The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

example, are pollinated by wind; their graceful spikes are in reality clusters of small flowers, but they have no conspicuous petals or perfume, because the light, dry pollen is sumply blown from plant to plant. They have no use for insect visitors. ‘There are many other examples of wind-pollinated flowers, generally small and greenish or brownish in colour because they need not attract attention. The dangling catkins of hazel are clusters of tiny male flowers, that is to say, flowers containing stamens but no ovaries, and pollen is blown from these to the female flowers, containing ovaries but no stamens, which form bud-like clusters only distinguishable from ordinary hazel-buds by the tiny brush of soft crimson stigmas with which they are tipped. Moreover, water is used as a medium for this transfer by many of the plants that live

Fig. 108. Pollen grains sprouting in a drop of water.

They grow away from the edge (above), i.e., away from oxygen. This “* growth instinct”? would guide them in towards the egg-cell if they were on a stigma (highly magnified).

therein. In the Canadian waterweed, to which we have already made reference, the male flowers break bodily away from the plants and float about on the surface of the pond until they come up against female flowers, when pollination can occur.

And here we may mention a curious and important point. The buttercup, we have seen, is provided with an elaborate apparatus of petals and nectaries so that insects may carry pollen from one flower to another. Why should this be? Why should not the pollen from the stamens drop directly on to the stigmas, and so bring about fertilization without the bother of hiring an insect intermediary ?

Throughout the plant and animal kingdoms we find a general preference for crossfertilization ; an avoidance of the union of eggs with male elements from the same individual. In most animals and a few

160

TES, SCI IN Gls OF Illes;

CHAPTER 5

plants the individuals are sexed, either male or female, so that here there is no possibility of self-fertilization. But in those animals such as snails and earthworms and liverflukes, and in the great majority of plants, which are hermaphrodite (male and female in one), there are usually precautions to restrain self-fertilization. The male half of one individual seldom fertilizes its own female half, but is mated with the female half of another.

As a matter of fact there is no immediate physiological reason why self-fertilization should not occur. Indeed, in a number of plants it does. Primroses, and other flowers which open early in the spring before many insects are abroad, fall back on self-pollination if they have not been cross-pollinated. In the violet and the wood-sorrel, which, being modest flowers, are apt to be overlooked, special flowers are developed towards the end of the season that do not open at all, but are regularly and automatically self-pollinated. And many of the humbler flowering weeds, such as chickweed, shepherd’s purse, and groundsel, nearly always pollinate themselves. Apparently self-pollination is perfectly possible and workable, but crosspollination is in some way more desirable ; the former is a second-best thing, to fall back upon if the latter fails.

In contrast to the modest weeds that put up with this second-best is the upright meadow buttercup, the ovules of which cannot be fertilized by pollen from the same flower. They just do not respond. ‘This is not in any sense a sign that self-fertilization is generally an impossible method ; it is a device for securing that only the preferable method, cross-pollination, shall occur. An enormous amount of unfounded belief has accumulated round the process of self-fertilization and the closely similar process of inbreeding. They have been accused of producing a loss of vigour, degeneracy, and a number of vaguely defined phenomena of that kind. It is the exact study of heredity on the basis of Mendel’s laws—the most important advance that biological science has made in the twentieth century—that has made the story clear, and put it into precise terms. We cannot go into the details of the problem here ; they are best deferred to a later chapter when the essential principles of heredity will be set forth. But we may say that there is nothing