Comparative account of nectary structure in Hexisea imbricata (Lindl.) Rchb.f. (Orchidaceae)

Despite the number of orchid species that are thought to be pollinated by hummingbirds, our knowledge of the nectaries of these orchids is based solely on a single species, Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge. Nevertheless, it is predicted that such nectaries are likely to be very diverse and the purpose of this paper is to compare the nectary and the process of nectar secretion in Hexisea imbricata (Lindl.) Rchb.f. with that of Maxillaria coccinea so as to begin to characterize the nectaries of presumed ornithophilous Neotropical orchids.

Light microscopy, transmission electronmicroscopy and histochemistry were used to examine the histology and chemical composition of nectary tissue and the process of nectar secretion in H. imbricata.

The nectary of H. imbricata has a vascular supply, is bound by a single-layered epidermis with few stomata and comprises two or three layers of subepidermal secretory cells beneath which lie several layers of palisade-like parenchymatous cells, some of which contain raphides or mucilage. The secretory cells are collenchymatous and their walls have numerous pits with associated plasmodesmata. They contain the full complement of organelles characteristic of secretory cells as well as intravacuolar protein bodies but some of the secretory epidermal cells, following secretion, collapse and their anticlinal walls seem to fold. Nectar secretion is thought to be granulocrine and, following starch depletion, lipid droplets collect within the plastids. The nectar accumulates beneath the cuticle which subsequently forms swellings. Finally, nectar collects in the saccate nectary spur formed by the fusion of the margins of the labellum and the base of the column-foot. Thus, although the nectary of H. imbricata and M. coccinea have many features in common, they nevertheless display a number of important differences.