Studies in eupatorium (Compositae), III. Apparent wind pollination

Eupatorium is a large, chiefly American genus in which some species have deviated from the usual reproductive methods of insect pollination and outcrossing. Apomixis and autogamy are two reproductive specializations which have been reported previously. A third, anemophily, is suspected inE. solidaginifolium, E. solidaginoides, E. monanthum, and several other species. Our assumptions are based upon morphological modifications of the inflorescence, anther appendages, style branches, and pollen. Wind pollination in the Compositae has heretofore been known only in the tribe Anthemideae and the subtribe Ambrosinae of the Heliantheae; its occurrence inEupatorium is an independent evolutionary event. The occurrence of apomixis, autogamy, and anemophily in members ofEupatorium from the same geographical region probably represents (at least in part) correlated responses to the same environmental stress, a scarcity of insect pollinators.