226 BOTANY. 



from flower to flower, the insect is almost certain to transfer 

 part of the pollen carried off from one flower to the stigma of 

 another of the same kind, thus effecting pollination. 



That the fertilization of a flower by pollen from another is 

 beneficial has been shown by many careful experiments which 

 show that nearly always at least in flowers where there are 

 special contrivances for cross-fertilization the number of 

 seeds is greater and the quality better where cross-fertilization 

 has taken place, than where the flower is fertilized by its own 

 pollen. From these experiments, as well as from very numer- 

 ous studies on the structure of the flower with reference to 

 insect aid in fertilization, we are justified in the conclusion 

 that all bright-colored flowers are, to a great extent, dependent 

 upon insect aid for transferring the pollen from one flower 

 to another, and that many, especially those with tubular or 

 zygomorphic (bilateral) flowers are perfectly incapable of self- 

 fertilization. In a few cases snails have been known to be the 

 conveyers of pollen, and the humming-birds are known in some 

 cases, as for instance the trumpet-creeper (Fig. 121, J.), to 

 take the place of insects. 1 



At first sight it would appear that most flowers are especially 

 adapted for self-fertilization; but in fact, although stamens 

 and pistils are in the same flower, there are usually effective 

 preventives for avoiding self-fertilization. In a few cases 

 investigated, it has been found that the pollen from the flower 

 will not germinate upon its own stigma, and in others it seems 

 to act injuriously. One of the commonest means of avoiding 

 self-fertilization is the maturing of stamens and pistils at dif- 

 ferent times. Usually the stamens ripen first, discharging 

 the pollen and withering before the stigma is ready to receive 

 it, e.g. willow-herb (Fig. 113, Z>), campanula (Fig. 123, A, D), 



1 In a number of plants with showy flowers, e.g. violets, jewel- weed, 

 small, inconspicuous flowers are also formed, which are self-fertilizing. 

 These inconspicuous flowers are called " cleistogamous." 



