570 PLANT RESPONSE 



that the after-effect of absorbed stimulus in maintaining a 

 favourable tonic condition is more prolonged in some cases 

 than in others. In Biophytum, for example, the rhythmic 

 activity by which the leaflets are thrown into pulsation is 

 maintained only as long as the stimulating factors are acting. 

 In Desmodium, on the other hand, there is a considerable 

 capacity for storage of energy, and rhythmic activity persists 

 for a long time, even on the removal of external stimulating 

 agencies. 



We thus see that the question of turgidity-variation alone 

 is not sufficient to explain the action of light on a plant. 

 We have also to take into account the important element of 

 energy. We have briefly considered the effect of light on 

 tissues in an abnormal condition ; but our main inquiry 

 concerns itself with the precise nature of the stimulatory 

 effect of light on tissues which are in a normal tonic condition. 

 I propose to demonstrate the character of this action by 

 three independent lines of investigation. First, we shall 

 study the action of light on pulvinated organs, in order to see 

 whether or not this stimulus produces the same kind of 

 differential contractile effect as other forms of stimulation. 

 Secondly, discarding those complications which inevitably 

 result from the differentiated structure of the pulvinated 

 organ, I shall proceed to determine the precise nature of the 

 fundamental form-change undergone by a tissue when 

 excited by light. For this purpose I shall subject a radial 

 organ to diffuse stimulation of light, observing whether or not 

 under these circumstances it exhibits longitudinal contraction. 

 And, lastly, I shall study the effect of light in inducing 

 changes in the rate of growth, and shall also try to find out 

 whether the fundamental responsive action discovered in the 

 case of stationary, that is to say of non-growing, organs, 

 might not be capable of explaining the observed variations of 

 growth under the action of light. 



Effect of light on pulvinated organs. — If a strong 

 beam of light be applied to the pulvinus of the leaf of Mimosa 



