The germination of pollinium and the organization of germ furrow in some members of Asclepiadaceae

Abstract

The asclepiad pollinium germinates through a specific region of the pollinial wall designated as the germ furrow. In untreated and ungerminated pollinia of Asclepias and Calotropis, the germ furrow is indistinguishable, by light microscopy, from the rest of the pollinial wall. Under the scanning electron microscope, however, the furrow appears as a discontinuous region marked by apertures of various sizes and shapes. On acetolysis, the wall materials that separate or surround the various openings on the germ furrow region dissolve and a characteristic narrow continuous opening (referred to as the slit) becomes visible.The position of the germ furrow, as revealed by the slit on acetolysis, is genus specific in all the 14 genera screened in this study. This specificity is maintained even in the relatively large pollinia of colchicine-induced tetraploids of Asclepias curassavica.By culturing sealed and cut pollinia and by standard bioassay methods it was shown that the polarity of pollen tubes within the pollinium is governed by the permeability of the germ-furrow region and not by the previously attributed factors such as innate orientation of pollen grains or chemotropic factors.