The Pollination Biology of Dendrobium speciosum Smith: a Case of False Advertising?

Potential pollinators of Dendrobium speciosum are attracted to the plant by large, cream to yellow, finely segmented, aromatic inflorescences. Plants in natural populations flower synchronously, produc- ing a massive display. Osmophores scattered over the perianth produce a strong, sweet scent in sunny weather. Nectar-seeking insects are guided to the central, reproductive area of the flower by the colour gradation of the perianth, including an area of high U.V. reflection near the centre, and a bright yellow ridge along the labellum. A tube formed by the labellum and column directs the potential pollinators. The flower, however, produces no nectar. Flowers vary in size within the six recognised varieties of D. speciosum and are pollinated when visited by bees of appropriate size. Post-pollination closure of flowers prevents further access by potential pollinators. The dendrobium beetle and larvae feed on the plant and inflorescences and play no part in pollination. The low fruit set, characteristic of this species, is considered to be a combined result of self-infertility and the absence of rewards offered by the flower. The specific climatic conditions during which pollinators visit the flowers are also important. The attraction and pollination mechanisms are similar among all observed varieties of D. speciosum as well as some closely related species, possibly explaining the high occurrence of hybrids within this group. It is concluded that D. speciosum provides a visual and olfactory display through the production of a large number of hermaphrodite non-rewarding flowers on each inflorescence, and relies on infrequent outcrosses for successful pollination.