Fertilization of the ovule.
st, stigma; p, pollen; pt, pollen-tube; o, ovary; e, embryo-sac. After Bergen.
Fertilization.—As soon as the pollen grain lodges on the stigma, it begins to form into a pollen tube. In more or less time it makes its way through the style into the ovary. It then penetrates the opening at the apex of the ovule, reaches one of the cells and transfers its nucleus into an egg-cell. The latter begins at once to form cell-walls and increases by continued subdivision to the plant embryo. Only one pollen tube is necessary to fertilize each ovule. Still plants produce more pollen than ovules—the ratio is from 1:8 to 1:1000—because so many pollen are lost on their way to the ovules.
The mechanism of the forwarding of the pollen to the ovule varies in different plants. In self-fertilization or in those plants in which the ovule may be impregnated by the pollen of the same flower, fertilization is comparatively easy. But in a great many plants cross-fertilization is the rule. In order to accomplish the most successful fertilization, the pollen must come from another plant of the same species. Here nature has devised different ways to carry the pollen from one plant to another.
In the first place, there is the wind which accomplishes the task of forwarding the pollen. The wind-fertilized flowers have dry and powdery pollen, and the pistils are feathery, adapted to catch flying pollen grains. The flowers are characterized by their inconspicuousness. They are usually greenish, without any odor or nectar.
Another device to forward the pollen is by way of insects. Most of the showy, sweet-scented or otherwise conspicuous kinds of flowers are entirely dependent for fertilization on the transference of pollen from one plant to another by insects. The showy colors and odors serve to attract insects to visit them for their nectar. Insects and flowers are interdependent upon one another. For many insects depend mainly upon the nectar and pollen of flowers for their food. These insects usually visit only one kind of flower during the day and thus carry only one kind of pollen. Butterflies, moths, and most of the bees go straight from one flower to another and carry a good deal of pollen entangled in the scales or hairs of their bodies. On its way, the insect leaves a good deal of the pollen on the stigma of the pistil and becomes dusted with new pollen to be carried to other flowers.
The means to attract insects are threefold: nectar, odor and color. The nectar is a sweet liquid which the flower secretes by means of nectar glands. The latter are usually situated near the base of the flower. Other plants attract insect visitors by giving up a sweet scent. These are especially the small flowers, like the mignonette or evening primrose. The color is another means by which the flower attracts insects and birds. The color of the flower is, as a rule, due to showy petals. Different kinds of insects are especially attracted by different colors. Some flowers with very long tubulated corollas depend entirely upon birds with long beaks to carry their pollen for them.
In complete and perfect flowers, where stamen and pistil are present in the same flower, self-fertilization would be the rule, if there were not certain means for its prevention. In the first place the pollen of another plant frequently prevails over that which the flower may shed over its own pistil. When both kinds are placed over the stigma, at the same time, it is the foreign pollen which causes fertilization. Another means to prevent self-fertilization consists in the stamens and pistils maturing at different times. The insect visitor, on its way to the nectary, brushes against the ripe stamens of a certain flower in its earlier stage. It cannot deposit the acquired pollen upon the stigma of the same flower and thus cause self-fertilization because the pistil is not ripe yet. But, in flying to a flower in the later stage, when the stigma has already ripened and the stamens have shed all their pollen, the insect will lodge the pollen first acquired on the ripe stigma, and in this way produce the desired cross-fertilization.
Sex-differentiation.—When we rise higher in the scale of animal life we find that the egg-cells and sperm-cells are almost always produced by different individuals. Those which produce egg-cells or ova are called female, and those which produce sperm-cells or spermatozoa are called male animals. The formation of a new being results in these species from the conjugation of two cellular elements of two different animals. In this way the origin of the organism, or the zygote from which the new individual develops is composed of parts of two different individuals, and a difference between the offspring and parents is insured. The intermixing of body-substance from two distinct individuals and the development therefrom of the new individual produce variation.[Z]
When the scale of animal life is reached where egg-cell and sperm-cell have their habitats in different individuals, the attraction between ovum and spermatozoön, which is based upon a kind of erotic chemotropismus, is transferred to the two hosts who harbor the two different sex-cells. Thus the erotic chemotropismus between two cells has now grown to sex-attraction between two animals. But even at this stage the attraction sometimes exists only between the sex-cells, namely when fertilization takes place outside of the mother’s body. The best example of outside fertilization are the fishes. The female fish contains the roe, which is a mass of small eggs. At the proper time the female lets the roe fall on the ground of rivers or the dark bottom of lakes, etc., at a favorable place called spawning bed, secure against enemies. The male fishes swim over the spawn and pour their semen or milt over it. The two kinds of cells attract each other like iron and magnet. When the milt has reached the spawn, the eggs are fertilized and develop into new fishes.