The floral fragrance glands (osmophores) of 18 species of Stanhopea and Sievekingia were examined through a series of developmental studies at light and electron microscope levels including late bud stages through postanthesis. Various characters were identified to be of potential systematic value and were recorded for each species. These characters included: texture of the osmophore surface, number of distinct cell layers comprising the osmophore, nature of lipid inclusions in osmophore cells, and presence or absence of plastoglobuli in osmophore amyloplasts. These characters were combined with traditional features of floral lip morphology for cladistic analysis. Sievekingia was the postulated outgroup. Stanhopea ecornuta showed the largest number of plesiomorphic characters. Stanhopea pulla, S. annulata, and S. candida were only slightly more derived. Stanhopea anfracta, S. gibbosa, S. martiana, S. oculata, S. radiosa, S. ruckeri, S. saccata, S. shuttleworthii, S. tigrina, S. vasquezii, and S. wardii form a monophyletic group that can be recognized by a labellum with an articulated epichile and a bicornuate mesochile (or hypochile). Stanhopea tricornis may be a hybrid between a species of Sievekingia and Stanhopea.
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