Comparative anatomy of putative secretory floral structures in the camaridium cucullatum complex and nitidobulbon (orchidaceae: Maxillariinae)

Abstract

Most members of the Neotropical orchid subtribe Maxillariinae lack floral rewards (e.g. nectar, food-hairs, resin or wax) and attract pollinators by food- or sexual-deceit. Of those that produce floral rewards, relatively few produce resin. Members of the Mexican and Central American Camaridium cucullatum complex are considered to be entirely rewardless. Here, we demonstrate that they produce lipoidal or resin-like labellar secretions. They are also characterized by the possession of a prominent, and often brightly coloured, thickened transverse ridge located at the base of the column, above the point of attachment to the labellum. Although the transverse ridge is considered a putative synapomorphy for members of the C. cucullatum complex, a similar structure was observed in the related genus Nitidobulbon. Anatomy and secretory activity of the labella and presumed transverse ridges of both taxa are investigated and compared by means of light and fluorescence microscopy, in conjunction with histochemistry and scanning and transmission electron microscopy. Of the representatives of core Maxillariinae investigated, only members of the C. cucullatum complex possessed a transverse ridge, strengthening claims for this synapomorphy. The similar structure in Nitidobulbon comprised the proximal part of the column. Because both these structures were non-secretory and located in juxtaposition to the secretory region of the labellum, where their bright colours contrasted with those of the perianth segments, it is proposed that they act as focal points, guiding insect visitors further into the flower, and thus facilitating pollination. It is also hypothesized that they compensate for a lack of obvious nectar-guides on the labellum, or where these are obfuscated by heavy secretions of resin. Because the transverse ridge was not prominent in some members of the C. cucullatum complex, and not always easily distinguishable from superficially similar structures in related genera (e.g. Nitidobulbon), we recommend it be used in taxonomy only in conjunction with other more reliable characters.