386 VEGETABLE PHYSIOLOGY 
Leaves in many cases show a similar sensitiveness, but 
the position they assume is different again. They place 
themselves so as to present their upper surfaces at right 
angles to the incident rays. 
These phenomena, thus associated with the incidence 
of a lateral light, are spoken of as heliotropism, aphelio- 
tropism, and diaheliotropism respectively. Sometimes the 
terms positive, negative, and transverse heliotropism are 
employed. The purposeful character of the response is 
generally obvious; the heliotropism of a stem places its 
leaves in the most favourable position for the action of the 
chlorophyll in the 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 m coming into contact 
with a support about which they can twine. The aphelio- 
tropism of the peduncles of Cyclamen, which are bent down- 
wards in a hooked fashion, enables them to grow towards 
the soil, into which they press the capsule, thus burying the 
seeds. . 
The actual stimulus appreciated by a stem appears to 
be the difference of the illumination on the two sides of the 
organ turned towards and away from the light. It seems 
as if the plant has the power of comparing such intensities 
of illumination. 
The response to the stimulus varies sometimes with the 
age of the organ. The hypocotyl of the Ivy is heliotropic 
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 movement of 
