122 PRACTICAL BOTANY 



By cross-pollination is meant the process of transferring for- 

 eign pollen to the stigma. The effect in fertilization is the 

 same whether the pollen is carried by the wind or otherwise. 



115. Advantages of cross-pollination. As already stated 

 (Sect. 113), foreign pollen is usually more effective than the 

 pollen from the same individual. Charles Darwin, the great 

 English naturalist, showed by experiments continued through 

 eleven years that in many cases the plant derives great advan- 

 tages from cross-pollination. 1 He found that in plants the 

 flowers of which are not especially suited to self-pollination 

 if left to themselves, but which he pollinated thoroughly by 

 hand, the plants grown from the seeds of cross-pollinated 

 flowers usually much exceeded in height, weight, and fertil- 

 ity those from self-pollinated flowers. It was found, for in- 

 stance, that when the yellow monkey flower (Mimulus luteus) 

 was self-pollinated to the ninth generation the plants thus 

 produced were -ffa the height of plants which came from 

 those self-pollinated to the eighth generation and then cross* 

 pollinated with a plant of another stock. In fertility the two 

 kinds (self-pollinated to the ninth generation and cross-polli- 

 nated at the end of the eighth generation) were in the ratio T -|^. 



Cabbages were raised by Darwin from seeds of a third 

 self-pollinated generation and also from those of the second 

 self-pollinated generation crossed with a plant from a distant 

 garden. The self -pollinated cabbages were only -ffo the weight 

 of the cross-pollinated ones. These two examples may serve as 

 extreme instances of the benefits of cross-pollination. In many 

 cases less advantage is gained by it, and there is a considerable 

 group of plants which seem to be indifferent to the source from 

 which the pollen comes, that from the same flower answering 

 as well as that from another individual of the same species. 

 The practical value of a knowledge of the effects of different 

 kinds of pollination is often very great (Chapter XXIII). 



1 See Darwin, The Effects of Cross and Self Fertilisation in the Vegetable 

 Kingdom, chaps, i and vii-ix. D. Appleton and Company, New York. For 

 facts about flowers which do not need cross-pollination see Sects. 121 and 126. 



