276 CHAP. XXVIII. GENERAL REVIEW 



give concordant results. The loss at 22 C. is about 

 4 per cent. 



The temperature-coefficient is determined on the 

 Differential Method by the formula 



A T -A; 



K = 



T-t 



The value of the coefficient is about 20. The photosyn- 

 thetic activity of Hydrilla becomes doubled under a rise of 

 temperature of 7 C. 



The activity at a higher temperature is found by the 

 formula 



A T =A; + K(T-*) 



The Tonic Factor in Photosynthesis 



The condition of a plant can be gauged by the Tono- 

 meter ; the ratio of response to stimulus measures the 

 physiological tone of the plant. In photosynthesis the 

 ratio, K, is found by dividing the increment of activity by 

 the increment of the factor which induces it. 



The tonic condition of a plant is modified by seasonal 

 variation, by traces of chemical substances, by the starch- 

 and chlorophyll-content of the cell, and by the action of 

 stimulus. 



Seasonal variation exerts a considerable influence on the 

 tonic condition ; the photosynthetic coefficient in spring is 

 nearly double that in winter. 



Infinitesimal traces of certain chemical substances induce 

 a heightening of the tonic level as seen in the great enhance- 

 ment in the rate of photosynthesis (p. 71). 



The photosynthetic activity is greater in a young leaf 

 than in a very young or old leaf. The chlorophyll-content 

 is at its maximum in a young leaf. 



In a sub-tonic specimen, stimulus stirs up the rela- 

 tively inert tissue to activity. The tonicity is raised and 

 the response becomes enhanced. In a vigorous specimen, 



