Thread-forming structures in angiosperm anthers: their diverse role in pollination ecology

Abstract

This paper reviews the origin, nature, systematic distribution, and the respective function of the highly variable and diverse thread-forming structures in angiosperm anthers (including some- what similar, rare features in ferns and gymno- sperms). On one hand, such threads may function as pollen-connecting vectors in forming pollen dispersal units, as sporopollenin threads (viscin threads), e.g. in Onagraceae, or sporopollenin-less threads in surprisingly many other angiosperm families. On the other hand, as is known from the Impatiens- "pollen basket", threads or ropes may be involved in pollen presentation. In addition, for the first time two new examples of "pollen baskets" in Boraginaceae and Scrophulariaceae are reported. In Echium the basket is formed by cellular elements from the modified septal regions, whereas in Esterhazya a similar effect is achieved in an analogous manner by trichomes of the epidermal layer of the thecal wall. There is obviously a different function of these seemingly very similar baskets: in Echium the feature acts preferably as a pollen presentation agent, whereas in Esterhazya the primary function is to prevent all the pollen from being dispersed too soon.