XVIl] BARBERRY: HORSE-CHESTNUT 167 



the base of the petals from rain. Honey collects between 

 the stamens and the ovary, and in striving to get this any 

 insect may touch the base of a stamen : the latter has an 

 irritable zone at the base of its filament, and the slightest 

 touch causes the hitherto horizontal stamen to spring 

 sharply up to the ovary, like a trigger, dusting both the 

 head of the insect and its own stigma. The insect, on 

 dipping its pollen-dusted head into another flower, rubs 

 some grains on to the rim-like stigma. 



Examples of flowers with explosive mechanism i.e. 

 irritable stamens, or bursting carina, &c. are : 



In the Horse-chestnut (Fig. 45) the mechanism for 

 ensuring cross-pollination depends on the fact that the 

 stigma is mature and in position to be dusted by pollen 

 carried by humble-bees visiting the flower, when the 

 anthers are as yet immature i.e. the flower is prote- 

 rogynous ; and also on the relative position of the parts. 



When a humble-bee alights on a recently opened 

 flower it finds the seven immature stamens depressed and 

 pendant beneath its thorax as it probes the flower. The 

 style, on the contrary, is sweeping stiffly forwards and 

 upwards, so that its stigma, already mature, just touches 

 the hinder part of the insect's abdomen. Hence if any 

 pollen is dusted on the latter, some of it is rubbed on 

 to the stigma as the insect moves during its probing 

 exertions. 



Later on the stamens mature, and sweep upwards and 

 forwards so as to bring the pollen-laden anthers exactly 

 where the stigma was in the last stage, and a visiting bee 

 would now have pollen dusted on to its abdomen in 



