The avoidance of interference between the presentation of pollen and stigmas in angiosperms II. Herkogamy

Abstract

Herkogamy is the spatial separation of pollen presentation and pollen receipt within or between blossoms of an individual plant. Several classes of herkogamy are recognised; these are defined by whether all blossoms are identical (homomorphic herkogamy), all blossoms dispatch and receive pollen but reciprocal forms occur (reciprocal herkogamy), or some or all blossoms perform only one function (interfloral herkogamy). Within homomorphic herkogamy, unordered herkogamy, in which pollinator contacts with stigmas and pollen within a blossom are many and occur in no particular sequence, is distinguished from ordered herkogamy in which contacts are few and ordered. For ordered herkogamy further divisions are based on the operation of the pollination mechanism. Herkogamy is usually interpreted as a mechanism which reduces self-fertilisation and promotes outcrossing. However, as many herkogamous plants are also self-incompatible, it is suggested that the various classes of herkogamy also function in part or solely as mechanisms which avoid interference between pollen receipt by stigmas and pollen dispatch from anthers. Although for herkogamous blossoms these two functions are separated in space, many such flowers control pollinator behaviour so that both pollination surfaces are contacted during a single visit. In dichogamous blossoms separation of pollen dispatch and receipt is temporal rather than spatial and this difference has several important consequences for the pollination biology of dichogamous and herkogamous blossoms.