paOTOTROPiSM ^]5 



(6) The effect of intensity of light. 



(7) The effect of duration of stimulation. 

 (S) The transmitted effect of light. 



(9) The effect of unequal excitability in ditlerent 



zones of. the organ. 



(10) The effect of transverse conduction in modifica- 



tion of the sign of response, 



(11) The effect of temperature on phototropic action. 



(12) The modification of ret^ponse due to differential 



excitability of the organ. 

 (liJ) Nastic and tropic reactions. 

 (14) The torsional effect of light. 



The sketch given above will give ns some idea of the 

 complexity of the problem. In this and in the following 

 papers I shall ilescribe the investigations I have carried 

 oat on the subjects detailed above. 



ACTION OF LIGHT. 



I have shown that there is no essential difference 

 between the responses of pulvinated and growing organs, 

 that diminution of turgor induced by stimulus brings 

 about contraction in the one, and retardation of the rate 

 of growth in the other. Indirect stimulation, on the other 

 hand, induces an expansion and acceleration ot the rate of 

 growth. The experimental investigation on the tropic effect 

 of light may therefore be carried out both with pulvinated 

 and growing organs. 



As regards the effect of direct stimulus of light on 

 growing organs we found (p. 20S) that it induces an in- 

 cipient contraction, seen in diminution of the rate of growth : 

 this incipient contraction culminates in an actual contrac- 

 tion under increasing intensitv of light. The contraction 



