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
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)
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
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-§a 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
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. +