FLORAL MECHANISMS 381 



anything strong enough pushes against the plate formed of 

 the two sterile anther lobes, this moves inwards and the 

 'fertile anther halves swing down. This is what happens 

 when a bee pushes into the flower seeking nectar, with the 

 result that the insect's back is dusted with pollen at a definite 

 spot. As the bee leaves, the stamens swing back into their 

 former position. At this stage of the flower's development 

 the stigmas, just projecting from the upper lip, are not 

 receptive, and are not touched by the bee. Later, when the 

 stamens have shed their pollen, the style grows out and down 

 so that the stigmas occupy the position reached by the 

 anthers when they touch the back of a visiting bee. If a 

 bee visits the flower at this stage the stigmas will touch its 

 back where it is dusted with pollen from other flowers. 

 Pollination can thus take place only in one particular way, 

 and it is carried out only by heavy bees sufficiently powerful 

 to work the mechanism. 



It will be further noted that only cross-pollination can 

 take place ; this is ensured both by the floral mechanism 

 and by the fact that the stamens and stigmas are mature at 

 different times. The number of methods by which cross- 

 pollination is more or less certainly effected is very large. 

 This, taken along with the breeder's experience of hybrid 

 vigour, is the chief support of the opinion cautiously ex- 

 pressed by Darwin in the Origin of Species^ that "it is a 

 general law of nature (utterly ignorant though we be of the 

 meaning of the law) that no organic being self-fertilises 

 itself for an eternity of generations ; but that a cross with 

 another individual is occasionally — perhaps at very long 

 intervals — indisp ensable . ' ' 



The investigation of floral mechanisms in relation to 

 pollination, particularly by insect agency, has been carried 

 out largely under the influence of this belief, which has 

 perhaps tended to influence conclusions unduly. Along 

 with some further examples of floral mechanism we may 

 consider this problem of cross- and self-pollination. 



