Structural and histochemical characterisation of the colleters of Rodriguezia venusta (Orchidaceae)

Colleters are secretory structures present in many dicotyledon families, but they have not yet been reported for the Orchidaceae. However, the orchid Rodriguezia venusta was observed to possess secretory trichomes on the adaxial side of the bracts subtending floral buds that can be interpreted to be colleters. The present work describes the anatomy and histochemistry of these structures. These bracteal trichomes are multicellular, uniseriate, and densely spaced near the region where the bract attaches to the inflorescence. The secretions of these trichomes are mucilaginous and apparently exclusively constituted by carbohydrates, and they appear to act as a lubricant that facilitates the young floral bud to slide over the bract as it grows. In light of these characteristics, it is proposed that these secretory trichomes should be considered to be ‘colleters’.