398 A TEXT-BOOK OF BOTANY. 



While most hermaphrodite flowers are self-pollinated, there are 

 some that are not, and this is brought about in several ways : 

 ( I ) As already pointed out, the pollen may germinate better on 

 the stigma of another flower than on the stigma of the same 

 flower. (2) The anthers and pistils of the same flower may mature 

 at different times, and this is one of the commonest ways of 

 preventing self-pollination. Usually in such cases the stamens 

 mature first. The common plantain (Plantago) furnishes an 

 example of the maturing of the stigma before the anther. The 

 flowers of this plant are arranged in spikes (Fig. 230, illus. 3 

 and 4) which belong to the indefinite class, and hence the lower 

 flowers on the spike expand first. As stated, the pistil of each 

 flower matures first, and after it withers the stamens protrude and 

 discharge their pollen. It is evident that the flowers can not be 

 self-pollinated, nor is it likely that one flower will be pollinated 

 by another of the same spike. (3) The stamens and pistils of the 

 same flower may vary in length, as in Polygonum (Fig. 230, illus. 

 I and 2) and Lythrum (Fig. 230, illus. 5), or stand in such other 

 relation to each other that self-pollination will not be effected, 

 as in some of the irregular or zygomorphic flowers, like those of 

 Orchids. In these several cases the pollen grains either fall upon 

 or are carried by various agents to the stigmas of other flowers, 

 and this is known as cross- pollination, and the fertilization 

 which follows as cross-fertilization. 



Cross-fertilization is an advantage to the species, for usually 

 the seeds which result from this process give rise to plants which 

 are more vigorous and otherwise superior to those which result 

 from self-fertilization. In some cases, in order to insure the pro- 

 duction of fruit, hand-pollination is practised, as by the growers 

 of vanilla and some other tropical plants of economic importance. 



In the case of unisexual flowers, or those in which the stamens 

 and pistils are in separate flowers, there is, of course, no chance 

 for self-pollination. Here, as in the case of cross-pollinated her- 

 maphrodite flowers, pollination may be more or less close or it 

 may be remote, as between flowers of the same cluster or inflores- 

 cence, between flowers of different clusters or inflorescences on 

 the same plant, or between flowers on different plants. 



In buckwheat (Fig. 230, illus. i and 2) and partridge berry 



