The Quantum Efficiency of Photosynthesis 



83 



produced by means of the Steinheil monochromator, entered the side of the ther- 

 mostat through a two walled window and was reflected by a mirror onto the bot- 

 tom 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 completely absorbed. To accomplish 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 /i\ of cells in each vessel. 



100 Watt incandescent 

 (White hght) 



Winao»/ ot thermoitct 



Rea light ot 



measurea intensity 



To manometer 



vented 

 Stoppers- 



Q= 



Mirror 



No influence of the cell concentration on the yield was observed when light ab- 

 sorption was complete and shaking adequate. By this method, both Oi and CO2 

 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 uncer- 

 tainties concerning this ratio (}') were eliminated. 



VI. Intermittency of Illumination 



The cross-section of the light beam entering the vessels was about 3 cm 2 , that is, 

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

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

 distance of about 1 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 (compensa- 

 tion point for O2). Then the oxygen development in the absorbing volume of the 

 cell Suspension 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 maxi- 

 mum or high yields when the vessels are shaken as described at not only compen- 

 sating but even considerably higher intensities, when the latter are provided by 

 white light. This proves that under our shaking conditions the cells alternate so 



