Comparative labellar anatomy of resin-secreting and putative resin-mimic species of Maxillaria s.l. (Orchidaceae: Maxillariinae)


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Flowers of most species formerly included in Maxillaria are rewardless. Some, however, reward pollinators with nectar or pseudopollen, whereas the labella of a relatively small number of taxa produce lipoidal floral rewards that include resins and waxes. To date, the labellar secretory tissues of few species have been examined in detail, and some rewardless species having glossy labella are thought to mimic taxa that produce lipoidal rewards. Here, the labellar anatomy of a range of resin-secreting, wax-secreting and putative resin-mimic species is compared using light microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry, in order to investigate how such secretory tissues may have evolved. Comparisons are also made with other resiniferous and wax-secreting Maxillariinae s.s., and with rewardless control species and the oil-secreting elaiophores of certain Oncidiinae. The labella of all Maxillariinae species investigated displayed similar anatomy, the differences largely being restricted to the adaxial epidermis and subepidermal layer. The labellar secretory epidermis of resiniferous Maxillariella elatior was composed of epithelial palisade cells, whereas that of other resin- and wax-secreting species investigated consisted of secretory papillae or trichomes. Putative resin-mimic species also produced resin, albeit in small quantities, and, here, the epidermal cells were cuboidal or rectangular in section. Secreted material on the labellar surface of control species was probably fragrance residues. Resin was often heterogeneous, and fluorescence microscopy revealed that its composition could change throughout the course of anthesis. Although the resiniferous floral tissues of Maxillariinae s.s. may be epithelial or trichomal, and thus closely resemble the epithelial and trichomal elaiophores of oil-secreting Oncidiinae, they differ in a number of important ways, necessitating the use of more exact descriptive terminology when distinguishing between oil-, resin- and wax-secreting floral structures. Similarity between the resin- or wax-secreting floral structures of Maxillariinae s.s and the oil-secreting elaiophores of Oncidiinae is probably a result of parallelism. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 170, 405–435.