THE NATURE OF PHOTOSYNTHESIS 105 



TABLE 19 



Rate of Photosynthesis of "Fontinalis" at Six Different Light Intensities 

 AND Four Different Concentrations of Carbon Dioxide. (From Harder.) 



-Light Intensity (in Meter Candles) 



KHCO3 167 667 2000 6000 18,000 36,000 



0.01 per cent... 0.12 0.42^^ *^-^^ ^'^^ ^'^^ ^'^^ 



0.04 per cent... 0.26 88^'^^ ^'^^ ^'^^ ^'"^^ ~ 



0.16 per cent... - i?f(l.lO 3.45 6.40 li;47 i 11.35 - 



1-1^ > 10.75 J 



^„ . 17^ IfioK^O 8.60 15.83 15.20 16.645 



0.32 per cent... — 1.^3 4.oU> 15.70 J 



If the table is read from left to right it becomes evident that each 

 increase in light intensity results in an increased photosynthetic rate ; also 

 if the table is read down it can be seen that each increase in carbon dioxide 

 concentration produces an increase in the rate of photosynthesis. Harder 

 therefore maintains that the rate of photosynthesis under all circumstances 

 is dependent upon the light intensity as well as upon the concentration of 

 carbon dioxide. He takes the stand that Blackman's conception of the 

 principle of limiting factors as well as Liebig's law of minimum have no 

 application to the photosynthetic process. 



There is apparently little doubt that conditions exist in which the in- 

 tensity of two or possibly more factors determines the rate of photo- 

 synthesis. It is here that the difficulty with Blackman's theory arises. 

 Blackman and Smith" state: "When several factors are possibly con- 

 trolling a function, a small increase or decrease of the factor that is limiting, 

 and that factor only, will bring about an alteration of the magnitude of 

 the functional activity." Contrary to this dictum Harder's experiments 

 show that changes in both carbon dioxide-concentration and light intensity 

 are able to eilect changes in the photosynthetic rate. The intensity of 

 one factor influences the effect produced by changes in intensity of some 

 second factor. Or, what amounts to about the same, the effect produced 

 by changes in intensity of this second factor is not the same for all in- 

 tensities of the first factor. Thus, for example, the increase in rate of 

 photosynthesis produced by augmented light intensity is dependent upon 

 the concentration of carbon dioxide; the effect prodticed by augmented 

 light intensity is greater the higher the concentration of carbon dioxide. 



When determining the influence of a single factor on the rate of photo- 

 synthesis, all other factors being kept constant, the amount by which an 

 increase in this single factor augments the photosynthetic rate depends, 

 according to Harder, in every case on the concentration of the factors kept 

 constant. The higher the concentration of these factors, the higher is 



"Blackman and Smith, Proc. Roy. Soc. London, B 83, 397 (1911). 



