Receptive surfaces of angiosperm stigmas have been classified by cyto-physiological and histological criteria. The main categories distinguished are wet or dry, with the receptive surface papillate or smooth in each category. In most families or even orders examined, the characteristics of each category were found to be uniform. The criteria used in this classification are mainly significant for processes initiated once the pollen grains have landed on the stigma. Morphological stigma characters, on the other hand, such as shape, size, positioning, orientation, and distribution of the receptive area are likely to be relevant for capturing of pollen grains. In the last four decades most studies on stigmas were related to their interactions with pollen grains and to incompatibility, while studies of pollination paid little attention to stigma morphology. The present work attempts to assess the variation of morphological stigma traits at various levels of taxonomic hierarchy. Is variation in these traits similar to the variation in characters significant for pollen-stigma interactions? A preliminary survey of the literature revealed dissimilar patterns of diversity for characters involved in the two functions of the stigma. Numerous examples were encountered in which details of stigma morphology relevant for pollination were among diagnostic criteria used for distinguishing between sub-families, tribes, genera, sections, and species. Diversity of stigma morphology among taxa of the same rank is mostly associated with uniformly wet or dry and papillate or non-papillate receptive areas. Properties of the receptive area functioning in pollen-stigma interactions appear to be more constant than properties involved in stigma moulding in accordance to requirements of pollen acquisition. Diversification of morphological properties frequently occurs in taxa with specialized pollination mechanisms, e.g., secondary pollen presentation or buzz-pollination. It may also serve as an isolation mechanism between sympatric species.

Key words: pollination, stigma characters, taxonomy