NEGATIVE HELIOTROPISM 605 



siderable. From this it was concluded that sensitiveness to 

 light was mainly confined to the upper part of the plant, and 

 that this determined the curvature ; but to this conclusion, 

 that it was the upper rather than the lower part that was 

 sensitive to light, Darwin found and recorded several excep- 

 tions, which he regarded as inexplicable. In the case of six 

 seedlings, for instance, of which the upper parts were covered 

 with opaque shields, there was as much curvature induced as 

 in seedlings which were unshielded. In these, therefore, there 

 must have been sensitiveness in the lower parts also, thus 

 negativing the conclusion, drawn from other and more 

 numerous experiments, that it was characteristic of the upper 

 alone. 



In order to see if these discrepancies were not capable of 

 explanation, I undertook an investigation into the heliotropic 

 action of light on the seedlings of A vena sativa. The method 

 of screening the upper part of the seedlings from light, which 

 has usually been employed by Darwin and others, labours 

 under the disadvantage that the weight and contact of the 

 tinfoil or the blackened glass tube are not unlikely themselves 

 to set up a certain mechanical irritation, which may have the 

 effect of causing an unknown disturbance in the result. I 

 was therefore desirous of keeping the delicate seedling free 

 from the irritating contact of caps in the course of my own 

 experiments. For this reason I arranged for the localised 

 application of light on upper or lower or both parts of the 

 organ at will, by the method which has already been described 

 of throwing a pencil of light on the required spot in the plant 

 placed in the heliotropic chamber (p. 592). The resultant 

 movement of the organ was now continuously observed and 

 recorded, by means of the Recording Microscope which has 

 been described. 



For the sake of clearness I may here forestall matters, 

 by saying that the observed results fall under two types, 

 according to whether the given specimen possesses feeble or 

 high conductivity — that is to say, power of transmitting 

 stimulus to a distance. Taking first the case in which the 



