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Anals of the Missouri Botanical Garden





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Pollination in Viola results both from sternotribic and nototribic pollinator responses, the frequencies of which vary according to the plant species. Syndromes of floral characters determine these frequencies, and each is adaptive to a group of pollinators, of variable taxonomic heterogeneity, characterised by particular morphology and behavior. Some syndromes elicit equal frequencies of sterno- and nototribic responses, while others elicit chiefly, or exclusively, one type, and coevolution with a single insect genus may be in progress. One effect of the differing syndromes is the partitioning of pollinator resources among simult a-neously flowering species. An evolutionary sequence for the floral syndromes is proposed which, in broad agreement with systematic conclusions, considers yellow-flowered, sternotiribic Chamaemelanium violets as the most ancient and blue-flowered sterno- or nototribic violets of the sections Rostellatae and Plagiostigma as the most recently evolved. Species of the section Melanium (the pansies), in which cleistogamy is almost absent, appear to be an early offshoot leading to multicolored, nototribic flowers. The "generalist" syndromes were probably vital to the success of the genus in (a) invading the northern and southern temperate zones and (b) adapting to widespread habitat disturbance by man. The basic structure of the violet flower is adaptive to a temperate pollinator complex to be found over very large geographic areas. Floral variations demonstrate adaptive radiation to many ecological conditions.

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