An exine architecture model for viscin threads

Abstract

Abstract Pollen grains with adhering viscin threads only occur in three unrelated angiosperm families (Onagraceae, Ericaceae, Caesalpiniaceae). Viscin threads consisting of sporopollenin are long, thin, flexible ropes on the pollen surface, and extended between adjoining single pollen grains or pollen tetrads; they should be seen as parts of the exine. The model presented deduces that the manifold viscin thread configurations and sculptures may be attributed to a single, simple exine architecture principle: Distally branched—either smooth or sculptured—columellae form rib-vaultings or arches; if the “ribs” (i.e. the columellar branches) are thick and at some distance from one another, they form muri, as in the Caesalpiniaceae; if the “ribs” are thin and close together, they form a ± compact “tectal network” like in the two other families. Some columellar branches do not contribute to the vaultings, but are enormously elongated, forming simple or (by distortion or by entanglement) complex viscin threads. A...