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 in Bulbophyllum are rare. Here, we investigate the structure of the labellum and secretion of labellar food-rewards for ten Asian species of the genus, representing six sections. Labellar tissue of each of these species was examined using LM, SEM, and TEM, and compared with our previous results for representatives of sect. Racemosae and two African species of Bulbophyllum. The study indicated that only members of sections Sestochilos and Stenochilus produced true nectar, and this was present on the adaxial surface of sepals. Lipids occurred only in the labellar secretions of members of sect. Recurvae. In members of sections Sestochilos, Brachyantha and Desmosanthes, labellar secretion was absent. In most investigated sections, secretion, if present, was produced by relatively unspecialized epidermal cells lining a median labellar groove. The labella of most sections contained both ground parenchyma and aerenchyma. This study demonstrates, in conjunction with the results of previous studies, that the structure of the labellum in this genus is highly diverse, especially with respect to the labellar adaxial epidermis, which represents various degrees of secretory specialization.