HELIOTEOPISM 493 



The same stimulus may also produce in other oases the 

 opposite effect. When a young root is exposed to it, it curves 

 so as to place itself in the same position with regard to the 

 incident rays, but with its growing apex placed in the opposite 

 direction. Stems are said accordingly to grow towards, and 

 roots away from, the light-source. This behaviour is not, 

 however, confined to roots, it is exhibited by the tendrils of 

 Bignonia capreolata, by the peduncles of Cyclamen persicii/m, 

 and by many other organs. 



Leaves in many oases 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 three movements in response to the stimulus of the 

 lateral light are spoken of as heliotropism, wph^liotropism, and 

 diaheliUropism respectively. The advantages thus secured are 

 in some cases very obvious ; thus the heliotropism of the stem 

 places its leaves in the most favourable position for the action 

 of the chlorophyll in decomposing the C0„ they absorb ; the 

 apheliotropism of the 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 may twine, 

 while 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 responsato the stimulus varies sometimes with the age 

 of the organ. Thus the hypocotyl of the Ivy is heliotropic when 

 young, but becomes apheliotropic when old. 



The sensitiveness varies very greatly in different organs. 

 Some of the seedlings oiPhalaris examined by Darwin responded 

 to such a feeble degree of illumination 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 apheliotropism is usually much slower than 

 that of heliotropism. 



The bending is not caused by a direct interference of the 

 light with the actual growing part. It would seem at first as if 

 the retarding effort of light upon growth might explain the 

 bending of the organ towards the light-source, the non-illu- 

 minated side continuing to grow while the illuminated one is 



