280 TEXTBOOK OF BOTANY 



vigorous as though the seeds had been produced by cross- 

 pollination. 



287. Self -Pollination. This is by no means rare, in 

 spite of the fact that so many features in the structure of 

 flowers seem to favor cross-pollination. Even in species 

 that are usually cross-pollinated, self-pollination is often 

 provided for in case cross-pollination should fail. An in- 

 stance of this kind is seen in the tiger lily, whose style, if 

 pollen is not brought in from another flower, bends so as to 

 bring its stigma into contact with an anther. There are 

 also not a few plants in which self-pollination is the rule, 

 and in which cross-pollination, if it happens at all, does so 

 only rarely and accidentally. This is true, for example, of 

 wheat, of most varieties of peas, and of some varieties of 

 barley. 



Some other plants produce small flowers that never open 

 and in which therefore cross-pollination is quite impossible. 

 Illustrations of this sort are the hog-peanut and some of 

 our common violets. In the spring the violets bear the 

 familiar flowers which open and which may be cross-polli- 

 nated. After these spring flowers disappear, and from time 

 to time during the summer, small, green, bud-like structures 

 appear at the ends of short stalks. These bud-like bodies 

 are really flowers that never open. When the pollen-grains 

 of these, flowers are ripe, they germinate in the pollen sacs; 

 the pollen tubes grow out through the walls of the sacs, 

 find -the stigma of the pistil, and grow through the style into 

 the ovary. Thus, by means of these flowers the violet is 

 independent of the visit of insects and is sure to produce 

 seed ; indeed, some of the violets form most of their seeds in 

 the ovaries of these little closed flowers. 



288. Hybrids. Hybrids or crosses are plants produced 

 by the union of gametes that belong to different species or 

 varieties. In seed plants, such a union of different gametes 

 of course results from the pollination of a flower by pollen 



