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

• Background and Aims Despite the number of orchid speciesthat are thought to be pollinated by hummingbirds, our knowledgeof the nectaries of these orchids is based solely on a singlespecies, Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge.Nevertheless, it is predicted that such nectaries are likelyto be very diverse and the purpose of this paper is to comparethe nectary and the process of nectar secretion in Hexisea imbricata(Lindl.) Rchb.f. with that of Maxillaria coccinea so as to beginto characterize the nectaries of presumed ornithophilous Neotropicalorchids. • Methods Light microscopy, transmission electronmicroscopyand histochemistry were used to examine the histology and chemicalcomposition of nectary tissue and the process of nectar secretionin H. imbricata. • Key Results and Conclusions The nectary of H. imbricatahas a vascular supply, is bound by a single-layered epidermiswith few stomata and comprises two or three layers of subepidermalsecretory cells beneath which lie several layers of palisade-likeparenchymatous cells, some of which contain raphides or mucilage.The secretory cells are collenchymatous and their walls havenumerous pits with associated plasmodesmata. They contain thefull complement of organelles characteristic of secretory cellsas well as intravacuolar protein bodies but some of the secretoryepidermal cells, following secretion, collapse and their anticlinalwalls seem to fold. Nectar secretion is thought to be granulocrineand, following starch depletion, lipid droplets collect withinthe plastids. The nectar accumulates beneath the cuticle whichsubsequently forms swellings. Finally, nectar collects in thesaccate nectary spur formed by the fusion of the margins ofthe labellum and the base of the column-foot. Thus, althoughthe nectary of H. imbricata and M. coccinea have many featuresin common, they nevertheless display a number of important differences.