Botany: The Science of Plant Life

by Norman Taylor

We have already made clear that all plants are divided upon the basis of whether they bear flowers and their mating goes on before the world, or whether they bear none and the process is accomplished in more secret ways. Because flowers are so much better known, and it is simpler to see how the act is consummated in them than in the cryptogamous plants, we shall first consider the phanerogams or flowering plants, and in the second section of this chapter the cryptogams or flowerless plants.

1. Visible Marriage of Flowering Plants

In the first chapter, under the section devoted to flowers, we found that the stamens are the male and the pistils the female organs of reproduction. As the period for mating draws near there is developed in the anther, which is the enlarged tip of the stamen, a fine, usually yellow, powder known as pollen. This matures in the anther, and when ripe is discharged from tiny pores.

Pollen is made up of individual pollen grains, which are very often stuck together so that we see only the mass, not the individual pollen grain. Sometimes the pollen is not sticky, as in the case of pine trees or in the ragweed—a fertile cause of hay fever. In these, and hundreds of other plants, the wind will blow great clouds of pollen through the air. When we stop to consider that a single, or at most a very few pollen grains are all that are necessary—in fact, are all that can be of real service—the enormous wastage of the male fertilizing substance, in order that mating be secured, gives us some idea of how prodigal is nature in this supreme function.

The pistil, or female organ of reproduction, is more cautious in the expenditure of its resources. As we have seen, it is composed of a swollen base, the ovary, a slender shank, the style, and a swollen or branched tip, the stigma. In some plants the ovary is divided into several compartments or cells, each with one or more ovules, which are only immature or unfertilized seeds, often very tiny, but usually quite easily seen if the ovary is cut open. It is the entrance of the pollen grain into this ovule that consummates the act of fertilization. As the ovule is carefully secreted within the ovary of the flower, and as the male fertilizing stuff or pollen is found only on the anther, it is obvious that some method of bringing the two together must be provided for.

In some plants this is accomplished by the anthers being just above the stigma, and when the pollen is ripe and the ovule ready, the stigma is found to be covered with a sticky substance. As the falling pollen grains touch the stigma, they are caught in this sticky substance just as surely as flies are caught once they touch a fly paper. But just here one of the most wonderful processes of nature begins. The pollen grain begins, slowly at first, to grow, and in the act it penetrates the outer coat of the stigma with a minute pollen tube. This slender threadlike tube, carrying with it the male germ, grows straight down through the stigma, into the narrowed style, and through this to the ovule. Once the pollen is caught on the stigma, nothing is so sure of fulfillment as that this male fertilizing stuff will ultimately reach the ovule. For the hitherto virgin ovule this impregnation starts a new phase in life. It means the beginning of the end, but in the process fruit and seed will be developed, and the young bride, already a mother, has triumphantly accomplished that for which she exists.

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.

Certain structural features of flowers have been so developed that fertilization of the ovary by the pollen of the same flower is impossible. The commonest case is in those flowers where the stamens are shorter than the pistils, as they always are in the common snowdrop, hyacinth, the sassafras tree, and in hundreds of others. There can be no consummation of the reproductive process in such flowers without some outside aid. More futile still without this aid are those flowers where the stamens are well above the pistils, but the time of maturing in both differs by a few days or even hours. Nothing could be more helpless than the pistil under these circumstances, for if its instinct for maternity were ever so strong, it would be doomed to barren sterility by the premature development of the males. Sometimes, too, the female is prematurely ripe for impregnation, and the stamens lag behind a day or two. Her time passes and with it her only chance of fertilization—by her own haste she has rendered impotent the now useless pollen which appears doomed to fall aimlessly upon the unreceptive stigma.