4 DYNAMICS OF PHOTOSYNTHESIS 



sunlight; in the second, the indicator was added to the sea water 

 before the exposure began. In the hitter case there was a possibility 

 that the presence of the indicator might affect the amount of photo- 

 synthesis but it was found by control experiments that this was not 

 the case with the concentrations employed in these experiments. 



There is an advantage in adding the indicator at the start since this 

 permits us to compare the times required to produce a given amount 

 of change under different conditions. A comparison of the values 

 thus obtained is more valuable than a comparison of the amounts 

 of CO2 abstracted during equal times, for the former procedure com- 

 pares the reaction velocities accurately while the- latter may not. 

 In case the indicator is added at the end, instead of at the beginning, 

 curves may be constructed (plotting CO2 against time) from which the 

 time required for equal changes in alkalinity may be obtained by inter- 

 polation. 



In case any substance is added to the solution which changes its 

 buffer value, due allowance must be made for this fact. An apparatus 

 for determining the buffer action of added reagents has recently been 

 described by one of us.'^ 



It was found by preliminary experiments that the amount of COo 

 abstracted by the plant was an approximately linear function of the 

 pH value (in the range here employed, between pH 8.1 and pH 8.3). 



It was necessary to ascertain whether the degree of alkalinity pro- 

 duced was a reliable measure of the amount of photosynthesis. This 

 was done by making simultaneous determinations of the degree of 

 alkalinity and the amount of oxygen evolved (by a modification of 

 Winkler's method recently described by us^). The results show that 

 the amount of photosynthesis, as indicated by the evolution of oxygen, 

 is approximately a linear function (in this range) of the change in the 

 pH value of the sea water. This being so we can measure the amount 

 of photosynthesis by determining the change in pH value regardless 

 of any possible complications, such as excretion of alkali by the plant. 



Since the plants produce CO2 by respiration this must be taken into 

 consideration. Experiments conducted under precisely the same con- 

 ditions, except that light was excluded, showed that the respiration was 



^ Osterhout, W. J. V., /. Biol. Chcni., 1918, xxxv, 237. 



SQsterhout, W. J. \., and Haas, A. R. C, /. Biol. Chem., 1917, xxxii, 141. 



