1008 



THE LIGHT FACTOR. I. INTENSITY 



CHAP. 28 



where saturation sets in at the surface of the vessel and spreads inward with 

 increasing intensity of iHumination. 



Figure 28.20 shows, in a quaUtative way, the expected differences in 

 the plots of the absolute rate of photos>Tithesis, P, and the relative satura- 

 tion, p/p™'^^-^ against the incident energy flux, I, and the absorbed energy, 



la. 



If the rate of photosynthesis, P, is plotted against the incident light 

 intensity, /, as independent variable (curves 1), the initial slopes of the 



R 



Dense 



'Thin 



Fig. 28.20. Effect, of optical density of a cell suspension on shape of light curves. 

 Heavy and thin double arrows represent the "linear range" of dense and thin suspen- 

 sion, respectivel}'; y is the angle that determines the maximum quantum yield. 



curves vary in proportion to optical density, but the extension of the linear 

 range must be practically independent of optical density (since the curve of 

 the dense suspension must bend as soon as saturation begins in the surface 

 layer). But if P is plotted against the absorbed energy, la (curves 2), 

 the initial slopes must be the same, but the linear range of the thin suspen- 

 sion must be shorter than that of the dense one. On the other hand, if we 

 plot the relative saturation, p/p"^^^-^ against either/ or/g, (curves 3 and 4), 

 the curves of the thin suspension will remain linear much closer to full 

 saturation. 



As an illustration, figure 28.21 shows the light saturation curves, 



