IRRITABILITY. 499 



upwards was exposed to light of optimum intensity (p. 432), 

 falling perpendicularly upon one side of it, it curved towards 

 the light so that its direction of growth was at an angle of 45 

 with the vertical. Taking the direction of growth as the 

 resultant of the action of gravity and of light, it appears that 

 in this case the geotropic is equal to the heliotropic irritability. 

 In the case of the primary shoot of the Vetch-seedling, the 

 organ curved, whatever its original position and whatever the 

 intensity of light, so that its long axis became parallel with 

 the direction of the incident rays of light. The heliotropic 

 irritability of this organ is much greater than its geotropic 

 irritability, so much greater in fact that the latter leads to no 

 perceptible response when the directive influence of light is in 

 operation. This is even more strikingly shewn in Wiesner's 

 comparative observations on the occurrence of the first indica- 

 tion of heliotropic curvature in Vetch-shoots, some of which 

 were growing erect whilst others were rotating horizontally on 

 the clinostat. Muller-Thurgau had observed in the case of 

 other shoots that the first indication of heliotropic curvature 

 could be perceived somewhat earlier when they were rotating 

 horizontally on the clinostat than when they were growing 

 erect. In the Vetch-shoots Wiesner failed to detect any such 

 difference. 



The relation between geotropic and heliotropic irritability, 

 and the influence of the intensity of light, is very clearly brought 

 out by Wiesner's observations on the shoots of Cress-seedlings. 

 Some of these rotating horizontally on the clinostat and others 

 growing erect were placed at a distance of 2*5 metres from the 

 source of light (Wiesner's normal gas-flame), a distance at 

 which the intensity of the light was the optimum. In 35 

 minutes all the shoots had begun to curve heliotropically, and 

 in 45 minutes more they had all curved so that their long axes 

 were parallel to the directions of the incident rays. When 

 removed half a metre nearer the gas-flame, or half a metre 

 further away from it, the same curvature took place in all 

 cases, but more slowly, and the shoots on the clinostat curved 

 about an hour earlier than those which were growing erect. 

 At greater distances from the optimum-position the shoots 



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