application of pollen to pistils in the same flower is close-fertilizationor self-fertilization. It will be seen that the cross-fertilization relationship may be of many degrees—between two flowers in the same cluster, between those in different clusters on the same branch, between those on different plants. Usually fertilization takes place only between plants of the same species or kind.
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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 many cases there is, in effect, an apparent selection of pollen when pollen from two or more sources is applied to the stigma. Sometimes the foreign pollen, if from the same kind of plant, grows, and fertilization results, while pollen from the same flower is less promptly effective. If, however, no foreign pollen is present, the pollen from the same flower may finally serve the same purpose.
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
