The genus Phyllanthus (Euphorbiaceae), with over 800 described species of trees, shrubs, and herbs, has a pantropical distribution. To better understand the range of pollen morphology and its evolution in the neotropical taxa, and to elucidate their relationships, 22 species from 11 sections have been examined with scanning electron microscopy. Special attention has been paid to the subgenus Conami because of its apparent close affinity to the African subgenus Kirganelia, and the variation in apertures and exine sculpturing among closely related species. Of particular interest are clypeate pollen grains, in which the exine is differentiated into polygonal shields. These are best known in the large subgenus Xylophylla, with over 80 species; however, a unique type of clypeate grain has evolved, apparently independently, in section Cyclanthera. Indeed, the extraordinary grains of P. lindenianus appear to be unique in the Euphorbiaceae and in all seed plants. Phylloclade-bearing species of section Xylophylla in the West Indies have clypeate pollen grains, while phylloclade-bearing Brazilian species (section Phyllanthus) have 3-colporate pollen grains with reticulate exine; the pollen evidence suggests the convergent evolution of similar vegetative morphologies in neotropical Phyllanthus.

Key words: Euphorbiaceae, Phyllanthus, phylogeny, pollen morphology, scanning electron microscopy