POLLEN MORPHOLOGY AND EVOLUTION IN HEDYOTIS SUBGENUS EDRISIA (RUBIACEAE)

Primarily on the basis of aperture structure, 31 species of Hedyotis subg. Edrisia (Houstonia) are separable into 5 palynological groups. Group 1 is characterized by a simple os circumscribed by varying nexinous thickenings, groups 2–4 combine this os with a crassimarginate one to form a compound os, while group 5 is known only with the crassimarginate os. The first type is considered primitive, the compound os specialized and more advanced, and the third type reduced and highly advanced. Other characteristics of the pollen, including size and shape, aperture number, thickness of sexine and nexine, and reticulum, are discussed in relation to the apertures, and their probable primitive and advanced expressions are outlined. The data agree with the phyletic trends found in the sporophyte on the basis of results from morphology, chromosome number and size, and distribution, and they support and add to an earlier phylogenetic scheme proposed for the subgenus. The evidence from palynology also supports the treatment of Hedyotis and Houstonia as congeneric.