356 LIFE MOVEMENTS IN PLANTS 



Owing to the difference in the time relations of the 

 two opposing activities, A and D, a phase difference often 

 arises in their respective maxima. It is probably on this 

 account that rhythmic tissues originally at standstill, exhi- 

 bit under continued stimulation a periodic up and down- 

 movement, which persists even on the cessation of the 

 stimulus. The persistence of after-oscillation depends, more- 

 over, on the intensity and duration of previous stimu- 

 lation.* 



The facts given above cannot bs explained by the 

 prevalent theory that stimulus acts merely as a releas- 

 ing agent, to set free energy which had been previously 

 stored up by the organism, like the pull of a trigger 

 causing explosion of a charged cartridge. It is true that 

 in a highly excitable tissue, the external work performed 

 and the run down of energy are disproportionately 

 greater than the energy of stimulus that induces it. But 

 in a sub-tonic tissue,- stimulus induces an effect which is 

 precisely the opposite ; instead of a depletion, there is an 

 enhancement of potential energy of the system. Thus the 

 resjionding leaf instead of undergoing a fall becomes 

 erected ; growing organs similarly exhibit a ' building up ' 

 and an acceleration of rate of growth, in contrast with 

 the usual ' break down ' and depression of the rate. It is 

 obvious that these new facts relating to the action of 

 stimulus necessitate a theory more comprehensive and 

 satisfactory than the one which has been in vogue. 



THE COMPLETE PHOTOTROPIC CURVE. 



I have explained the characteristics of the simple photo- 

 tropic curve in which the tropic curvature, on account of 

 the favourable tonic condition and strong intensity of 



* " Plant Response "—p. 293, etc. 



