases these are achieved by the antibiotic forming complexes with, or otherwise inserting itself into, a nucleic acid. Antibiotics have been widely used as clinical drugs. Their use has generated new selection pressures for the target microorganisms
ANTIBODY (IMMUNOGLOBULjN) COOH COOH . Diagram of IgG structure; rectangles are protein subunits and hypervariable regjons are shown as dark lines. Antigen-binding sites are within dotted lines; other domains exist for complement fixation, for binding to F, receptors, neutrophiisand K cells. See A N n BOD Y for details (see DEUTEROMYCOTINA), resulting’in the spread of ANTIBIOTIC R‘ESISTANCE ELEMENTS inwhatresembles an ARMS RACE.
ANTIBODY (IYMUNOGLOBULIN). Class of glycoprotein produced by vertebrate white blood cells (B - CELL s), after maturation into plasma cells. Their main function is to bind highly selectively to foreign
molecules (antigens), which then clump together (agglutinate) so that phagocytic white cells can engulf them. Five major classes differ principally in their type of heavy protein chain, and the degree to which. the molecule is a polymer of immunoglobulin ‘monomers’. Each immunoglobulin unit comprises two
identical H- (heavy) and two identical L- (light) polypeptide chains forming mirror images off each other’’ and joined by a flexible hinge region involving disulphide bridges. They bind to antigen at specific antigen-binding regions provided uniquely by the combination of H- and L-chain amino-terminal portions , which are extremely variable in their amino acid sequences between different antibodies, in contrast to constant regions at their carboxy-terminal portions. Only about 20-30 amino acids of the variable regions of H- I and L-chains contribute to the antigen-binding site, these being located in three short hypervariable regions of each variable region. These lie themselves within relatively invariant ‘framework regions’

ANTIBODY DIVERSITY Of the variable regions. The other biological properties of the molecule G4ii
are determined by the constant domains of the heavy chains. Digestion of antibody with sap& produces two identical (antigen-binding) fragments and one Fc (crystallizing) fragment. The latter region in the intact Ig (immunoglobulin) molecule is responsible for determining which component of the immune system the antibody ill bind to. The Fc region of IgG may bind phagocytes and the first component of COMPLEMENT. Only the IgG antibody can cross the mammalian placenta. IgM is the major Ig type secreted in a primary immune response, but I G dominates in secondary immune responses (see B-CELL).
Transformation j otB-cells into differentiated antibody-producing plasma cells generally requires both antigen-presenting cells and a signal from a helper T-cell (see T-C E L L). Because B-cells have only a
a few days’ life in culture they are not suitable for commercial antibody production: however, if an antibody-producing B-cell from an appropriately immunized mouse is fused to an appropriate mutant tumour B-cell, the hybrid cell formed may continue dividing and producing the particular antibody required. The resulting HYBRIDOMA can be sub-cloned indefinitely, giving large amounts of antibody. Initial isolation of the appropriate B-cell follows discovery of the required antibody in the growing medium, The purity of the resulting monoclonal antibody and its production in response to what is possibly a minor component of an impure antigen mixture are both desirable features of the technique. See A N T I B 0 D Y D IV E R SITY,ANTIGEN-ANTIBODYREACTION, IgA-IgM.

ANTIBODY DIVERSITY (A, VARIATION). Production of different A N T I BODY molecules by different B - c E L L s. Light and heavy chains are encoded by different gene clusters. In humans, light chain genes lie on chromosomes 2 and 22, heavy chain - “L‘ genes on chromosome 14, and the light chains of a particular immunoglobulin molecule are encoded either by chromosome 2 or chromosome 22, not both. Any’ particular B-cell assembles in a line all the heavy chain genes needed to make its own unique antibody 3, type, joining first the genes for variable (V), hypervariable (HV) and joining (J) regions of the molecule, then linking this combination to the genes for the constant (C) regions of the molecule, with different constant regions for different immunoglobulin classes (see IMMUNOGLOBULIN references). The enzymes bringing together genes from different parts of a chromosome are performing a form of RECOMBINATION. Diversity arises from the randomness with which particular genes from heavy and light chain clusters are brought together. In addition, extra short nucleotide sections (N segments) get inserted, probably in some rule-following way, into the DNA encoding the antigen-binding regions of the molecule, and this together with variation in RNA PROCESSING of the hnRNA transcript

ANTICLINAL increases still further the total antibody diversity, often classified as follows: (1). Allotypic: variation in the Cnl, Cn3 and CL antibody regions caused by allelic differences between individuals at one or more loci for a subclass of immunoglobulin chains; (2) Idiotypic: variation in the VL and Vn regions (especially in the hypervariable regions) that are generally characteristic of a particular antibody
clone, and therefore notdpresent in all members of a population; (3) Isotypic: variation in the CL, and in the Cnl-3 antibody regions, determined by loci whose representative alleles are shared by all healthy members of a population.

ANTICLINAL. (Bot.) Alignment of the plane of cell division approximately at right angles to the outer surface of the plant part. Compare PERICLINAL.

ANTICOAGULANT.. Any substance preventing blood clotting. Blood naturally contains such substances: fibrin and antithrombin III a absorb much of the thrombin formed in the clotting process and HEPARIN inhibits conversion of prothrombin to thrombin. Bloodsucking animals (leeches, insects, bats, etc.) frequently produce antiacoagulants in their saliva. Artificial anticoagularits (e.g. dicumarol) are either helpful to patients, or prevent blood samples from clotting in blood banks (e.g. EDTA). The,rat poison warfarin is an anticoagulant. See BLOOD CLOrTrNG.

ANTICODON: The triplet sequence of ‘tRNA nucleotides‘ capable of ’ base-pairing with a codon triplet of an mRNA molecule. See PROTEIN SYNTHESIS.

ANTIDIURETIC HORMONE (-ADH, VM~PRESSIN). Ring-structure octapeptide hormone produced by hypothalamic neurosecretory cells and released into posterior pituitary circulation if blood water potential drops below the homeostatic norm. Has marked vasoconstrictor effects on arterioles, raising blood pressure, and increases water permeability of collecting ducts and distal convoluted tubules to the
l0-20% of the initial glomerular filtrate still remaining (see KIDNEY), resulting in water retention. See osMoREGuLATIoN, 0xYTocIN.

ANTIGEN. Molecule (often protein or glycoprotein) which induces production of specific ANTIBODY, and to which the latter binds at a specific configurational domain of the antigen molecule called the antigenic determinant, or epitope. See ANTIGEN-ANTIBODY REACTION,
ANTIGEN-PRESENTING CELL, IMMUNITY.
ANTIGEN-ANTIBODY REACTION. Non-covalent bonding between antigenie determinant of ANTIGEN and antigen-combining site on an immunoglobulin molecule (see A N T I B 0 D Y). Several such bonds form
simultaneously. The reactions show high specificity but cross-reactivity may result if some determinants of one antigen are shared by another. Antibodies seem to recognize the three-dimensional con33
ANTLER figuration and charge distributian of an antigen rather than its chemical make-up as such. Such reactions form the basis of hum&aland of many cell-mediated immune responses. See AGG LUTININ,
COMPLEMENT,PRECIPITIN,IMMUNITY.
ANTIGENIC VARIATION. Ability of some’ pathogens, notably viruses, bacteria and protozoa, to change their coat antigens during infection. Trypanosomes and some stages‘ in the malarial life-cycle achieve it,
making the search for vaccines to some devastating human diseases very difficult.
ANTIGEN-PRESENTING CELL (APC). Few antigens bind directly to antigen-sensitive T - CELL s or B-c ELLS but are generally ‘presented’ to these lymphocytes on the &u-faces of other cells, the antigenpresenting
cells. Dendritic cells with a large resultant surface area for antigen-attachment are widely distributed in the human body and trap antigens. Those in spleen’ and lymph nodes trap lymph- and bloodborne antigen and present it to lymphocytes there. Other antiginbearing dendritic cells migrate from non-lymphoid-regions to lymphnodes. Clonal expansion of appropriately stimulated B-lyrhphocyte
classes occurs, resulting in specific antibody production. See IMMUNITY. .
ANTI GIBBERELLINS. organic compotinds of varied- structure causing plants to grow with short, thick stems or with appearance opposite to that obtained with GIBBERELLIN, which can reverse thl action of
most of these compounds, Of agriculturtil importance, they include phosphon and maleic hydrazide (retarding growth of grass, reducing frequency of cutting).
ANTIPODALS. Three (sometimes more) cells of the mature EMBRYO SAG, located at thg end opposite the micropyle.
ANTIPORT. See TRANSPORT PROTEINS.
ANTlCEPTlC,Substance used on a living surface (e.g. skin) to destroy microorganisms and sterilize it. Ethyl and isopropyl alcohol, diluted 70% with sterile water, kill vegetative bacteria and some viruses, but
not spores of bacteria or fungi. Iodine (dissolved with potassium iodide in 90% ethanol) is rapidly bactericidti’, killing both vegetative cells.and spores. It does, however,SEE DISINFECTANT,AUTOCLAVE.
ANTISERUM. SERUM, containing antibodies with affinity for a specific antigenic determinant (see ANTIGEN) to which they bind. May result in cross-reactivity(see ANTIGEN-ANTIBODY REACTION) within Recipient.

ANTLER. Bony projection from skull of deer. Unlike HORN (>which is matted hair) they are often branched, are shed annually, and are confined to males (except in reindeer).
ANURA (SALIENTIA). Frogs and toads. An order of the Class AMPHE$IA.Hind legs modified for jumping and swimming; no tail; often vocaL
ANUS. The opening of the alimentary canal to the exterior through which egested material, some excretory material and water may exit. When present, the gut is said to be entire. Absent from coelenterates and platyhelminths. See PROCTODAEUM. AORTA. Term applied to some major vertebrate arteries. See AORTA, DORSAL;AORTA,VENTRAL;AORTICARCH.
AORTA, DORSAL. Major vertebrate (and cephalochordate) artery through which blood passes to much of body, supplying arteries to ,most major organs. In sharks a single dorsal aorta collects oxygenated .blood from the gills, but in bony fish paired dorsal aortae on either side in the head region perform this task before uniting as a single median vessel. Oxygenated blood then passes backwards to the body; but in fish too blood flowing up through the third AOR TI c k R c H tends to pass anteriorly through the aorta(e) rather than posteriorly (see CAROTID ARTERY). In adult tetrapods, those parts of the single or paired dorsal, aortae between the third and fourth aortic arches tend to disappear, blood from the fourth (systemic) arch(es) passing back within two uniting dorsal aortae (terrestrial salamanders, lizards) or within a single dorsal aorta (most reptiles, birds and mammals) derived from the right arch (reptiles and birds) and from the left arch (mammals). Protected throughout in vertebrates by proximity of bone above (typically vertebrae).

AORTA, VENTRAL. Large median artery of fish and embryonic amniotes leading anteriorly from ventricle of heart, either giving off branches to gills or running uninterrupted as AORTI c ARCHES to dorsal aorta(e). In lungfish, branches differ in this respect. In living amphibians it has disappeared, while in other tetrapods it serves merely as a channel supplying blood to aortic arches III, IV and VI. AORTIC ARCHES. Paired arteries (usually 6, but up to 15 in hagfishes) of 6 vertebrate embryos connecting ventral aorta with dorsal aorta(e) by running up between gill slits or gill pouches on each side, one in each
v I s C E R A L A R CH. The study of their comparative anatomy in embryos and, where they persist, in adults provides striking support for macroevolutionary change. Each is given a Roman numeral, beginning anteriorly. Arches I and II do not persist in post-embryonic tetrapods, but arch II at least is present in sharks, some bony fish and lungfish. Arch III usually serves (with parts of the dorsal aortae) as the tetrapod carotid arteries, but in fish is usually interrupted by gills; arch IV is separated from the anterior arches in most tetrapods and becomes the systemic arch (see A 0 R T A, D o R s A L); arch V is absent from adult tetrapods other _ than urodeles, but serves as the D u c TuS ARTERY osus in development prior to lung function; arch VI then shifts to supply the lungs.
APICAL MERISTEM AP& General term for HOMINOI D primates of families Hylobatidae (gibbons, siamangs) and Pongidae (‘great, apes’). See A N T H R o P o I D

APE,

APETALOUS . Lacking petals, e.g. flower of wood anemone.

APHANIPTERA. See SIPHONAPTERA (fleasj

APHID. Green fly,or black fly. Homopteran insect (Superfamily Aphidoidea) notorious for sucking plant juices, for transmitting plant viral diseases, and for phenomenal powers of increase by viviparous PARTHENOGENESIS.

APHYLLOUS. Leafless.

APICAL DOMINANCE. (Bot.) Influence exerted by a terminal bucl in suppressing growth of lateral buds. See A UXINS.

APICAL MERISTEM. Growing point (zone of cell division) at tip of root and stem in vascular plants, having its origin in a.single cell (initial), e.g. Pterophyta, or in a group of cells (initials), e.g. Anthophyta% In the latter, the growing point apex (promeristem) consists of actively dividing cells. Behind this, division continues and differentiation begins, becoming progressively greater towards mature tissues. One (older) concept of growing point organization in flowering plants recognizes differentiation into three regions (histogens): dermatogen, a superficial cell layer giving rise to the epidermis; plerome, a central core of tissue giving rise to the vascular cylinder and pith; and ’ periblem, tissue lying between derrnatogen and plerome, that gives rise to. cortex. It is now evident that respective roles assigned to these histogens are by” no means universal; nor can periblem and plerome always be distinguished, especially in the shoot apex. Becoming widely accepted is the tunica-corpus concept, an interpretation of the shoot apex recognizing two tissue zones in the promeristem: tunica, consisting of one or more peripheral layers, in which the planes of cell division are predominantly anticlinal, enclosing the corpusor central tissue of irregularly arranged cells in which the planes of cell division vary. No relation is implied between cells of these two regions and differentiated tissue behind the apex as in the histogen
concept. Although epidermis arises from the outermost tunica layer, underlying tissue may originate inunica or in corpus, or in both, in different plant species. Besides providing for growth in length of main axis, apical meristem of stem is the site of origin of leaf and bud primordia. In roots, two types of apical meristem occur, one in which vascular cylinder, cortex and root cap can be traced to distinct layers of cells in the promeristem, and a second type in which ali tissues have a common origin in one group of
promeristem cells. In contrast to those of stems, apical meristems of roots provide only for growth in length, lateral roots originating some distance from apex and, endogenously, from pericycle.

APOCARPOUS. (Of the gynoecium of flowering carpels, e.g. buttercup. See FLO WER. plants) hav.ia ng separate