| ||||||||||||||||||||||
Protein-rich reward pulls the pollinators
Orchids never cease to amaze when it comes to interaction with their pollinators. In describing those interactions we use terms such as attraction, deceit and reward, which may equally apply to human interactions. In respect of reward, no orchid allows its pollinators to obtain pollen, because the pollen is contained in pollinia and is inaccessible to the pollinating insect. However, there are several genera of orchid that provide a pollen-like reward in the form of 'pseudopollen'. Davies et al. (University of Wales, Cardiff and Kew Gardens, pp. 477-484) have made an extensive study of pseudopollen production in the large genus Polystachya. Pseudopollen originates from food hairs situated on the flower's labellum. 'True pseudopollen' (as the authors call it) occurs when these food hairs fragment during floral maturation to form individual cells that may, in general appearance, resemble pollen. This study suggests that true pseudopollen is confined to just one taxonomic section within the genus. However, in many species of Polystachya, food hairs do not break into individual cells but become detached as complete structures. To the unaided eye both appear as a floury powder on the labellum, but they are readily distinguished under the microscope. Chemical analysis of both types reveals that the main nutrient is protein although in many species starch-containing amyloplasts are also present. The authors note that pseudopollen and/or food hairs occur in several orchid genera, and based on the relationships between those genera they conclude that these features have evolved several times within the Orchidaceae. It has not yet been possible to show how effective pseudopollen is in attracting pollinators but it is reasonable to conclude that the cost of laying down nutrients in food hairs surely must confer an advantage. The attraction of pollinators as part of the reproductive strategy is likely to be involved in that advantage.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
| ||||||||||||||||||||||