240 



THE LIFE OF THE PLANT 



by pollen from the same flower is less successful, and gives 

 a less vigorous posterity than when the pollen comes 

 from another flower. Moreover, there are plants in 

 which self-pollination is absolutely fruitless, e.g. Cory- 

 dalis. There are also plants with two or even three 

 kinds of flowers, e.g. the primrose, the purple loosestrife, 

 and the different species of flax. The pistils and stamens 

 of these flowers are of different lengths, and moreover 

 long pistils occur in the same flower with short stamens 

 (fig. 68 A) and vice versa (B). For successful fertilisa- 

 tion, it is necessary to transfer pollen to the stigma from 



OoOO 

 



X 250 



Fig. 68. 



stamens corresponding in height, which means that the 

 pollen must always be taken from another flower.^ 

 There are plants where fertilisation with the pollen of 

 another variety turns out to be more fruitful than self- 

 fertilisation. Finally, quite trustworthy investigators 

 have described cases in which self-fertilisation acts like 

 poison ; the stigma of a self-pollinated pistil appears as 

 if singed, and the flower dies without forming fruit, 

 whereas pollen taken from other flowers effects fertilisa- 

 tion. Thus a number of facts prove empirically the 

 existence of a general law, according to which cross- 

 fertilisation is beneficial and self-fertilisation compara- 

 tively harmful to the organism. 



1 In the middle of fig. 68 the pollen grains, which are of different sizes, 

 are shown. 



