Labellar secretory structures and pollinator food-rewards in representatives of Old World Bulbophyllum Thouars.

Detailed studies of the floral anatomy, morphology, ultrastructure, and the occurrence of floral food-rewards inBulbophyllum are rare. Here, we investigate the structure of the labellum and secretion of labellar food-rewardsfor ten Asian species of the genus, representing six sections. Labellar tissue of each of these species was examinedusing LM, SEM, and TEM, and compared with our previous results for representatives of sect. Racemosae and twoAfrican species of Bulbophyllum. The study indicated that only members of sections Sestochilos and Stenochilusproduced true nectar, and this was present on the adaxial surface of sepals. Lipids occurred only in the labellarsecretions of members of sect. Recurvae. In members of sections Sestochilos, Brachyantha and Desmosanthes, la-bellar secretion was absent. In most investigated sections, secretion, if present, was produced by relativelyunspecialized epidermal cells lining a median labellar groove. The labella of most sections contained bothground parenchyma and aerenchyma. This study demonstrates, in conjunction with the results of previousstudies, that the structure of the labellum in this genus is highly diverse, especially with respect to the labellaradaxial epidermis, which represents various degrees of secretory specialization.