negative phototropism of roots i39 



Supposed Phototropic Ineffectiveness of Sunlight 



A brief reference may be made here of the apparently 

 anomalous phenomenon that ' direct sunlight is too bright to 

 bring about heliotropic curvature ; only diffuse and not direct 

 sunlight has the power of inducing heliotropic movements.' ^ 

 It is inconceivable that sunlight should have lost all photo- 

 tropic power because it is so bright. The experiments just 

 described give an adequate explanation of the apparent 

 ineffectiveness of bright light. It has been shown that the 

 tropic curvature, under moderate intensity of light, does 

 not undergo any neutralisation, but that under very high 

 intensity of bright light transverse conduction occurs, which 

 causes the undoing of the curvature. This is demonstrated by 

 the record already given of the continuous action of strong 

 light, showing that the normal positive curvature at the 

 beginning became neutralised later. 



Negative Phototropism of Roots 



The abohtion of geotropic reaction in the root, after 

 amputation of the tip, has led to the conclusion that the 

 tip is the perceptive organ, the responding organ being at 

 the growing region at a short distance from the root-tip. 

 On the analogy of the opposite geotropic responses of shoot 

 and root, the hasty generalisation has been made that the 

 sign of response of root to light is opposite to that of the 

 stem, a negative instead of a positive curvature. The 

 conclusion was apparently supported by the negative 

 phototropic curvature of the root of Sinapis. The supposed 

 analogy is, however, false ; for while in the case of the 

 root the stimulus of gravity acts only on the restricted 

 area of the tip, the stimulus of light is not necessarily so 

 restricted, since it can act not only on the tip, but also on the 

 region of growth. That there is no universal analogy between 

 the action of light and gravitation is seen from the fact that 

 while gravitation induces in the root a movement opposite 



1 Jost, ibid. p. 464. 



