FERTILIZATION OF FLOWERS. 357 



or no seed was produced. In order to reach the honey, which is 

 secreted at the bottom of the long tube of the corolla, the bee 

 thrusts its proboscis down the tube and in so doing brushes past 

 the stigma and the anthers. If a pin-eyed flower be visited the 

 pollen from the anthers is.deposited comparatively low down on 

 the proboscis. If a thrum-eyed flower be visited the pollen is 

 deposited much higher up, in accordance with the more elevated 

 position of the anthers. Thus the bees carry about two different 

 kinds of pollen on two different parts of the proboscis, a fact which 

 has been established by microscopic examination of the proboscis 

 itself with the attached pollen. 



If it be remembered that in the two different kinds of flower 

 the relative positions of the anthers and stigma are reversed, it 

 will be obvious that when a bee sucks honey from a long-styled 

 flower the stigma will be touched and pollinated by that part of 

 the proboscis which touches the anthers of a short-styled flower, 

 and vice versa. Hence it follows that pin-eyed flowers will be 

 fertilized by pollen from thrum-eyed, and thrum-eyed flowers by 

 pollen from pin-eyed. 



Here, then, we have a very precise adaptation for a special kind 

 of cross-fertilization, and Darwin further proved by his experiments 

 that this is the only kind of fertilization which results in complete 

 fertility. Cross-fertilization in these plants is, however, possible 

 in no less than four distinct ways : (1) A pin-eyed flower may 

 be fertilized by pollen from another pin-eyed ; (2) a thrum-eyed 

 flower may be fertilized by pollen from another thrum-eyed ; 

 (3) a pin-eyed flower may be fertilized by pollen from a thrum- 

 eyed ; (4) a thrum-eyed flower may be fertilized by pollen from 

 a pin-eyed. The first two methods have been termed by Darwin 

 "illegitimate unions" and they result in incomplete fertility; the 

 last two have been termed " legitimate " and result in complete 

 fertility. 



The benefit derived from the existence of the two kinds of 

 flower lies in the intercrossing, not merely of two distinct 

 flowers, but of two distinct plants, for it will be remembered that 

 each plant bears only one kind of flower. Self-fertilization is 

 not absolutely prevented in this case, for the anthers and stigma 

 are mature at the same time in the same flower, but it is not 

 likely to take place, and if it does it results in incomplete fertility 

 and weakly offspring. But even if pollen should accidentally find 

 its way to a stigma of its own plant it need not necessarily 



