A Dictionary of Biology [antikvár]

ABAXiAL (Dorsal). (Of a leaf surface), facing away from the stem. Cf. Adaxial. abdomen. In vertebrates: region of the body containing the viscera other than heart and lungs (i.e. intestine, liver, kidneys, etc.); in mammals, but not in other vertebrates, bounded anteriorly by diaphragm. In arthropods: posterior group of segments similar to each other. ABDUCENS NERVE. Sixth cranial nerve of vertebrates. Almost entirely motor, supplying external rectus eye-muscle. See Eye-muscles. A ventral root. ABIOGENESIS. Spontaneous generation (q.v.). absgisig acid (abscisin, dormin). Grovrth-inhibiting plant hormone (a sesquiterpene). Present in a variety of plant organs -leaves, buds, fruits, seeds and tubers. Promotes senescence and abscission of leaves, induces dormancy in buds and seeds. Antagonizes influences of growth promoting hormones. Believed to act by inhibiting nucleic acid and protein synthesis. abscission layer. Layer at base of leaf-stalk in woody dicotyledons and gymnosperms, in which the parenchyma cells become separated from one another through dissolution of the middle lamella before leaf-fall. ABYSSAL. Inhabiting deep water (roughly below i,ooo metres). acanthodii. Group of fossil fish, the earliest known Gnathostomes, mainly Devonian, 350 to 400 million years ago, but lasted from Silurian to Permian. Bony skeleton. Spines at front margin of fins, and a row of spines between pectoral and pelvic fins. Heterocercal tail. acarina (acarida). Order of Arachnida including mites and ticks; some are important parasites, e.g. Sarcoptes causing scabies, cattle tick carrying redwater fever. accessory nerve. Eleventh cranial nerve of tetrapod vertebrates. Really a branch of the vagus, clearly separate only in mammals. acclimation, acclimatization. Slow change in the physiology of an organism, as a result of its exposure to a changed en-vhronment (e.g. a lowered temperature), which improves its ability to adjust to the new conditions. accommodation. Changing the focus of the eye. In man and a few other mammals occurs by changing curvature of lens; at rest, lens is focused for distant objects; it is focused for near objects by becoming more convex with the contraction of the ciliary muscles in ciliary body (q.v.) (see Fig. 3, p. 107). Few mammals can kaccommodate. Most birds and reptiles accommodate by changing curvature of the lens; in fish and amphibians, lens is moved back-