The Facts On File Dictionary of Botany

activation energyrainfall, which causes leaching of limefrom the surface layers, or over acid substratasuch as granite or sand. In the acidicconditions, decomposition of organic materialin the soil is slow. Acid soils oftencontain substantial concentrations of ironand aluminum hydroxides. See brownearth; podsol.Acrasiomycota (cellular slime molds) Aclass of protoctists in the phylum Rhizopoda.Formerly considered to be fungi,they are made up of independently livingamebas that feed on bacteria, which theyingest by phagocytosis. Under conditionsof food shortage or other stresses, theycome together to form a slug-like aggregationof amebas called a pseudoplasmodium,which acts like an individualorganism.acrocarpous Describing mosses inwhich the reproductive organs are borne atthe top of the main axis, which is usuallyerect, so that subsequent growth is sympodial.acrocentricSee centromere.acropetal Describing a process (such asgrowth or development) that progressesfrom the base or point of attachment, sothat the oldest parts are at the base and theyoungest are at the tip. Compare basipetal.actin A globular contractile protein (Gactin)that makes up 10–15% of the totalcell protein in eukaryotic cells. It is a majorcomponent of the CYTOSKELETON. It alsocontrols motility, cellular movements,and cyclosis. G-actin can polymerize intohelical strands that coil together to formmicrofilaments called fibrous actin (Factin).Actinobacteria (ray fungi; actinomycetes)A phylum of Eubacteria that contains thetrue actinobacteria and the coryneformbacteria. The true actinobacteria formbranching filaments that resemble smallfungal mycelia, with hyphae usually lessthan 1.5 m m in diameter; the coryneformbacteria are rodlike structures. TheActinobacteria are distinguished by producingactinospores: entire cells encystedin thick walls to form resistant spores.actinodromous (palmate; digitate) Describinga form of leaf VENATION in whichthree or more primary veins radiate fromthe base of the lamina toward the margin,as in sycamore (Acer pseudoplatanus), resultingin a leaf that has several large lobesor several leaflets all originating at thesame point.actinomorphyactinomycetesSee radial symmetry.See Actinobacteria.actinostele A type of protostele inwhich the xylem is star shaped and thephloem lies between the points of thestar. Protosteles are found in the primaryroots of higher plants, and in some speciesof Lycopodium and Psilotum. Seestele.action spectrum A graph showing theeffect of different wavelengths of radiation,usually light, on a given process. It is oftensimilar to the absorption spectrum of thesubstance that absorbs the radiation andcan therefore be helpful in identifying thatsubstance. For example, the action spectrumof photosynthesis is similar to the absorptionspectrum of chlorophyll. It showsthe net assimilation plotted against wavelength.See absorption spectrum. See illustrationoverleaf.activation energy The minimum extraenergy that must be put into a system to enablea reaction to occur. It increases the energylevels of participating molecules, andhence raises their reactivity, often by raisingelectrons to an excited state. ENZYMESenhance reaction rates by lowering the activationenergy. This is often achieved bythe attraction of the reactants to the enzyme’sactive site, thus coming into closerproximity, or by the enzyme causing conformationalor electrostatic changes in oneor more of the reactants.3