606 PLANT RESPONSE 



organ possesses feeble conductivity, I have found that when 

 the extreme tip was stimulated unilaterally, by using a pencil 

 of light from a sixteen candle-power incandescent electric lamp, 

 the result was a negative movement — that is to say, a move- 

 ment away from light. This is exactly parallel to the 

 response to stimulus of the unopened flower-bud of Crocus 

 and the root-tip of Sinapis, in both of which the indirect 

 effect at the growing region, of stimulus applied on the tip, 

 was seen to be a convexity of the side acted upon, with 

 consequent negative movement of the tip. 



I next applied unilateral stimulus to the same specimen 

 a little lower down, and now, owing to the better conductivity 

 of this part of the tissue, the excitation itself was transmitted 

 to the growing region, inducing concavity and positive 

 heliotropic movement. The same effect was found to be 

 produced when stimulus was applied on the growing region 

 itself. It must be remembered that in this case the con- 

 ducting power of the tissue is not high, hence there is no 

 transmission of stimulus to the distal side, by which, as 

 we have seen, the positive curvature would be neutralised. 

 The long-continued action of light on one side here tends 

 only to increase the positive curvature to a maximum. Thus 

 when one side of the entire seedling is acted upon by light, 

 while the response of the extreme tip tends to induce a slight 

 negative, all the other parts, from immediately below it to 

 the growing region, conspire together to exhibit a much 

 stronger positive heliotropic action. The result is therefore a 

 movement towards the light. Thus we see that in the case 

 of seedlings having feeble conductivity the curvature will 

 be positive, whether it is the lower part only or the entire 

 plant which is exposed to the one-sided action of light. In 

 this fact we find the explanation of those exceptional cases 

 observed by Darwin, in which the seedling was found to bend 

 towards the light, in spite of the upper part being covered. 



We shall next take up that type of response in which the 

 tissue of the specimen is rather better conducting. In this 

 case, when the upper part of the organ is locally stimulated, 



