84 The Quantum Efficiency of Photosynthesis 



frequently between darkness and illumination that the concentrations of the parti- 

 cipants 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 

 light action. Thus the yield determinations were in a sense determinations of inhi- 

 bited or diminished respiration. We have changed this Situation by illuminating 

 the vessels from above the thermostat by a 100-watt constant-voltage incandescent 

 lamp (as diagrammed in Fig. 1), 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 light added in the usual manner from below the vessel. The intensity 

 of the white light 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 inten- 

 sives it was possible to eliminate repiration as an experimental quantity, and to Start 

 the yield experiments at positive rather than negative pressures, and yet still obtain 

 (as experience showed) virtually as good yields from the red light, whether the 

 base line were darkness or the white light. 



Another limiting feature of the earlier yield experiments was the short duration 

 of not only the periods of illumination (lOminutes) but also the total length of the 

 experiment (commonly less than one hour). By the use of white light we have now 

 succeeded in extending the duration of the manometric yield experiments up to at 

 least 10 hours, if not indefinitely. The effects of this important advance are several. 

 In general, the yields may now be determined under nearly the same conditions as 

 obtain during the growth and cultivation of the cells, since the light intensity, 

 temperature, medium, and gas phase during the growth and manometry are essen- 

 tially the same, and furthermore we have found that the shaking does not change 

 the cells under these conditions. 



VIII. Examples of Data 



Protocols 1, 2, and 3 provide examples of the data obtained. 



PROTOCOL No. 1 



Experiment of V-26-49. 20 r, C. 630—660 mu. 5% C0 2 in air. 260 //l of cells per vessel. Each 

 vessel alternating 10' in dark and 10' red light; thus when vessel No. 5 was dark, vessel No. 3 was 

 illuminatid, and vice versa. 



