328 HOW PLANTS LIVE AND WORK. 



shall obtain a due supply of pollen grains, either from its own 

 stamens or from those of some other flower of the same species. 

 It has been conclusively proved by various botanists — more parti- 

 cularly Darwin, Sprengel, and H. Miiller— that a flower produces 

 more, and also better, seed when it is fertilised by pollen from 

 another flower. Such cross-fertilisation is chiefly performed by 

 insects, and, as we shall see later, some flowers have adopted 

 devices, which are perfect marvels of ingenuity, for bringing about 

 this object. The inducements offered by the Wallflower to bees 

 are not of a very complicated or subtle character, but they are 

 quite effective. 



A bee comes to a Wallflower for the sake both of honey and 

 pollen, the " bee-bread." As the bee thrusts its proboscis down 

 between stamens and pistil in search of the honey in the sepal 

 pouches, its head is pretty certain to come in contact with, and to 

 brush off, some of the pollen-dust which is hanging loose on the 

 inner faces of the anthers, for we have seen that the anthers of the 

 Wallflowers burst on their inner sides. When the bee flies off to 

 another flower and continues its search for honey, it almost 

 invariably leaves some of the pollen from the first upon the hairy 

 stigma of the second. 



Flower and bee are thus mutually helpful. The flower pro- 

 vides the bee with " bee-bread " and honey, and in order to make 

 the insect's search easier it displays gaily coloured and attractively 

 scented petals, actually marking them in some cases with lines 

 which guide the animal to the nectaries. The nectaries, however, 

 are placed in such a position that before the bee can reach the 

 honey, it must perform the service of transferring pollen to the 

 stigma. It is still possible that some few Wallflowers may not be 

 visited by insects. In order to provide for such a contingency, 

 the anthers of the four inner stamens grow up so as to overhang 

 the stigma, and some of their pollen drops on it. This self-ferti- 

 lisation is not so good as the cross-fertilisation brought about by 

 the visits of insects, but it is much better than nothing. 



The first great essential is now U7i fait accompli. The pollen 

 has gained access to the stigma of the pistil. As has been men- 

 tioned above, the top of the stigma is sticky with a sugary 

 solution. This sugary solution stimulates the pollen grains to 



