CH. XII] DICHOGAMY. 159 



that either cross- or self-fertilisation may occur. There 

 are however a variety of characters found in flowers which 

 are apparently adapted to favour cross-fertilisation, that is 

 to render it more probable that the plant shall be cross- 

 than self-fertilised. 



Experiment has shown that the offspring of cross- 

 fertilisation is more vigorous than that of self -fertilised 

 flowers, so that any adaptation which favours cross- 

 fertilisation is an advantage to the species. These 

 experiments make it possible to understand why so 

 many flowers present arrangements by which cross- 

 fertilisation is favoured. Such modifications will be 

 preserved in the struggle for existence because they 

 increase the general effectiveness of the species. 



Dichogamy. 



In the nettle, as above mentioned, cross-fertilisation is 

 a necessity, because the pollen-grain and the egg-cell are 

 the product of different individuals. The gyncecium is 

 separated in space from the androecium. In the pheno- 

 menon known as dichogamy the separation is one of time, 

 not of space. 



The plantain is a good example of this state of things. 

 The head or inflorescence of the plantain bears a series of 

 flowers of graduated ages, those at the base are the oldest, 

 while the free end of the spike bears the youngest flowers. 

 The younger flowers (fig. 72 F) show a stigma projecting 

 beyond the corolla, but no stamens are to be seen. On 

 dissection they will be found in an immature condition, 

 tucked away within the flower. In this stage the flower 



