POSITIVE HELIOTROPISM 595 



(4) When the stimulus ceases to act, the active contraction 

 which forced the water against tension to the distal side comes 

 to an end, and there is a rebound of the expelled water to 

 the proximal side. Thus the increased growth of the distal 

 falls, and the decreased growth of the proximal rises, to the 

 normal rate of growth. The unequal tensions on the two 

 sides, which previously maintained the curvature, being now 

 equalised, the organ shows a tendency to straighten itself. 



(5) I have also shown that a tissue which has been 

 subjected to stimulus, having absorbed energy and held it 

 latent, exhibits it on the cessation of external stimulus, in 

 the form of a temporary negative after-effect, that is to say 

 an acceleration of growth above the normal. As a result o 

 this fact, the stimulated side of the organ will show an active 

 tendency to neutralise the previous curvature, and return to 

 the straight line. 



It is thus seen, from facts which I had already established 

 regarding the nature of the after-effect of stimulus, that the 

 recovery of the organ is fully explained, without postulating 

 the existence of any specific power, such as that of recti- 

 petality. 



Summary 



The responsive movement of the plant-organ towards 

 light is due to the excitatory contraction of the side acted 

 upon. The curvature of a growing organ towards light is 

 brought about by the joint action of the induced concavity 

 of the proximal and the convexity of the distal sides. The 

 former is the result of the contraction, negative turgidity- 

 variation, and retardation of growth caused by the stimulus. 

 The latter comes about by the positive turgidity-variation 

 due to forcing-in of expelled water, expansion and accelera- 

 tion of growth of the distal side. 



Such responsive movements take place in organs previously 

 devoid % of circumnutation. 



The sensitiveness of certain plant-organs to heliotropic 

 stimulus is very great. The terminal leaflet of Desmodium 



QQ2 



