Experimental Evidence Concerning the Role of the Corpusculum in Asclepias Pollination

Abstract

Experimental manipulation of Asclepias tuberosa flowers indicates that intact pollinaria significantly increase successful insertions. The hypothesis that the groove of the corpusculum of an intact pollinarium serves as an attachment point for the translator arm is examined experimentally. Significantly fewer pollinia are inserted into flowers with the pollinaria removed than into flowers with the pollinaria intact. For complexity of floral morphology the Asclepiadaceae, or milkweed family, is rivalled only by the Orchidaceae. Yet the number of studies concerned with the pollination biology of the Asclepiadaceae have been comparatively few. Quantitative studies of the reproductive biology of Asclepias were attempted in the 1940's when potential commercial uses of milkweeds were investigated (Fischer, 1941; Whiting, 1943; Stevens, 1945; Moore, 1946, 1947; Sparrow and Pearson, 1948). Recent attempts to quantify and explain pollination biology in evolutionary terms (e.g., Faegri & van der Pijl, 1966; Heinrich & Raven, 1972; Proctor & Yeo, 1972) have again stirred interest in the reproductive biology of these plants. Reexaminations of the floral morphology of Secamone spp. and Asclepias curassavica L. by Safwat (1962) and Galil and Zeroni (1965, 1969), respectively, have dealt with the functional and evolutionary significance of the extremely complex floral modifications in the Asclepiadaceae. The exaggerated corona of Calotropis gigantea (Willd.) R. Brown has been explained by Wanntorp (1974) as an adaptation for pollination by carpenter bees (Xylocopa). Macior (1965) has shown that species of milkweeds may differentially attach pollinaria on various parts of a pollinator's body and has suggested that this type of mechanical isolating mechanism may have played a role in speciation in the genus. The floral ecology of two Californian species, A. solanoana Woodson and A. californica Greene, has recently been studied by Lynch (1977, in press), who has related rates of removal and insertion to pollinator behavior. A controversy has arisen surrounding the general assumption that there is no specific mechanism for pollinia insertion in asclepiads (Brown, 1833; Delpino, 1865; Hildebrand, 1866; Woodson, 1954). Wyatt (1976) has suggested that insertion, far from being a chance process, is effected by a very specific mechanism. Using A. tuberosa L., he attempted to demonstrate the operation of this mechanism by comparing the observed number of insertions into chambers flanked by intact pollinaria or into 1 Botany, Duke University, Durham, NC 27706; Present address: Biology, Texas A&M University, College Station, TX 77843.