Stigma characteristics and angiosperm taxonomy

When a classification based upon certain morphological and physiological characters of the stigma was applied to ca. 1,000 species of some 900 genera of angiosperms, several taxonomic regularities emerged. Most families proved to be relatively homogeneous in stigma type, whilst others were strikingly diverse. Among the latter were certain supposedly primitive families such as the Liliaceae, a fact that may have some phylogenetical significance. Sometimes where a family proved to be homogeneous in respect to the more general characters of the stigma, fuller investigation revealed great variation in detail. An example is given by the Boraginaceae, a family in which stigma characters are of potential taxonomic importance. The stigma papillae in this family are often capitate, with heavily cutinised heads, and pollen has to be inserted forcibly between the papillae. There is a strong correlation between pollen size and stigma–papilla size, so that certain types of cross pollination are impossible. These specialisations can thus play a part as isolating mechanisms. The form of the stigma papillae is probably also important in genera of the Boraginaceae adapted to arid conditions, since the contiguous cutinised caps overarch the actual receptive surfaces and so. presumably, offer protection from desiccation.
In both monocotyledons and dicotyledons, clear relationships exist between stigma type and physiology, and notably with the type of self–incompatibility system. It is already apparent that stigma characters can have predictive value in the investigation of breeding systems.
The paper includes a comprehensive literature list, covering most ot the principal sources of structural, taxonomic and other data bearing upon angiosperm stigmas.