VARIATION IN POLLEN WALL ULTRASTRUCTURE IN NEW-WORLD SENECIONEAE (ASTERACEAE), WITH SPECIAL REFERENCE TO PACKERA

The structure of the pollen wall as revealed by transmission electron microscopy is presented for 34 species representing two subtribes and 12 genera of New World Senecioneae. The genus Packera (=aureoid Seneci o), with the exception of Packera zimapanica, is characterized by the helianthoid wall structure. In light of these results, the disposition of the latter species requires review. The genera Robinsonecio and Te lanthophora of the subtribe Tussilagininae also possess helianthoid po llen. All other taxa surveyed have senecioid pollen. So far as known n o taxa exist within the tribe Senecioneae with a base chromosome numbe r of n = 20 and helianthoid pollen. This suggests that the evolution o f Packera may have involved hybridization between members of the two s ubtribes Senecionineae and Tussilagininae.