POLLINATION 279 



an excess of pollen is applied to the stigmas of flowers the tissues 

 of the pericarp are stimulated to develop more extensively and the 

 fruit is consequently larger, than when a small amount of pollen 

 is applied. 



5. Pollination : self-fertilisation and cross-fertilisation. — It 

 will be understood from the foregoing account that among plants 

 with completely closed carpels the fertilisation-process is preceded 

 by and dependent upon the deposition of the pollen-grain on the 

 stigma of the carpel of a flower. Although the pollen-grains 

 may be induced to germinate on other parts of the carpel, the 

 pollen-tubes have no power of penetrating the tissues of the 

 latter except when placed on the specially receptive stigma. 

 This necessary transference of pollen-grains from the anthers of 

 the stamens to the stigmas of the carpels is termed pollination. 



Where the stigma receives pollen from the anthers of the same 

 flower the latter is said to be self-pollinated : frequently, however, 

 the stigma in one flower receives pollen from a flower growing 

 on another distinct plant, in which case the flower receiving the 

 pollen is spoken of as cross-pollinated. 



A simple term is needed for the intermediate case where the 

 pollen of a flower is transferred to the stigma of another flower 

 growing on the same plant. 



Where self-pollination is followed by fertilisation the plants 

 are said to be self-fertilised or close-fertilised; the term cross- 

 fertilisation is applied to cases where the fertilising pollen is 

 derived from another distinct flower of the same species of plant. 



Since most plants have their sexual organs close together in the 

 same flower it might be imagined that self-fertilisation would be 

 the rule among flowering-plants. A number of plants with open 

 flowers are undoubtedly self-fertilised and certain plants such as 

 pansy, violet, wood-sorrel and barley, possess cleistogamous flowers 

 which never open and which therefore insure certain self- 

 fertilisation. 



Extensive and careful observation, however, shows that a large 



