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 in 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 



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 



