VOL. 4 (1950) QUANTUM EFFICIENCY OF PHOTOSYNTHESIS 339 



through a two walled window and was reflected by a mirror onto the bottom of a vessel, 

 alternately in the one or the other by either shifting the mirror or the manometers, 

 depending on the design of the experiment. The red light entering the vessel was com- 

 pletely absorbed. To accompUsh this, the amount of cells must be sufficiently great. 

 The amount depends upon the chlorophyll content of the cells. It was found safe, to 

 avoid loss of light, to have 300 /J of cells in each vessel. No influence of the cell con- 

 centration on the yield was observed when Hght absorption was complete and shaking 

 adequate. By this method, both O2 and COg exchanges were obtained simultaneously 

 and independently for any and every desired period of measurement, and every yield 

 determination was connected with an experimental determination of the relationship 

 CO2/O2, so that earlier uncertainties concerning this ratio (y) were eliminated. 



VI. INTERMITTENCY OF ILLUMINATION 



The cross-section of the light beam entering the vessels was about 3 cm^, that is, 

 3/8 of the bottom area, of the vessel. It can be calculated, if we disregard the scattering 

 of light, that the major part of the red light (75%) is absorbed within a distance of about 

 I mm from the bottom of the vessel. This means that the light absorbing volume is 

 only about 1/20 of the 7 ml of the cell-suspension. 



Let now the intensity of the red light be so strong, that the oxygen consumption 

 of the whole cell suspension is compensated by the oxygen evolution (compensation 

 point for Og). Then the oxygen development in the absorbing volume of the cell sus- 

 pension may approach 20 times the point where the cells become saturated with light 

 and the increment yield zero (with our cell conditions the saturation intensity is about 

 30-40 times the compensation intensity) . But we obtain maximum or high yields when 

 the vessels are shaken as described at not only compensating but even considerably 

 higher intensities, when the latter are provided by white light. This proves that under 

 our shaking conditions the cells alternate so frequently between darkness and illumina- 

 tion that the concentrations of the participants of all dark reactions virtually retain 

 their dark values — a consideration which shows the methodological importance of 

 the kind and rate of shaking. 



VII. YIELD DETERMINATIONS ABOVE THE COMPENSATION POINT 



A limiting feature of most earlier yield determinations was the low total light 

 intensity, so low that only a fraction of the respiration was compensated for by the Hght 

 action. Thus the yield determinations were in a sense determinations of inhibited or 

 diminished respiration. We have changed this situation by illuminating the vessels from 

 above the thermostat by a lOO-watt constant-voltage incandescent lamp (as diagrammed 

 in Fig. i), at such a distance that the pressure changes in the vessels become zero or 

 positive; yield determinations were then made with measured amounts of red Hght 

 added in the usual manner from below the vessel. The intensity of the white Hght 

 at the vessel surface was considerably smaller per unit area than that of the red light 

 but covered a many fold greater area and hence provided much more total effective 

 light than did the red beam. Owing to this relationship of intensities it was possible 

 to eliminate respiration as an experimental quantity, and to start the yield experiments 

 at positive rather than negative pressures, and yet still obtain (as experience showed) 

 References p. 346. 



