STIMULATION AND ITS EESULTS 379 



process of photosynthesis of carbohydrate material ; the 

 same object is secured by the diaheliotropism of such 

 leaves as exhibit it ; the apheliotropism of a root assists it 

 in penetrating into the crevices of the soil. The tendrils 

 of Bignonia are aided by it in coming into contact with a 

 support about which they can twine. The apheliotropism 

 of the peduncles of Cyclamen, which are bent downwards 

 in a hooked fashion, enables them to grow towards the 

 soil, into which they press the capsule, thus burying the 

 seeds. 



The response to the stimulus varies sometimes with 

 the age of the organ. The hypocotyl of the Ivy is helio- 

 tropic when young, but becomes apheliotropic when old. 



The degree of sensitiveness varies very greatly in 

 different organs. Some of the seedlings of Phalaris 

 examined by Darwin responded to a degree of illumination 

 so feeble that it was hardly sufficient to cast the shadow 

 of a pencil upon a piece of white paper held close behind 

 it. The rapidity of the response also varies, some organs 

 bending almost immediately, while others do so much more 

 slowly. To this point we shall return later. The move- 

 ment of apheliotropism is usually much slower than that 

 of heliotropism. 



The bending is not caused by a direct interference of 

 the light with the part actually growing. It would seem 

 at first as if the retarding effect of light upon growth 

 might explain the bending of the organ towards the 

 light-source, the non-illuminated side continuing to grow 

 and the illuminated one being prevented from doing so. 

 This explanation is directly contradicted by the phenome- 

 non of apheliotropism. It is moreover proved to be an 

 insufficient explanation by the fact that the part which is 

 sensitive to the stimulus is not the part which actually 

 bends. Darwin showed this by preventing the access of 

 the light to a small region about one-tenth of an inch in 

 length close to the tip of the seedling, when he found that 

 the heliotropic curvature did not take place, although the 



