The structure and function of orchid pollinaria

Cohesive masses of pollen known as pollinia have evolved independently in t wo plant families - Orchidaceae and Asclepiadaceae. Yet, the bilateral symm etry of orchids has allowed a greater degree of specialization in pollinati on systems and a much greater diversity in the morphology of pollinaria - u nits comprising the pollinia(um) together with accessory structures for att achment to the pollinator. Pollinaria differ in the degree of cohesion of p ollen in the pollinium, which may be soft, sectile (comprised of sub-units known as massulae) or hard. A single hard pollinium may contain more than a million pollen grains, yet pollen:ovule ratios in orchids are several orde rs of magnitude lower than in plants with powdery pollen due to the lack of wastage during transport to the stigma. Attachment of pollinia to the poll inator is usually achieved by means of a viscidium that adheres most effect ively to smooth surfaces, such as the eyes and mouthparts of insects and be aks of birds. The stalk connecting a pollinium to the viscidium may be comp rised of a caudicle (sporogenous in origin) and/or a stipe (derived from ve getative tissue), or be lacking altogether. Caudicles and stipes may underg o a gradual bending movement 20 s to several hours after withdrawal from th e flower, the main function of which appears to be to reduce the possibilit y of geitonogamous pollination. Other mechanisms that promote outcrossing a nd pollen export in orchids include pollen carryover (achieved by sectile o r soft pollinia), temporary retention of the anther cap, protandry and self -incompatibility (rare among orchids). Pollinaria ensure that large pollen loads are deposited on the stigma, thus enabling the fertilization of the l arge numbers of ovules in the flowers of Orchidaceae. Pollinaria also ensur e efficient removal of pollen from the anther, minimal pollen wastage durin g transit, and a high probability of deposition on conspecific stigmas.