Pollen Wall Ultrastructure and Its Bearing on the Systematic Position of Blennosperma and Crocidium (Compositae)

Electron microscopy is used to help resolve the question of the taxonomic relationships of the genera Blennosperma and Crocidium. Ultrastructural features of their pollen walls indicate that these taxa, long placed in the tribes Helenieae and Senecioneae, respectively, are closely related and that their tribal affinities are with the Senecioneae. Additional ultrastructural observations are made for 40 or more species in the Helenieae and Senecioneae. Three major pollen wall types can be recognized: Anthemoid, Helianthoid and Senecioid. The distribution and significance of these pollen types among the Astereae, Heliantheae, Helenieae, Anthemideae and Senecioneae are discussed.