The Facts On File Dictionary of Botany

air bladderstage of the chain is used to form ATP duringa coupling process (see oxidative phosphorylation).There is a net production of38 ATPs (2930 kJ) per molecule of glucoseduring aerobic respiration, a yield of about19 times that of anaerobic respiration (300kJ), and the mechanism of the majority oforganisms. Compare anaerobic respiration.aerotaxis (aerotactic movement) ATAXIS in response to an oxygen concentrationgradient. For instance, motile aerobicbacteria are positively aerotactic, whereasmotile obligate anaerobic bacteria are negativelyaerotactic. Some photosynthetic eukaryoticunicells are also aerotactic.aerotropism A TROPISM in which theorientating stimulus is oxygen.afforestation The establishment of foreston land not previously forested, eitherby natural succession or by planting.aflatoxin One of a group of carcinogenictoxins produced by fungi of the genusAspergillus, especially A. flavus. Thisspecies is a common contaminant of crops,such as peanuts. Contaminated feed causesserious outbreaks of disease among livestock.after-ripening The collective name forprocesses that are necessary before germinationcan take place in certain seeds, eventhough external conditions may be suitable.For example, a period of dormancymay be imposed on the seed, preventingpremature germination before an unfavorableseason such as winter. After-ripeningis common in areas with marked seasonality.agamospermySee apomixis.agar A gelling agent prepared from seaweed(normally a red alga, Gelidium), usedto set liquid nutrients. It can withstandsterilization at high temperatures and is resistantto attack by most bacteria. Agargels are extensively used for growingmicroorganisms and tissue cultures.Agaricales (agarics) An order of basidiomycetesthat contains the mushroomsand toadstools. The cap of the fleshy fruitingbody – the mushroom or toadstool –bears a series of parallel gills on its lowersurface that greatly increase its surfacearea. These are covered in the sporebearinglayer (hymenium). Agarics arecommon in leaf litter and rotting wood,where they live as saprobes.aggregate fruit (compound fruit) A fruitlikestructure that is made up of several individualfruits derived from the carpels of asingle flower, e.g. strawberry (Fragaria),blackberry (Rubus fruticosus). These maybe achenes, follicles, berries, or drupelets.An aggregate fruit is sometimes called anetaerio, but this term may also be restrictedto an aggregate of drupelets. A fruit thatdevelops from a group of flowers is termeda multiple fruit, e.g. pineapple (Ananas comosus)and hop (Humulus lupulus).Agrobacterium A genus of soil bacteria,the species A. tumefaciens being thecausative agent of CROWN GALL, a type oftumor in plants. A segment of DNA (transferredDNA, T-DNA) from a PLASMID inthe bacterium is transferred into the hostDNA and induces tumor formation. Sincethe plasmid is capable of independentreplication in host cells of many dicotyledonousplants, it has been used as a cloningvector in GENETIC ENGINEERING.agroforestry A system of cultivationcommon in many parts of the tropics, especiallyin rainforest regions, in whichforestry and arable farming are mixed.This reduces soil degradation, as freshlytilled soils are shaded and protected fromheavy rains and run-off by the trees. Thetrees help to draw nutrients up from thedeeper soil layers into the zone occupied bythe roots of the crop plants.agronomyA branch of agriculture deal-7