734 PLANT RESPONSE 



and that (2) geotropic stimulus is always of moderate in- 

 tensity. As regards the first of these two differences, it has 

 been shown that, on applying unilateral heliotropic stimulus 

 of moderate intensity to the tip of the shoot, we obtained the 

 same negative response of indirect stimulation as is given by 

 the root-tip. In the case of geotropic stimulus, however, 

 there can be no phenomenon corresponding to this, inasmuch 

 as in the stem the statolithic particles appear to be diffused, 

 instead of being localised at the tip. The second point of 

 difference between the two responses arises from the fact that 

 heliotropic stimulus may be of any degree of intensity. 

 Hence the direct excitatory effect of strong unilateral stimula- 

 tion of the root-tip may in the case of light be transmitted to 

 the growing region, and there induce a positive response, or 

 movement towards stimulus. This accounts for the fact that 

 while roots in general give one kind of gravitational response 

 of so-called positive sign (but really negative), some roots 

 give negative response to light, and others positive. 



Turning next to the direct action of unilateral heliotropic 

 stimulus on the growing region, we find, as explained in 

 the summary of responsive action in general (p. 535). that 

 the effect is modified by the intensity of stimulus, by the 

 transverse conductivity of the organ, and by its existing 

 anisotropy. Thus in the case of a radial organ, such as the 

 hypocotyl of Sinapis, moderate stimulus, its effect remaining 

 localised on the proximal side, has been shown to evoke a 

 positive responsive movement. Stronger or long-continued 

 stimulus, reaching the distal side by transverse conduction, 

 neutralises this first effect, and the organ thus remains at 

 right angles to the incident light, or in a dia-heliotropic 

 position, apparently unaffected by it. In other instances, 

 again, owing to the alternate excitation of the two sides, the 

 organ may oscillate to and fro about a mean position. With 

 still stronger stimulus, however, an anisotropy is induced, by 

 which the proximal side becomes, through fatigue, the less 

 excitable, and the internally diffused stimulus causes greater 

 contraction and resultant concavity of the distal side ; that is 





