378 VEGETABLE PHYSIOLOGY 
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 
