STRUCTURE AND FUNCTION IN ASCLEPIAD POLLINATION

Aspects of floral biology in relation to floral structures are investigated in seven species of Asclepiadaceae. Pollinaria removal and insertion by the insect pollinator is aided by various structural devices such as hairs or corona elements guiding the proboscis towards the entrance of the anther slit. The nectary is situated directly below this opening, thus increasing the probability of the proboscis being introduced into the anther slit during retraction. The anther slit itself often displays structural differentiation in adaptation to the guiding of the proboscis and the insertion of a pollinium in cross pollination. These mechanisms are highly elaborated so that they work as well in closed flower types, in which the pollinator does not enter the flower directly, but only introduces its proboscis (Sisyranthus, Microloma). A special feature of pollinaria removal is the variety of ways of fixing the corpusculum against the force of the proboscis when it is drawn into the cleft. - The stigma of all species except Secamone is located on the lower surface of the stigma head where it is appressed to the filament tube. In some cases a direct contact of the inserted pollinium with the stigmatic surface does not seem to be necessary for pollen tube germination, probably moistening by nectar suffices in these flowers. With Tylophora spec., a case of in situ pollen tube germination has been studied. - Concerning the path of the pollen tubes, two different strategies are followed. Secamone and Tylophora develop a structurally defined compitum, while the other five species have strictly divided stylar canals. In these higher evolved species, the pollen tubes of one inserted pollinium are all directed into one ovary only, thus enabling the plant to allocate its resources to one carpel only in the frequent case of fertilization by one pollinium.