BIOLOGY OF THE FLOWER 



191 



touched by the stigma and then receives a shower of pollen. 

 The wings and keel are dislocated ; they hang vertically 

 downwards, and cannot return to their former position. 

 If the bee is dusted with pollen from another flower, cross- 

 pollination will occur ; if not, self-pollination may take 

 place as the insect leaves the flower. Imitate the action of 

 the bee by depressing the keel with a pencil and watch the 



Fig. 132. Gorse. I, leaf ; s, spine. 



explosion. Such explosive flowers can profit only by one 

 visit. 



Summary of the various grades of flower- structure. Flowers 

 may be regarded from two points of view : (1) the 

 Biological, which is concerned with the functions of the 

 flower and its relations to the outside world, especially wind 

 and insects. In the series of flowers which we have studied, 

 we find an increase in complexity of structure from simple 

 wind-pollinated flowers to more complex insect-pollinated 

 flowers, and in the latter, commencing with flowers in the 

 form of an open cup, and accessible to the lower orders of 

 insects, we find a tendency to form a deeper flower-tube, 



