=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. [Illustration: 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. [Illustration: 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.