THE WALLFLOWER 123 



further development. It can only become an embryo 

 or young plant after union with another cell, namely, 

 with one of the generative cells formed from the pollen- 

 grain. The contents of the pollen-grains, therefore, 

 must in some way be conveyed to the ovules in the 

 ovary, in order that fertilisation may take place. 



The first stage in this process is that the pollen- 

 grains have to be deposited on the stigma. This is 

 called pollination, and is a necessary preliminary to 

 fertilisation. It is easy to prove experimentally that 

 pollination is necessary for the production of ripe 

 seeds, i.e. seeds which contain embryos. In order to 

 prove this, the stamens of some of the flowers are cut 

 off before the anthers have dehisced to shed their 

 pollen. It is, of course, essential to do this in good 

 time, and the flower-buds must be opened artificially 

 in order to perform the operation, for as soon as the 

 flower opens naturally the pollen is shed. 



The plant must be kept under a bell-glass or under 

 very fine gauze, so that no insects may reach it which 

 might bring pollen from other flowers. There is no 

 danger of pollen being brought by the wind, as the 

 Wallflower pollen is too sticky to be blown about. It 

 is therefore better to use gauze than a bell-glass, as 

 the plant might become unhealthy if kept from the 

 air. It will be found that in all the flowers which 

 have had their stamens removed, and which are not 

 allowed to receive pollen from other wallflowers, the 

 pistils wither away without setting their seeds, and 

 if we examine their shrivelled ovules we find that they 

 contain no embryos. We can prove that it is not the 



