April 26, 1918] 



SCIENCE 



421 



pletely covered the inside of the tube for the 

 greater portion of ita length. Fronds were 

 chosen which were sufficiently stiff so that 

 their own elasticity caused them to remain 

 closely and evenly pressed against the inner 

 surface of the glass tube even when liquid 

 was poured in and out or shaken back and 

 fourth in the tube. 



The glass tube was sealed off at one end, 

 while at the other it was furnished with a 

 short piece of rubber covered with paraffin.' 

 The covering of paraffin was continuous and 

 care was taken to renew it each time the tube 

 was used. 



After placing the frond in the tube, the lat- 

 ter was filled with sea water (at the tempera- 

 ture of the bath) and the rubber tube was 

 clamped shut. In some cases a small bubble 

 of air was left in the tube to act as a stirrer : 

 in other cases the tube was completely filled 

 with sea water and the stirring was effected 

 by a small piece of paraffin or by a glass bead 

 covered with paraffin. 



The tube was then placed in a large water 

 bath in direct sunlight. The tube was slanted 

 80 as to receive the sunlight nearly at right 

 angles. The light passed through a sufficient 

 amount of water to filter out most of the heat 

 rays. Some light was reflected from the sur- 

 face of the water but this was practically con- 

 stant during any one experiment. The tem- 

 perature of the bath was kept constant within 

 1° in most of the experiments. 



In order to determine the degree of alkalin- 

 ity produced by photosynthesis two methods 

 were used. In the first the indicator was added 

 to the sea water containing Ulva after a defi- 

 nite exposure to sunlight; in the second the 

 indicator was added to the sea water before the 

 exposure began. In the latter case there was a 

 possibility that the presence of the indicator 

 might affect the amount of photosynthesis, 

 but it was found by control experiments that 



« It is necessary to use paraflSn which will not 

 give o£f measurable quantities of acid during the 

 time of the experiment. For this purpose paraffin 

 of a high melting point is usually advantageous. 

 Eubber should be used which gives off the mini- 

 mum amount of acid; the rubber used in these ex- 

 periments was repeatedly boiled before using. 



this was not the case with the concentrations 

 employed in these experiments. 



It was also necessary to ascertain whether 

 the degree of alkalinity produced was a reli- 

 able measure of the amount of photosynthesis. 

 This was done by making simultaneous de- 

 terminations of the degree of alkalinity and 

 the amount of oxygen evolved (by Winkler's 

 method). The results show that the amount 

 of photosynthesis, as indicated by the evolu- 

 tion of oxygen, is approximately a linear func- 

 tion (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 de- 

 termining the change in PH value regardless 

 of any possible complications such excretion 

 of alkali by the plant. 



Since the plants produce CO, by respiration 

 this must be taken into consideration. Ex- 

 periments conducted under precisely the same 

 conditions except that light was excluded 

 showed that the respiration was practically 

 constant. It is, therefore, easy to make a 

 correction for it. 



In order to ascertain how much photosyn- 

 thesis had taken place after a definite time the 

 pink color produced by the Ulva was matched 

 against the colors of a series of 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 pro- 

 duced may be easily ascertained and since this 

 corresponds to the amount of oxygen evolved 

 it gives us a direct measure of photosynthesis, 

 provided we know the amount of CO, or of O, 

 corresponding to the observed changes in alka- 

 linity. These may be determined in various 

 ways which can not be discussed here. 



7 For buffer solutions see : Sdrensen, Biochem. 

 Zeit., 21: 131, 1909; Ergeb. d. Physiol., 12: 392, 

 1912. Hijber, R., Physik. Chem. d. Zelle u. d. 

 Gewebe, 4te Aufl., 1914, 8. 169. Bayliss, W. M., 

 "Principles of General Physiology," 1915, p. 203. 



For the PH values needed in these investiga- 

 tions mixtures of .05 M borax and 0.2 M boric acid 

 (to each liter of boric acid 2.925 gm. NaCl ia 

 added) are useful. The following table gives the 



