VOL. 4 (1950) QUANTUM EFFICIENCY OF PHOTOSYNTHESIS 337 



in mm^. Then, when the oxygen developed by ilhiminating the green algae is n id 

 and the oxygen absorbed in the actinometer for the same time and beam of light is 

 »'//l, the quantum requirement per mol of O.^ developed in photosynthesis is simph' 



lj(f' = n':}i. 



IV. COMMENTS ON THE 2-VESSEL MANOMETKIC METHOD 



— CO., 



If the vield q? and the assimilatorv quotient, y = , are to be determined 



simultaneouslv, two vessels must be employed. If H be the pressure change in vessel 

 I and H' that in vessel II, the x^g and Xco2 values can be calculated by well known 

 equations (see ^ and section 8). 



The 2- vessel method, simple when the gas-exchanges in the dark are determined, 

 recjuires special attention when applied to illuminated cells. As will be shown later, the 

 illumination of the cells is an illumination with intermittent light. This intermittency 

 should be equal in the two vessels, and this is attainable if the liquid volumes are equal 

 in both vessels. Furthermore, the respiration in most cell suspensions gradually changes 

 with time, so that the pressure changes in light will also change with time. Thus the 

 two vessels should be darkened and illuminated simultaneously so that the conditions 

 of the aforementioned equations are fulfilled, namely 



•^02 ^ -^ 02 



^C02 = ^ CO2 



where the primed magnitudes refer to one vessel and the non-primed to the other. 



These conditions may be satisfactorily met by the method of alternately shifting 

 the mirror under the two vessels at periods of, e.g., 10 minutes, as indicated in Fig. i, 

 and discussed in the next section. After two or more cycles, the pressure readings for 

 each vessel for light and dark periods mav be averaged and the light action calculated 

 from the differences between the pressure changes in light and dark. A possible error 

 involving noncomparability of time periods is thus eliminated. This error has been one 

 of the main sources of difticulty in r///o;'t'//cf-photosynthesis experiments with the 2- 

 \-essel method. 



V. PROCEDURE 



Simple H.\ldane-Bakcroft constant-volume manometers with small capillaries 

 (0.8 mm diameter) with rectangular vessels attached were shaken horizontally (not by 

 arc motion) at 140-180 (usually 150) cycles per minute at an amplitude of 2.0 cm in a 

 water bath at 20° C. The two rectangular vessels of about 2.2 <3.8 td inside width and 

 length and 13-14 and i8-ig ml volume respectively, were filled with 200-400 /d cells in 

 7 ml, thus the liquid volumes were identical and the gas spaces differed. The vessels 

 (with capillary sidearm vents) were gassed on the bath, simultaneously with aid of a 

 manifold, and with shaking. The horizontal (not arc) shaking was so effective that 

 physical after-effects of gas equilibration in the transition periods of dark to light and 

 vice versa were not appreciable even when the illumination produced photosynthesis 

 far above the compensation point and pressure changes of 5-10 mm per minute were 

 involved. The manometers were usually read without stopping. The end of the mano- 

 Rejerences p. 346. 



