SYSTEMATIC SIGNIFICANCE OF POLLEN NUCLEAR NUMBER IN EUPHORBIACEAE, TRIBE EUPHORBIEAE

Pollen nuclear number is determined in 139 species of 5 genera in the Euphorbieae, subtribe Euphorbiinae. The 111 new determinations are tabulated along with previous reports, and the results indicate that the distribution of binucleate (II) and trinucleate (III) pollen is strongly associated with the taxonomic groupings within the Euphorbieae. Although binucleate pollen is probably primitive within the tribe Euphorbieae, as suggested by the nuclear condition in Neoguillauminia , the situation in Euphorbia still requires further elucidation. Within Euphorbia , the morphologically most primitive species studied have III pollen despite the fact that II pollen is presumably the original condition for the subtribe Euphorbiinae. In Euphorbia , II pollen only is reported from nine sections and III pollen only from ten sections, while in four sections ( Esula, Goniostema, Aphyllis , and Deuterocalli ) both II and III pollen have been found. The New World species of Euphorbia nearly all have III pollen, whereas the vast majority of the African succulents have II pollen. The genera of New World origin, Chamaesyce and Pedilanthus , have III pollen, while the African genera Monadenium and Synadenium have II pollen. Independent derivations of III pollen from II pollen appear to have occurred in sections Goniostema, Aphyllis , and Deuterocalli (all of subg. Euphorbia ). There is no evidence that reversals from III to II pollen have occurred.