I46 CHAP. XVIII. THE TONIC FACTOR IN PHOTOSYNTHESIS 



plant may be estimated from the amplitude of response to 

 a definite testing stimulus. 



The Tonometer 



In my previous work I have explained how the in- 

 ternal condition of the plant may be revealed through 

 the quantitative relation of response to the stimulus which 

 provokes it. The response may be recorded in diverse 

 ways, as a mechanical or an electrical response, or as a 



"D 



variation in the rate of growth. We thus obtain a ratio -g, 



in which R represents the response and S the stimulus. The 

 response under the same stimulus is increased when the tonic 

 level P is in any way raised ; contrariwise, when the internal 

 condition or tone of the organ becomes depressed, the ratio 



"^ is correspondingly decreased. If we keep the testing 



stimulus constant, the amplitude of response R affords a 

 measure of the changed tonicity of the plant. In photo- 

 synthesis the tonic condition is similarly gauged by the 

 coefficient, which is the ratio of the increment of activity 

 to the increment of stimulus. 



I will now describe the effects of external and internal 

 changes on the tonic condition of the plant ; of the condi- 

 tions which modify tonicity, some are under our control, 

 while others are not. The following are the investigations 

 which have been carried out on the subject : 



(1) The effect of season has already been considered in 

 previous chapters. 



(2) The effect of traces of chemical substances on photo- 

 synthesis is fully discussed in Chapter IX. 



(3) The effect of the chlorophyll- and starch-content of 



the cell. 



(4) The effect of stimulus in modifying the tonic con- 

 dition as exhibited (a) by mechanical response, (b) by 

 autonomous response, and (c) by photosynthetic response. 



