AOBPreview originally published online on March 30, 2006
Annals of Botany 2006 97(6):975-984; doi:10.1093/aob/mcl065
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A Rudimentary Labellar Speculum in Cymbidium lowianum (Rchb.f.) Rchb.f. and Cymbidium devonianum Paxton (Orchidaceae)
SKA21 School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK, 2 Department of Botany, Agricultural University, Akademicka 15, 20-950 Lublin, Poland and 3 School of Biosciences, Cardiff University, Park Place, Cardiff CF10 3TL, UK
* For correspondence. E-mail kevinldavies{at}btinternet.com
Received: 4 January 2006 Returned for revision: 7 February 2006 Accepted: 10 February 2006 Published electronically: 30 March 2006
• Background and Aims The labellar ‘hairs’ of some Cymbidium spp. are said to be thin-walled and to contain ‘plasma’, oil and sugars and it has long been speculated that they may function as food-hairs. However, the present authors' preliminary studies showed that certain atypical papillae may have a different role and, by reflecting light, function as a speculum. The purpose of the paper is to test this hypothesis.
• Methods Light microscopy, scanning electron microscopy, transmission electron microscopy, histochemistry and ultraviolet photography were used to investigate the structure, food content and light-reflecting properties of these papillae.
• Key Results and Conclusions The labellum of Cymbidium lowianum (Rchb.f.) Rchb.f. is densely clothed with obconical to conical papillae with wide bases and pointed tips. However, on either side of the median axis of the lip occur silvery patches comprising papillae with truncated tips and it is thought that these reflect light and thereby attract insect pollinators. Similar patches are also found in Cymbidium devonianum Paxton, and in both species, they are set against a reddish background, which, since bees cannot perceive this colour, probably appears dark to the insect thus enhancing the visual impact of the light-reflecting patches. In Cymbidium tigrinum Parish ex Hook. and Cymbidium mastersii Griff. ex Lindl., however, the labellum is mainly white and no light-reflecting patches were observed. Instead, unlike C. lowianum and C. devonianum, these species are highly fragrant and the attraction of insects probably depends to a greater extent on olfactory cues. In C. lowianum both types of papillae contain protein, starch and lipid bodies but only protein is seemingly present at elevated concentrations. However, lipoidal material also occurs upon the surface of the labellum and it is possible that this may be gathered by insects as reported for C. iridifolium A. Cunn (syn. C. madidum Lindl.). The labellar papillae of C. lowianum, thus, have the potential to function as food-hairs, although direct evidence for this is lacking.
Key words: Cymbidium lowianum, Cymbidium devonianum, floral rewards, food-hairs, labellum, light absorption, light reflection, papillae, speculum
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