. A textbook of botany for colleges and universities ... Botany. 854 ECOLOGY pollen and stigma are to play a part in pollination; this is an apparent disadvan- tage as compared with horaogamous flowers. Heterostyly. — A highly specialized condition that opposes close pollination and favors cross pollination is that known as heterostyly, in which the stigmas and the anthers in different flowers occupy different positions. Most such flowers are actinomorphic, and they are illustrated by the primrose, flax, forget-me-not, and bluets. In the primrose some plants have flowers with long styles, the
. A textbook of botany for colleges and universities ... Botany. 854 ECOLOGY pollen and stigma are to play a part in pollination; this is an apparent disadvan- tage as compared with horaogamous flowers. Heterostyly. — A highly specialized condition that opposes close pollination and favors cross pollination is that known as heterostyly, in which the stigmas and the anthers in different flowers occupy different positions. Most such flowers are actinomorphic, and they are illustrated by the primrose, flax, forget-me-not, and bluets. In the primrose some plants have flowers with long styles, the stamens being attached toward the base of the corolla tube, while other plants have flowers with short styles and with the sta- mens attached toward the upper part of the corolla tube (figs. 1180, 1181). In Lythrum Salicaria there are three kinds of flowers, one with long styles and in- termediate and short sta- mens, another with short styles and intermediate and long stamens, and a third with intermediate styles and long and short stamens. The same part of the insect that comes in contact with the lower sta- mens will touch the stigma of a short-styled flower, while pollen from the upper stamens will come in contact with the stigma of a long-styled flower, thus insuring cross pollination. Commonly the upper stamens of heterostyled flowers have large pollen grains corresponding to the large long-haired stigmas of the long-styled flowers, while the lower stamens have small grains corresponding to the small smooth stigmas of the short-styled flowers; the corollas and other organs also may differ considerably. Some investigators regard the large size of the pollen grams of the upper stamens as advantageous, since their pollen tubes have to traverse a greater distance upon germination; this view, which is doubtful a priori on account of their parasitic nourishment, has been experimentally disproven. Impotent and prepotent pollen. —• So far as the prevention of close pollinat
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