_Most flowers are so constructed as to increase the chances of cross-pollination._ We have seen that the stigma may have the power of choosing foreign pollen. The commonest means of necessitating cross-pollination is the _different times of maturing of stamens and pistils in the same flower_. In most cases the stamens mature first: the flower is then =proterandrous=. When the pistils mature first, the flower is =proterogynous=. (_Aner_, _andr_, is a Greek root often used, in combinations, for stamen, and _gyne_ for pistil.) The difference in time of ripening may be an hour or two, or it may be a day. The ripening of the stamens and pistils at different times is known as =dichogamy=, and flowers of such character are said to be dichogamous. There is little chance for dichogamous flowers to pollinate themselves. Many flowers are _imperfectly dichogamous_--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. [Illustration: FIG. 198.--FLOWER OF HOLLYHOCK; proterandrous.] [Illustration: 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 irregular and provided with nectar and strong perfume are usually pollinated by insects._ Gaudy colors probably attract insects in many cases, but perfume appears to be a greater attraction. [Illustration: FIG. 200.--FLOWER OF LARKSPUR.] [Illustration: FIG. 201.--ENVELOPES OF A LARKSPUR. There are five wide sepals, the upper one being spurred. There are four small petals.]