THE ORDER CENTROSPERMAE

Perhaps no other order of flowering plants of its size is as well investigated morphologically, ultrastructurally, and chemically as is the Centrospermae. The betalain pigment discoveries of the early 1960s were followed by the more recent discovery of unique protein depositions in the sieve-element plastids of members of this order. These and other molecular data, including DNA-RNA hybridization results, have permitted a circumscription of the order based on 11 core families, including all 9 betalain families: Aizoaceae, Amaranthaceae, Basellaceae, Cactaceae, Chenopodiaceae, Didiereaceae, Nyctaginaceae, Phytolaccaceae, and Portulacaceae, as well as two anthocyanin families: Caryophyllaceae and Molluginaceae. Several smaller betalain taxa (including Gisekia, Halophytum, Hectorella, and Dysphania) which are sometimes treated as independent families or as members of one of these 11 core families also clearly belong to the order. Other families such as the Bataceae, Gyrostemonaceae, Vivaniaceae, and Theligonaceae are excluded from the Centrospermae. The betalain evolutionary line of centrospermous families may have originated from a centrospermous ancestor which lost the ability to produce anthocyanins and then subsequently gained the two or three steps required to produce betalains. Pollen morphology of centrospermous taxa and the widespread occurrence of C4 photosynthesis in the Centrospermae are also discussed.