THE PHOTOSYNTHETIC CURVE 275 



cases = 1 ; but in acid plants it is less than i, and in extreme 

 cases it may be zero. 



The fact that photosynthesis may occur in the absence 

 of C0 2 renders the method of estimating photosynthesis 

 by the absorption of carbon dioxide less reliable than the 

 method by the evolution of oxygen. 



The catabolic activity is far more pronounced in 

 summer, which may account for the acid condition of the 

 Hydrilla in that season. The oxidation produces organic 

 acids, and finally carbon dioxide under complete physio- 

 logical combustion. When the leaves containing organic 

 acids are acted on by daylight, the building up process 

 begins, the acid taking the place of carbon dioxide. The 

 result is more or less the same, whether the plant is pro- 

 vided with an external supply of carbon dioxide or whether 

 it assimilates the organic acids produced within itself. 



Photosynthetic Curve under Variation of 

 Temperature 



An essential condition in the accurate determination 

 of photosynthesis under thermal variation is the adjustment 

 of temperature without any sudden change which causes 

 a physiological shock. This was secured by a device for 

 the gradual rise of temperature ; arrangements were also 

 made for the maintenance of constant temperature during 

 the period of the experiment. 



The increase of photosynthetic activity in Hydrilla is 

 uniform in the median range between 17 and 28 C. ; beyond 

 the optimum there is an abrupt decline. A remarkable 

 resemblance is observed between the curves of photo- 

 synthesis and of growth, under variation of temperature 

 (p. 140). 



A correction has to be made in respect of the respira- 

 tory loss in order to arrive at the absolute rate of photo- 

 synthetic evolution of oxygen. The loss due to respiration 

 has been determined by two independent methods, which 



