If fertilization of all flowers were as simple as this, there would be no need of what follows, but actually in surprisingly few plants are the stamens and pistils so arranged, the ripening of the pollen and readiness of the ovule for impregnation so timed that the act can be accomplished in such direct fashion. For it is quite obvious that in flowers in which the whole drama of mating goes on within the petals, without the interference or help of any outside agency, the result will be a crop of young who know no other characters than those of the parents, and have nothing to look forward to but a closely inbreeding progeny, very little, if at all different from themselves. In other words, such plants are pure bred, they lack the usually obvious virility that comes from crossing the male of one plant with the female of another. There are so many devices to prevent self-fertilization in flowers, so marvelous are the contrivances to see to it that only cross-fertilization can be effective, and, finally, the experience of breeders that strength and virility often or usually result from impregnating the ovules of one plant with the pollen of another, that we are forced to the conclusion that absolute purity in the sexual relations of flowers is rare indeed. It occurs, without peradventure of a doubt, only in those flowers whose petals never open and where fertilization is consummated, if not in private, at any rate without external help. In many violets the showy violet blossoms are often nearly infertile, while down near the ground are inconspicuous flowers which never open, but within which fertilization is so successful that the crop of seeds is far more plentiful than in the more showy ones that most people think are the only flowers ever borne by violets. These flowers that never open, or at any rate open so slightly that their sexual processes are modestly completed without intrusion, are known as _cleistogamous_ flowers (Figure 69). They have been found in a few plants, but overwhelmingly the greater number of flowers not only do, but must, rely on some outside agency to insure fertilization.