THE TONIC LEVEL 145 



complexity. Among these may be mentioned (i) the com- 

 position of the atmosphere surrounding the plant ; (2) traces 

 of chemical substances present in the soil ; and (3) the after- 

 effect of stimulus. 



The effect of differences in the composition of the atmos- 

 phere is shown in a striking manner by Biophytum sensi- 

 tivum : the plants growing seven miles outside Calcutta 

 exhibit motile sensibility in a high degree, whilst others in 

 my experimental grounds in Calcutta are found to be quite 

 insensitive. 



The effect of traces of chemical substances in the soil is 

 shown in the differences of growth in equally vigorous 

 transplanted seedlings : of these some show normal rate 

 of growth, while others exhibit either an exceptionally high 

 or an exceptionally low rate. The difference cannot be 

 accidental, but must be due to traces of substances 

 in the soil which it is impossible to detect by the most 

 refined chemical tests. The effect of infinitesimal quanti- 

 ties of chemical substances in enhancement of photo- 

 synthesis has already been described ; there is no doubt 

 that traces of chemical substances affect growth and other 

 activities of the plant. 



Special investigations will be described which will show 

 that the tonic condition of the plant is also modified by the 

 action of stimulus. 



The difficulties in the present investigation are accen- 

 tuated by lack of suitable terms for describing the tonic 

 condition, and also by the absence of means for measuring 

 it. I will use the term tonic level, of which P is to be the 

 symbol, to indicate the underlying protoplasmic activity 

 whose variation modifies the tonic condition. When the 

 tonic level is raised from P to P', the diverse physiological 

 activities undergo a corresponding enhancement ; the agents 

 or conditions which modify the activity are therefore to 

 be regarded as so many tonic factors. I have devised the 

 Tonometer for the measurement of the tonic condition and 

 changes induced in that condition. The tonic level of a 



