PROPULSION OF SAP AND ITS VARIOUS EFFECTS 4OI 



movement, namely, that of recovery, was an active and not a 

 passive process. Certain experiments that I am about to 

 describe will enable us to analyse response and recovery 

 still more closely. 



When an organ is locallycxcited, the contraction induced 

 results in an expulsion of water, thus reducing the turgidity 

 of the organ, and the consequence of this is a mechanical 

 fall of the leaf. Some internal activity now forces the water 

 back into the organ from 

 which it was expelled, and 

 we have the recovery or 

 erection of the leaf. That 

 it is the internal energy 

 of the plant to which this 

 recovery is due, will be 

 seen clearly from the 

 following experiment. A 

 leaf of BiopJiytiim falls, as 

 has been said before, when 

 it is stimulated, whatever 

 be the form of stimulus. 

 I shall explain in my 

 chapter on the effect of 

 stimulus of light, that the 

 effect of photic stimulus 

 is the same as that of 

 any other form, while its 

 special advantage is that 

 it may be applied without 

 causing any mechanical disturbance, and that its intensity can 

 be very easily regulated. 



On subjecting a petiole of BiopJiytiim, therefore, to the 

 action of sunlight, the leaf responded by depression, the 

 average rate of fall of the tip of the leaf being '16 mm. per 

 minute. In half an hour it had passed through almost 5 mm. 

 On now shutting off the light, the after-effect persisted for 

 another five minutes, when there followed recovery by slow 



D D 



Fig. 166. Record showing Recovery to be 

 Hastened by the Increase of Internal 

 Activity which is caused by Application 

 of Warm Water to the Roots 



The first part of the record shows the fall 

 of the leaf, due to direct stimulation by 

 sunlight during thirty minutes. Stimulus 

 is next cut off at point marked by 

 interruption of record. After-eflfect 

 persists for five minutes, and there is a 

 subsequent slow recovery. Warm water 

 applied to root at moment x , with the 

 result of quickening the rate of recovery 

 by fifteen times. 



