The Facts On File Dictionary of Botany

amino acidsalpha helix A highly stable structure inwhich peptide chains are coiled to form aspiral. Each turn of the spiral contains approximately3.6 amino acid residues. TheR group of these amino acids extends outwardfrom the helix. Hydrogen bondingbetween successive coils holds the helix together.If the alpha helix is stretched thehydrogen bonds are broken but reform onrelaxation. The alpha helix is found inmuscle protein and keratin.alpha-naphthol test (Molisch’s test) Atest for detecting the presence of carbohydratesin solution. The test solution isplaced in a test tube, and a small amount ofalpha-naphthol added; concentrated sulfuricacid is then trickled slowly down theside of the tube. If carbohydrate is present,a violet ring will form at the junction of theliquids.alpine Describing a BIOME (regionalcommunity) of plants above the treelineand below the snowline on high mountains.The lower limit of the alpine zonevaries in different mountain regions, accordingto the rainfall and other climaticand topographic factors, from 100 metersabove sea level in parts of Scotland to 3700meters in the western Himalayas. There areoften considerable differences between thenature and distribution of vegetation onnorth- and south-facing slopes and betweenwindward and leeward slopes.Alpine vegetation is often similar to TUN-DRA vegetation, being adapted to harsh climateconditions, including high windspeeds.alternate Describing a leaf arrangementin which there is only one leaf at each node,as in hazel. This is the commonest form ofleaf arrangement. See phyllotaxis.alternate host Any host other thanthe main (most common) one. For example,many rusts overwinter on alternatehosts. Some parasites and pests are able tolive on more than one host, and someneed a second host to complete their lifecycle.alternation of generations The occurrenceof two, or occasionally more, generationsduring the life cycle of an organism.In all plants and some algae there is an alternationbetween sexual haploid andasexual diploid stages. They usually differmarkedly in morphology. The haploidplant produces gametes mitotically and isthus termed the gametophyte while thediploid plant produces spores meioticallyand is called the sporophyte. The gametesfuse to form a zygote, which develops intothe sporophyte, and the spores germinateand produce the gametophyte, so forminga cycle. In bryophytes (mosses, liverwortsand hornworts) the haploid gametophyte isthe dominant phase of the life cycle and thesporophyte is represented only by the capsule,seta, and foot. In vascular plants thediploid sporophyte is the dominant phaseand in the ferns, for example, the gametophyteis a small prothallus. The concept ofan alternation of generations can be extendedto the flowering plants, in which theembryo sac and pollen represent the muchreduced female and male gametophyte generationsrespectively.ameboid Describing an organism thatresembles an ameba in shape and movement.amensalism An association betweentwo different species, at either the level ofthe individual organism or the populationlevel, in which one is harmed and the otheris unaffected. Compare commensalism;mutualism.amino acids Compounds containingboth carboxylic acid and amino groups intheir molecules. The amino acids have thegeneral formula RCHNH 2 –COOH. Here,the group R ranges from a simple hydrogenatom to complex ring structures. All aminoacids are white, crystalline, soluble inwater, and with the sole exception of thesimplest member, glycine, all are opticallyactive.Amino acids are the basic componentsof proteins, which consist of chains ofamino acids. Plants also contain some 20011