CHAPTER XVIII 



THE TONIC FACTOR IN PHOTOSYNTHESIS 



The effect of season on physiological activity — The tonic level — The 

 Tonometer — Measurement of tonicity from the ratio of response to 

 stimulus — Effect of season on photosynthesis — Induced change of the 

 tonic level under seasonal variation — Effect of minute traces of chemical 

 substances in modifying the tonic condition — Correlation of the 

 number of chloroplasts and photosynthetic activity — Effect of excess 

 of starch-content in the cell — After-effect of stimulus in modifying 

 the tonic condition exhibited (a) by mechanical response, (b) by 

 autonomous response, and (c) by photosynthetic response — Determina- 

 tion of physiological Hysteresis by the Method of Cyclic Curve — 

 Negative, Positive and Zero Hysteresis. 



The diverse activities of the plant are profoundly modified 

 by the internal physiological factor vaguely described as the 

 tonic condition. This is strikingly shown in the effect of 

 season : the mechanical response of Mimosa in winter is 

 very feeble, while a few weeks after, in spring, the response 

 becomes greatly enhanced. I also find that the velocity 

 of transmission of excitation in this plant is increased from 

 about 4 mm. in winter to 30 mm. in summer. Again, the 

 minimal intensity of stimulus which is effective in evoking 

 response in summer is quite ineffective in winter. 



Similar changes under variation of season are observed 

 in the autonomous activities of growth, which, arrested in 

 winter, become revived in spring. The pulsatory activity 

 of the leaflet of Desmodium gyrans is greatly depressed in 

 winter, while in spring or summer it becomes very vigorous. 

 Photosynthesis presents parallel variations, the photosyn- 

 thetic activity in spring being at least i-8 times greater 

 than that in winter. 



There are other factors, whose influence on the tonic 

 condition has not been fully recognised on account of their 



