POLLEN-CONNECTING THREADS IN GYMNOCALYCIUM (CACTACEAE) - THEIR ORIGIN, FUNCTION, AND SYSTEMATIC RELEVANCE

In dehisced anthers of all Gymnocalycium species (Cactaceae, subfam. C actoideae, tribe Trichocereae) investigated so far, a small percentage of pollen grains are aggregated by only a few threads. These threads are located at the margin of the dehiscing zone of stomium and septum. They are not part of the pollen grain wall, and they are not resistan t to acetolysis, thus indicating the absence of sporopollenin. The thr eads originate in part from modified stomium/septum cells (split cell walls, cytoplasmic remnants and lipid droplets) and in part from polle nkitt produced by the - now desintegrated - tapetal cells. This highly viscous, gum-like substance along the modified stomium and septum cel l walls may assume an irregular thread-like habit before or during deh iscence. Pollen aggregates may be formed frequently by different stick y or non-sticky agents. A detailed conspectus on origin, nature, signi ficance and function of various types of pollen-connecting agents form ing threads is presented. Pollen-connecting threads are normally effic ient pollination vehicles and are restricted to flowers pollinated by large insects or vertebrates. The genus Gymnocalycium is probably excl usively bee-pollinated, with most pollen connected by pollenkitt. The functional role of the threads in Gymnocalycium is uncertain: since on ly a few threads connect a small number of pollen grains per pollen-sa c they hardly play a significant role in pollination ecology. The thre ads most probably represent a character restricted to the genus Gymnoc alycium, as they have not been observed in closely related genera.