POSITIVE HELIOTROPISM 



537 



of the after-effect varies widely, depending on the condition 

 of the tissue as well as the intensity of stimulus. 



Orientation induced by light. — When the leaflet, with 

 its sensitive motile organ, is exposed to strong sunlight, the 

 heliotropic movement continues till the organ becomes par- 

 allel to the direction of light. The question now arises, 

 in this as in other cases of heliotropic movement, why should 

 the movement come to a stop when the organ reaches this 

 parallel position ? 



A partial answer to this question may be found in the 

 fact that such movements depend upon the effective intensity 

 of light which is absorbed, and this 

 effective intensity is greatest at perpen- 

 dicular incidence, and becomes reduced 

 to nearly zero as the rays of light are 

 rendered more and more oblique by the 

 responsive movement of the organ. This 

 consideration alone, however, would not 

 wholly explain the orientation of the 

 organ, parallel to the direction of light, 

 for we know that the directive impulse 

 caused by light persists for some time, 

 and this would cause the organ to over- 

 shoot the parallel position. In order, 

 therefore, to obtain a satisfactory explana- 

 tion, I undertook the following experi- 

 ment, which, as will be seen, completely 

 meets the difficulties of the case. 



I now took the same leaflet of Des- 

 modium as was used in the previous ex- 

 periments, and caused sunlight to strike 

 the pulvinus vertically, from below upwards, by means of a 

 suitably inclined mirror. I obtained, as will be seen (fig. 239), a 

 continuous responsive movement downwards — i.e. towards the 

 direction of the light. The average rate of movement of the 

 tip of the leaf was in this case about 2-5 mm. per minute. 

 This is somewhat greater than the upward rate of movement, 



Fig. 239. Response 

 of Terminal Leaflet 

 of Desmodium to 

 Sunlight acting from 

 Below 



The dotted portion of 

 the curve represents 

 the after-effect and 

 recovery on the 

 cessation of light. 



