The stigmatic orifice in Cassia , Senna , and Chamaecrista (Caesalpiniaceae): morphological variation, function during pollination, and possible adaptive significance

In species of Cassia, Senna , and Chamaecrista an orifice leading into a stigmatic cavity is present at the tip of the style. The size of the orifice, the occurrence and morphology of surrounding trichomes or papillae, and the occurrence of a drop of secretion at its outlet were recorded in 17 species belonging to the three genera of the subtribe Cassiinae. Much of the observed variation in these characters apparently reflects different modes of pollen capturing and varying degrees of secretion exposure. In some of the species studied no secretion was noticeable at the entrance to the stigmatic cavity. The unusual type of stigma in the Cassiinae does not fit into any of the recognized main groups of angiosperm stigmas. It is conjectured that in the buzz‐pollinated Cassiinae species with small stigmatic orifices, the secretory material is retained within the stigmatic cavity, to be extruded by vibratory movements of the style induced by a buzzing bee. It is also hypothesized that in the wet tropics, concealment of secretory materials within a cavity prevents their washing away or dilution by rain. Furthermore, in the Cassiinae, the considerable variation in features of the style and of the stigma may function as mechanisms isolating between sympatric, visitor‐sharing, concurrently blooming species.