LUTHER BURBANK 



related plants, the anthers lie along the roof of 

 the corolla tube, where they are brushed by insects 

 that pass down the tube in search of nectar. The 

 stigma holds a similar position, but is farther out 

 toward the mouth of the tube. The stigma is a 

 very interesting structure; it is composed of two 

 flattened lips, which respond to the slightest touch. 



When a bee, after visits to other flowers, enters 

 the tube, the hair-like appendages on its back 

 brush against the lower lip of the stigma, and the 

 irritation causes the lips to close tightly together, 

 coming thus in contact with and scraping the 

 pollen-dusted back of the bee. 



Whether or not the receptive lips have secured 

 any pollen, they remain closed for four or five 

 minutes, so there is no danger that they will 

 encounter the bee as it leaves the flower laden 

 with a fresh supply of pollen from the companion 

 anthers. But a few minutes later the stigma lobes 

 open again, like a trap set for the next visitor. 



Human ingenuity could not well devise a 

 mechanism better adapted than this to secure 

 cross-poUenation and ensure against the possi- 

 bility of self-fertilization. 



The foxglove (Digitalis) also has stamens and 

 pistils lying along the roof of the corolla tube. Its 

 device to prevent self-fertilization is the less 

 ingenious but equally etfective one of ripening 



[216] 



