868 EXTERNAL AND INTERNAL FACTORS CIL\P. 26 



ables. There is no theoretical reason to expect, and no actual experience 

 to indicate, that saturation with respect to, say, light intensity, or carbon 

 dioxide concentration, is always caused by the same "limiting factor." To 

 the contrary, clear indications can be found of saturation phenomena caused 

 by slow diffusion, slow carboxylation, various catalytic deficiencies, limited 

 light supply and limited supply of reductants (in bacterial photosynthesis) . 

 One may consider this as evidence of good adjustment of the photosynthetic 

 process as a whole, since it means that the different parts of the complex 

 mechanism of photosjTithesis have approximately the same maximum 

 capacity. One understands, in the light of this multiplicity of possible 

 rate-limiting steps, why repeated attempts to represent the kinetics of 

 photosynthesis by means of models consisting of a small number of reac- 

 tions, e. g., of one light reaction and one dark reaction only, could not have 

 led to more than very limited success. 



Saturation with carhon dioxide is reached at pressures three or four times 

 higher than the partial pressure of carbon dioxide in the free atmosphere 

 (c/. Table 27.1); while saturation with light is reached at light intensities 

 equivalent to 10-100% of full midday sunlight (c/. Table 28.1) and thus 

 about equal to the average light intensity to which freelj^ growing plants 

 are exposed in nature. The optimum temperature of photosynthesis is some- 

 what above the average summer temperature, at least in temperate zones. 

 An approximate adjustment of the photosynthetic mechanism to natural 

 conditions is thus obvious. Perhaps, this adjustment was achieved in 

 times when both the average temperature and the carbon dioxide content 

 of the atmosphere were somewhat higher than they are now. However, 

 this inference is by no means certain ; nature has seldom been able to de- 

 velop ideal solutions of its adaptation problems, and is usually satisfied 

 with more or less rough approximations. The present kinetics of photo- 

 synthesis may have been the best plants were able to evolve in response 

 to the now prevailing climatic conditions. 



(h) Origin of Kinetic Curve Systems of Different Types 



The distinctive feature of "Blackman type curves," described on page 

 860, are closely coincident initial, linear parts of the cui'ves. This charac- 

 teristic distinguishes them from the two other t}^es of curve systems, repre- 

 sented in figures 26.3 and 26.4. On the other hand, Blackman curves 

 (fig. 26.2) and Bose curves (fig. 26.3) have in common a wide spacing of 

 saturation plateaus, v.hile curves of the "third type" in figure 26.4 all ap- 

 proach a common saturation level. 



Coincidence or separation of the saturation levels depends on whether 

 saturation is imposed by the same parameter ¥-2, to which the set of curves 



