W. J. V. OSTERHOUT AND A. R. C. HAAS 5 



practically constant. It is, therefore, easy to make a correction for it. 

 It does not afifect the form of the curve of photosynthesis found in the 

 present investigation. That no acid other than carbonic is given off 

 by the plant is shown by experiments in the dark in which the acidity 

 produced was completely removed by a stream of hydrogen. 



In order to ascertain how much photosynthesis had taken place 

 after a definite time the pink color produced by the Ulva was matched 

 against the colors of a series of Pyrex glass tubes^ (of the same size) 

 containing the same concentration of indicator in a series of buffer 

 solutions of known alkalinity.^" The matching was done under a 

 "Daylight" lamp, which is invaluable for this purpose. 



In this way the degree of alkalinity produced may be easily ascer- 

 tained and since this corresponds to the amount of oxygen evolved, 

 it gives us a direct measure of photosynthesis, provided we know the 

 amount of COo corresponding to the observed changes in alkalinity. 

 This may be determined by the method referred to above. '^ 



In order to carry out such investigations as the present one or to 

 study the effects of temperature, light intensity, etc., it is not necessary 

 to know the amount of CO2 abstracted; it is sufficient to compare the 

 time required to produce the same change in the color of the indicator 

 under different conditions. 



The experimental procedure was as follows: Young, vigorous 

 plants of Ulva (deep green in color and not over 4 inches in diameter) 

 were collected in the afternoon and placed in running sea water in the 

 laboratory. In the evening they were covered with a dark screen 

 so that the morning light could not reach them. On the following 

 morning the plants were placed in a water bath at 27°C. and allowed 

 to come to the temperature of the bath. 



^ These tubes were prepared and clamped shut in precisely the same manner 

 as the tubes containing Ulva. 



10 Cf. Sorensen, S. P. L., Biochem. Z., 1909, xxi, 131; Ergebn. Physiol., 1912, 

 xii, 393. Hoeber, R., Physikahsche Chemie der Zelle und der Gewebe, Leipsic, 

 4th edition, 1914, 169. BayHss, W. M., Principles of general physiology, New 

 York, 1915, 203. 



For the pH values needed in these investigations mixtures of 0.05 m borax and 

 0.2 M boric acid (to each Hter of boric acid 2.925 gm. NaCI is added) are useful. 

 The following table gives the pH values of a series of mixtures (Pahtzsch, S., 

 Biochem. Z., 1915, Ixx, 3ic>; Compt. rend. lab. Carlsberg, 1916, xi, 199). Cf. 



