Note to the Teacher.—It cannot be urged too often that the specimens themselves be studied. If this chapter becomes a mere recitation on names and definitions, the exercise will be worse than useless. Properly taught by means of the flowers themselves, the names become merely incidental and a part of the pupil’s language, and the subject has living interest.
CHAPTER XIX
THE FLOWER—FERTILIZATION AND POLLINATION
Fig. 193.—B, Pollen escaping from anther; A, pollen germinating on a stigma. Enlarged.
Fertilization.—Seeds result from the union of two elements or parts. One of these elements is a cell-nucleus of the pollen-grain. The other element is the cell-nucleus of an egg-cell, borne in the ovary. The pollen-grain falls on the stigma (Fig. [193]). It absorbs the juices exuded by the stigma, and grows by sending out a tube (Fig. [194]). This tube grows downward through the style, absorbing food as it goes, and finally reaches the egg-cell in the interior of an ovule in the ovary (Fig. [195]), and fertilization, or union of a nucleus of the pollen and the nucleus of the egg-cell in the ovule, takes place. The ovule and embryo within then develops into a seed. The growth of the pollen-tube is often spoken of as germination of the pollen, but it is not germination in the sense in which the word is used when speaking of seeds.
Fig. 194.—A Pollen-grain and the Growing Tube.
Better seeds—that is, those that produce stronger and more fruitful plants—often result when the pollen comes from another flower. Fertilization effected between different flowers is cross-fertilization; that resulting from the application of pollen to pistils in the same flower is close-fertilization or 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.
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.
