Floral glands in asclepiads: structure, diversity and evolution

ABSTRACT

Species of Apocynaceae stand out among angiosperms in having very complex flowers, especially those of asclepiads, which belong to the most derived subfamily (Asclepiadoideae). These flowers are known to represent the highest degree of floral synorganization of the eudicots, and are comparable only to orchids. This morphological complexity may also be understood by observing their glands. Asclepiads have several protective and nuptial secretory structures. Their highly specific and specialized pollination systems are associated with the great diversity of glands found in their flowers. This review gathers data regarding all types of floral glands described for asclepiads and adds three new types (glandular trichome, secretory idioblast and obturator), for a total of 13 types of glands. Some of the species reported here may have dozens of glands of up to 11 types on a single flower, corresponding to the largest diversity of glands recorded to date for a single structure.

Keywords
anatomy; Apocynaceae; Asclepiadoideae; diversity; evolution; flower; secretory structures