THE WALLFLOWER 125 



of loose pollen is hanging, for the anthers are introrse, 

 i.e. they open on the inner side. When the bee flies 

 to another flower, it is pretty sure that some of the 

 pollen which has stuck to her head will be brushed 

 on to its stigma. If this happens, as it generally will, 

 cross-fertilisation will be effected, i.e. the stigma of one 

 flower will receive the pollen of another, and this will 

 often happen between the flowers of different plants. 

 The chief way in which bees and other insects are of 

 use to flowers is in bringing about cross-fertilisation, 

 which, as Darwin's experiments prove, produces more 

 and better seed than fertilisation by pollen of the same 

 flower. In the Wallflower, however, self-fertilisation 

 is also provided for ; the anthers of the longer stamens 

 rise up immediately above the stigma, and some of 

 their pollen will be almost certain to fall upon it. 

 It is also, of course, possible that the insect may 

 happen to brush some of the pollen on to the stigma 

 of the same flower. Probably this does not happen 

 very often ; for generally, as the bee works round the 

 flower, one side of its head will touch the anthers, 

 while the opposite side brushes against the stigma. 

 In any case the Wallflower is sure to be cross-fertilised 

 very often ; and if this fails, owing to bad weather and 

 the absence of insects, there is still self-fertilisation to 

 be relied on, which is a great deal better for the plant 

 than no fertilisation at all. Many other flowers have 

 much more elaborate arrangements ensuring cross- 

 fertilisation, and rendering self -fertilisation impossible, 

 but the simple mechanism of the Wallflower is effective 

 enough. 



