some of the anthers mature simultaneously with the pistils, so that there is chance for self-pollination in case foreign pollen does not arrive. Even when the stigma receives pollen from its own flower, cross-fertilization may result. The hollyhock is proterandrous. Fig. 198 shows a flower recently expanded. The center is occupied by the column of stamens. In Fig. 199, showing an older flower, the long styles are conspicuous.
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Fig. 198.—Flower of Hollyhock; proterandrous.
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Fig. 199.—Older Flower of Hollyhock.
Some flowers are so constructed as to prohibit self-pollination. Very irregular flowers are usually of this kind. With some of them, the petals form a sac to inclose the anthers and the pollen cannot be shed on the stigma but is retained until a bee forces the sac open; the pollen is rubbed on the hairs of the bee and transported. Regular flowers usually depend mostly on dichogamy and the selective power of the pistil to insure crossing. Flowers that are very
