The taxonomy, ecology and evolution of C3, C4 and CAM plants The majority of flowering plant species are C3 species, with C4 plants accounting for only about 1%, while species exhibiting CAM have been estimated at 5–10%. In assessing their importance, however, their overall biomass as well as species numbers must be considered, and in fact C4 and CAM plants are of considerable importance ecologically and to some extent also agriculturally. Both groups of plants are confined to a limited number of families and also have characteristic ecological distributions. C4 photosynthesis is particularly associated with tropical and semitropical grasses. Sixty per cent of all C4 species belong to the Poaceae (grasses including cereals); this includes the two very important and highly productive crops, maize (Zea mays) and sugarcane (Saccharum officinarum). It also includes eight grasses considered at least by some authors to be among the world’s most troublesome weeds. Another 32% of C4 species can be found in five other families: Amaranthaceae, Asteraceae, Chenopodiaceae, Cyperaceae and Euphorbiaceae, with the remainder scattered as odd genera or species amongst another dozen families. No flowering plant family is known to consist exclusively of C4 species. Almost all C4 species are natives of warm habitats; few are found in temperate zones and none in arctic regions. C4 plants are, as stated earlier, adapted to take advantage of high temperatures and high irradiance and to conserve water, which also can run short in warm climates CAM plants are found in about 30 flowering plant families, although 80% of the species belong to five families: Cactaceae and Crassulaceae (consisting exclusively of CAM species), Aizoaceae, Bromeliaceae, and Orchidaceae. Some families include examples of both CAM and C4 plants, e.g. Asteraceae and Euphorbiaceae
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