Osmophores of Stanhopea (Orchidaceae)

Species of the Neotropical orchid genus Stanhopea produce a fragrance comprising terpenoids and aromatics which attracts euglossine bee pollinators. The secretory tissue, called an osmophore, is located in the adaxial region of a sac formed near the proximal portion of the floral lip. This region is easily recognized in Stanhopea oculata and S. wardii because it is papillate. The osmophore in these two species includes all the cells of the papillae and those directly below, that grade into fundamental tissue. Osmophore cells are more densely cytoplasmic than cells in the adjacent tissue. Numerous amyloplasts and mitochondria are seen in these cells from the earliest bud stages we examined through anthesis. Smooth and rough endoplasmic reticulum are abundant, but dictyosomes are uncommon. Mitochondria of osmophore cells appear to be distributed with no apparent pattern during bud stages, although they tend to be aligned near the plasmalemma at anthesis. Osmophore cells are highly vacuolate after anthesis. STANHOPEA Frost ex Hooker is a Neotropical genus of Orchidaceae consisting of about 50 species. Members of Stanhopea are epiphytic herbs with ovoid or subcylindric, ribbed, unifoliate pseudobulbs (Fig. la). Leaves are plicate, elliptic to narrowly elliptic, cQnspicuously nerved, and distinctly petiolate. Inflorescences are sharply pendent from the base of the pseudobulb (Fig. 1-3) and bear 2-15 large, showy, short-lived, intensely aromatic flowers (Fig. 1, 4). The median sepal is coriaceous, erect, and free; lateral sepals are connate at their lower margins and adnate to the lip. Petals are narrower and less coriaceous than the sepals, and usually are reflexed strongly between them. The lip (Fig. lb, 4, 5) is thick, fleshy, glossy, and usually divided into a basal, saccate hypochile, a mesochile bearing conspicuous horns, and a flattened, distal epichile. The fragrance emitting portion of the flower, or osmophore, of S. oculata and S. wardii consists of a papillate layer of tissue situated in the pouch of the hypochile (Fig. lb, 5, 6). The column is elongate, usually broadly winged, and arched toward the lip. The anther is terminal with two pollinia. I Received for publication 1 April 1986; revision accepted 29 December 1986. We acknowledge Terry W. Lucansky, University of Florida, and Norris H. Williams, Florida State Museum, for critical reading of the manuscript and helpful suggestions for improvement; Ann L. Curry, University of Florida, for her generous technical assistance; and Alan A. Atchley, USDA, Beltsville, Maryland, for supplying classical literature. This study was supported in part by a grant from the National Science Foundation, DEB 82-19120. We recognize substantial assistance from the Florida Agricultural Experiment Station of which this paper is number 7209 in the Journal Series. Stanhopea is one of several Neotropical orchid genera pollinated by euglossine bees. There are published reports of the biology of these plants and their pollinating bees, of the chemistry of the compounds (mostly terpenoids and aromatics; Williams and Whitten, 1983) comprising the fragrances emitted by the flowers, and of the interactions between bee and flower. Dodson et al. (1969) and Dressler (1968, 1982) have studied the euglossine-bee-pollination syndrome in orchids. They reported that only male bees are associated with pollination, and that bees are attracted to the flowers by the strong fragrances they collect, presumably as a percursor for a sex pheromone they synthesize. Osmophore structure has been studied in orchids with light microscopy by Vogel (1 96 3), with scanning electron microscopy by Williams (1983), and with both scanning and transmission electron microscopy by Pridgeon and Stern (1983, 1985). The term osmophore (osmoforo from Gr. osmo, odor, and pherein, to bear) was first used by G. Arcangeli (1883) who applied it to the fragrant spadix of certain members of the Araceae (e.g., Dracunculus vulgaris, Amorphophallus rivieri). The external morphology of the osmophores in Stanhopea and other euglossine-bee-pollinated taxa varies from the numerous mounds of tissue in Clowesia and the trichomes of Polycycnis to the papillae in some species of Stanhopea and smooth tissue of other species. Williams ( 19 8 3) noted that in this group of orchids "the structure of the osmophore region is quite variable from species to species, and from genus to genus. "