600 PLANT RESPONSE 



strike the root unilaterally, say from the right side. This 

 pencil of light may be so thrown as to act locally on the root- 

 tip, or on the growing region, or on both at the same time. 

 The movement of the root towards or away from light is 

 observed through the microscope focussed on the tip. The 

 eye-piece end of the microscope has a disc of glass adjusted 

 at an angle of 45 to the vertical. The observer sees the tip 

 of the root directly through the transparent disc, and at the 

 same time the reflected image of the recording point of the 

 pen, lying against the revolving drum below. The two 

 images are at the beginning of the experiment coincident, 

 and the responsive movement of the tip of the root, which 

 takes place afterwards, is easily followed by the observer with 

 the recording pen. Thus we obtain the response-record on 

 the moving surface. This method of the recording microscope 

 can always be used when attachment to the Optic Lever is not 

 possible or not desired. 



Negative heliotropic curvature induced by stimulation 

 of the tips of root and shoot. — I have by this method ob- 

 tained various records of the responses of the root and shoot 

 to the unilateral stimulus of light applied at the tip. Of these 

 I shall give, as a typical example, the record of the root of a 

 seedling of Sinapis nigra, suitably mounted in the cubical 

 trough by means of a cork. The curve seen to the left of 

 fig. 245 represents the negative movement, or movement 

 away from light, of this root, when the tip alone was 

 unilaterally stimulated. This movement was due therefore 

 to the indirect action of the stimulus on the growing 

 responding region. After a period of rest in darkness I next 

 took a record of its movement resulting from the direct 

 unilateral illumination of the growing region. I now ob- 

 tained a positive responsive curvature, as seen to the right of 

 fig. 245. It will be noticed that this particular movement 

 was relatively smaller than the preceding. We must here 

 remember that the receptivity of an organ is not the same in 

 all its different parts, and the greater negative response 

 induced in this case by the indirect action of stimulus on the 



