HELIOTEOPISM 493 



The same stimulus may also produce in other cases 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 

 Bigyionia capi'eolata, by the peduncles of Cijclamen ])ersicuin, 

 and by many other organs. 



Leaves in many cases show a similar sensitiveness, but the 

 position they assume is different again. The3^ 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 heliotrojnsm, aplieliotropism, and 

 diahelictrojjism 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 tlie action 

 of the chlorophyll in decomposing the CO., the3' 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 Cj^lamen, 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. Thus the hypocotj'l of the Ivy is heliotropic when 

 young, but becomes apheliotropic when old. 



The sensitiA'eness varies very greatly in different organs. 

 Some of the seedlings of PJialaris examined by Darwin responded 

 to such a feeble degree of illumination that it w^as hardly 

 sufhcient 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 bendmg 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 



