894 CONCENTRATION FACTORS CHAP. 27 



It hardly needs repeating that the problem must be treated, not from the 

 point of view of an apodictic "law," but on the basis of the general princi- 

 ples of reaction kinetics, and that these principles admit of "limiting fac- 

 tors" only as approximations, useful under certain extreme conditions. 



Table 27.1 gives a summary of the most important experimental de- 

 terminations of the carbon dioxide curves of photosynthesis, since the time 

 of Blackman and Smith. As a general rule, these curves rise rapidly at 

 first, then more slowly and finally go over into "saturation plateaus." 

 At excessively high [CO2] values, the rate may decline again. Table 27.1 

 gives, in the last two columns, the concentrations found necessary to pro- 

 duce full carbon dioxide saturation and half saturation, respectively. 

 (When the approach to saturation is gradual, the second figure can often 

 be given with more precision than the first one.) 



We note that the observed saturating concentrations vary all the way 

 from 0.5 X 10 -s, to 400 X 10"^ mole/1. CO2. It will be shown in section 5 

 that the higher values are beyond doubt due to depletion of carbon dioxide 

 in the medium surrounding the plants, and consequent establishment of 

 large external concentration gradients. They can be strongly reduced by 

 accelerated circulation or buffering. The lower values, on the other hand, 

 may be determined either by diffusion resistance which is not affected by 

 buffering or stirring (e. g., that of the stomata, air channels, of adsorption 

 layers, cell walls and cytoplasm), or by intrinsic kinetic characteristics of 

 photosynthesis (such as the carboxylation equilibrium and the rate of car- 

 boxylation). 



In the general discussion of the kinetic curves of photosynthesis in 

 chapter 26, three types of curve sets, P = f[Fi], with F2 as parameter, were 

 considered and designated as the first (or "Blackman") type, the second 

 (or "Bose") type, and the third type (see figs. 26.2, 26.3 and 26.4, respec- 

 tively). It was stated that curves of {he first type must arise when the 

 parameter, F2, determines the maximum rate of a partial process that does 

 not depend on the independent variable, Fi, and therefore imposes a hori- 

 zontal "ceiling" on the curves P = f(Fi), without affecting the initial slope 

 of these curves. Curves of the third type are found when the parameter af- 

 fects only the initial slope of the light curves, for example, if it codetermines 

 the rate of a process that is also proportional to the independent variable, 

 Fi. In curve systems of the second type, the parameter affects both the 

 initial slope and the saturation level. Carbon dioxide curves of all three 

 types can be expected under appropriate conditions; theoretical examples 

 were given in chapter 26. So far, however, the only experimentally deter- 

 mined carbon dioxide curve sets have been obtained with light intensity as 

 parameter. Four sets of such curves, which appear comparatively reliable 

 as far as the measuring technique is concerned, are reproduced in figures 



