Floral morphology and pollination mechanisms in species of Oxypetalum R. Br. (Asclepiadoideae, Apocynaceae)

Abstract

Morphoanatomical and histochemical studies were carried out to better understand the floral morphology of Asclepiadoideae species and associate it to their reproductive strategies. This study, was analyzed the functional morphology of the flowers of seven Oxypetalum species in order to describe its pollination mechanisms. Floral structure was studied with fixed flowers following classical techniques. Besides, were observed the pollen tubes pathways in hand-pollinated flowers. The number of elements in each floral whorl and fusions of the elements follow the pattern observed in other Asclepiadoideae. Each pollination unit is delimited by a petal located oppositely to the anther slit, two adjacent corona elements and the gynostegium. The anther slit, formed by adjacent anther wings, represents a structure adapted to guide the pollinator mouthparts (guide rail), resulting in removal or insertion of pollinia. The nectar accumulates at the base of each petal and thus increases the chances of the mouthparts being “caught” by the anther slit during its retraction. The pollinium is inserted into the stigmatic chamber (hyperstigma), located behind the anther slit and formed by interstaminal tissue; each chamber is lined by secretory tissue (glandular epithelium, nectary). In the chamber, the pollen germinates and pollen tubes grow, following the glandular epithelium, towards the gynoecium. In the gynoecium of six species, the pollen tubes penetrate and obliterate the transmitting tissue of the solid style. Then obliterate the transmitting tissue of the hollow style corresponding to the solid style and reach the glandular epidermis of the placenta of the ovary corresponding; one follicle is produced per flower.