Fig. 195.—Diagram to represent Fertilization.
s, stigma; st, style; ov, ovary; o, ovule; p, pollen-grain; pt, pollen-tube; e, egg-cell; m, micropyle.
In order that the pollen may grow, the stigma must be ripe. At this stage the stigma is usually moist and sometimes sticky. A ripe stigma is said to be receptive. The stigma may remain receptive for several hours or even days, depending on the kind of plant, the weather, and how soon pollen is received. Watch a certain flower every day to see the anther locules open and the stigma ripen. When fertilization takes place, the stigma dies. Observe, also, how soon the petals wither after the stigma has received pollen.
Pollination.—The transfer of the pollen from anther to stigma is known as pollination. The pollen may fall of its own weight on the adjacent stigma, or it may be carried from flower to flower by wind, insects, or other agents. There may be self-pollination or cross-pollination, and of course it must always precede fertilization.
Fig. 196.—Anther of Azalea, opening by terminal pores.
Usually the pollen is discharged by the bursting of the anthers. The commonest method of discharge is through a slit on either side of the anther (Fig. [193]). Sometimes it discharges through a pore at the apex, as in azalea (Fig. [196]), rhododendron, huckleberry, wintergreen. In some plants a part of the anther wall raises or falls as a lid, as in barberry (Fig. [197]), blue cohosh, May apple. The opening of an anther (as also of a seed-pod) is known as dehiscence (de, from; hisco, to gape). When an anther or seed-pod opens, it is said to dehisce.
Fig. 197.—Barberry Stamen, with anther opening by lids.
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 the 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 centre is occupied by the column of stamens. In Fig. [199], showing an older flower, the long styles are conspicuous.