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
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
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