W. J. V. OSTEEHOUT AND A. R. C. HAAS 



297 



This is designated in the table as the apparent rate of photosynthesis 

 since it is not corrected for respiration. The correction is made by- 

 adding the rate of respiration^ to, the apparent rate of photosynthesis. 

 The sum gives the true rate of photosynthesis.'' The true rate is 

 seen to be 150 at 17°C. and 271 at 27°C., giving a temperature 

 coefficient of 1.81.^ 



• The question arises: How can the process have so high a tem- 

 perature coefficient? The answer is not far to seek. The writers 

 have suggested in a recent paper^ that photosynthesis involves cate- 

 nary reactions of the type S -^ M —^ P in which S represents a sub- 

 stance which, under the influence of light, breaks down to form M; 



TABLE I. 



Temperature Coefficient of Photosynthesis and Respiration in Ulva. 



Temperature 

 coefficient. 



Apparent rate of photosynthesis. 



Rate of respiration 



True rate of photosynthesis 



1.69 

 2.04 

 1.81 



Each figure represents the average of eight experiments. The rate of photo- 

 synthesis at 17°C. (which is taken as 100) represents a change from pH 8.1 to 

 pH 8.3 in 18.1 minutes. 



this in turn forms P, the amount of which is proportional to the amount 

 of photosynthesis. If the reaction S —^ M is more rapid than M -^ P- 



^ This was ascertained by repeating the experiment with the same pieces of 

 Ulva under precisely the same conditions except that light was excluded. It 

 is expressed as per cent of the rate of photosynthesis at 17°C. (which is taken as 

 100). 



"^ This involves the assumption that the rate of respiration is practically the 

 same in light and in dark (as long as the temperature remains the same) . There 

 seem to be good grounds for this assumption. 



*It is not likely that this result is affected by "limiting factors," since the 

 light was practically full sunlight, and since the temperatures were near the opti- 

 mum and the supply of CO2 was sufficient, owing to the presence of carbonates 

 and bicarbonates in the sea water. The eiifect of a limiting factor would be 

 to make the coefficient appear less than it is in reality. 



^Osterhout and Haas, /. Gen. Physiol., 1918, i, 1. 



