322 LIFE MOVEMENTB IN PLANTS 



experiment (the result of which is not given in tlie record) 

 the intensity of light was increased still further ; the 

 retardation now Itecame very marked, and it persisted for 

 a long time even on the cessation of light. 



We thus find that : 



(1) The immediate efl'ect of light of moderate 



intensity is a preliminary acceleration, followed 

 by normal retardation. The acceleration is the 

 effect of sub-minimal stimulation. The imme- 

 diate after-etl'ect is an acceleration above the 

 normal. 



(2) The immesliate eflCect of strong light is a retardation 



from the beginning ; the immediate after-effect 

 shows no acceleration, the growth rate being 

 gradually restored to the normal. 



(3) Under very strong light the induced retardation 



is very great, and this persists for a long time 

 even on the removal of light. 



The experiments described explains the reasons of com- 

 plete recovery after moderate stimulation, and also the 

 absence of recovery after strong stimulation. The imme- 

 diate after-effect of moderate stimulation is shown to be 

 an acceleration of rate above the normal. Returning tt) 

 tropic curvature, the contraction at the proximal side 

 induced by unilateral light is thus compensated by the 

 accelerated rate of growth on the cessation of light. There 

 is no such compensation in the case of strong and long 

 continued action of light ; for the after-effect of strong 

 light shows no such acceleration as the immediate after- 

 effect. 



We may perhaps go a step further in explaining this 

 difference. Stimulus was found to induce at the same 

 time two physico-chemical reactions of opposite signs (p. 144). 



