Perliaps no other order of flowering plants of its size is as well investigated morphologically, nltrastructurally, and chemically as is the Centrospennae. Tlie betalain pigment discoveries of the early 1960s were followed by the moie recent discovery of nni^ine protein depositions in the sieve-element plastids of members of this order. These and otlier 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 Giaekia, lialoplnjtum^ Hectorella, and Dtjsphania) 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, G>'rostemonaceae, Vivaniaceae, and Thchgonaceae 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 occiirrence of Cj photosynthesis in the Centrospermae are also discussed.Since all the review papers from a symposium on the "Evolution of Centrospermous Families/' presented in July, 1975 during the Xllth International Botanical Congress, Leningrad, USSR, have now been published (Mabry & Behnkc, 1976a), a summary of our current views of the Centrospemiac (or Caryophyllales) will suffice in this review. This account will emphasize the way our interpretations of the order have been shaped by molecular data.Since 1876 when Eichler (see Table 1) introduced the name for tlie order, the Centrospermae have always contained a core of about 8-12 famihes. Eichler (1876 and, in part, 1878) recognized most of what we now consider to be centrospermous families including, for example, the Cactaceae (in the 1876 treatment).In the 100 years following Eichler's work, most systematists also included those families now generally recognized on the basis of molecular data as belonging to the order but often included additional ones (compare Tables 2, 3 and 4). The molecular data which bear upon our current treatment of the order are suiumarized in the following sections.
Sieve-element PLAsxmsOf all the modern approaches for investigating the Centrospermae, none has, in my opinion, contributed more to our understanding of the circumscription of the order than the ultrastructural investigations of the sieve-element plastids.