154 CHAP. XVIII. THE TONIC FACTOR IN PHOTOSYNTHESIS 



possible ; this can be done, as previously stated, in the 

 course of less than an hour, during which short period the 

 external conditions can be maintained absolutely constant. 



Summary 



The tonic condition of a plant may be gauged by the 



Tonometer. 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. 



Seasonal variation exerts a marked effect on photo- 

 synthetic activity. The tonic level in spring is found to be 

 nearly double that in winter. 



Traces of certain chemical substances produce a height- 

 ening of the tonic level and enhancement of activity. 



A relation was found between the chlorophyll-content 

 of the cell and photosynthetic activity. The number of 

 chloroplasts in a fully developed but still young leaf was 

 considerably larger than in very young or old leaves ; the 

 photosynthetic activity exhibited by the still young leaf 

 was found to be relatively the greater. 



Excess of starch-content in the cell exerts a partial 

 inhibitory action on photosynthetic activity. 



In photosynthesis the after-effect of stimulus in modi- 

 fying the tonic condition is detectable by the Method of 

 Cyclic Curve. Three types of physiological hysteresis are 

 observed under definite conditions : first- the negative 

 hysteresis, where physiological activity becomes depressed 

 under strong and long-continued stimulation ; second, 

 the positive hysteresis, where the feeble activity of a sub- 

 tonic tissue is enhanced in consequence of stimulation ; 

 and third, the zero hysteresis, where the activity remains 

 unchanged. 



