106 PHOTOSYNTHESIS 



each point on the curve of the photosynthetic rate. Moreover, a change 

 in that factor which in comparison to the others is in minimum concen- 

 tration (or intensity) has the greatest efifect on the rate of photosynthesis. 

 Thus with high Hght intensity and low carbon dioxide-concentration, an 

 increase in the concentration of the latter produces the greatest change 

 and vice versa. Theoretically it is inconceivable that only one factor can 

 determine the photosynthetic rate, though Harder concedes that practically 

 it may occur that a single factor is so greatly in minimum concentration 

 compared to the others that it appears as though the concentration of this 

 one alone is determining the rate of photosynthesis. Harder also points 

 out (inconsidering primarily the influence of the two factors, carbon 

 dioxide-concentration and light intensity) that in every photosynthetic rate 

 curve there are two portions to be differentiated: the one in which light 

 intensity is the determining factor, the other in which carbon dioxide-con- 

 centration plays this role. Between these two portions of the curve there 

 is a point at which the two factors are of equal influence. This point 

 represents the most favorable relative combination of the two factors, and 

 Harder concludes that the product of the two intensities at this point is 

 lower than for any other combination of the two factors producing the 

 same photosynthetic rate. 



The interaction of the various factors is undoubtedly complex. Harder's 

 chief point is that the augmenting effect on the rate of photosynthesis of 

 a single factor is not directly proportional to the intensity of this factor at 

 all concentrations of the other factors. The higher the intensity of light 

 the greater is the augmenting effect of an increase in carbon dioxide- 

 concentration, and conversely the higher the carbon dioxide-concentration, 

 the greater is the augmenting effect of an increase in light intensity. Finally, 

 an important point in Harder's conclusions is this: that the augmenting 

 effect in the photosynthetic rate just described pertains to absolute rates. 

 Relatively the augmenting effect of an increase in carbon dioxide-concen- 

 tration is greatest with low light intensity as is also the relative effect of 

 light greatest with low carbon dioxide-concentration. 



Lundegardh " has formulated a theory which is essentially the same 

 as that of" Harder. To this Lundegardh has given the name Relativity 

 Law of Photosynthetic Factors. According to this theory the rate of 

 photosynthesis is determined by both the light intensity and the carbon 

 dioxide-concentration. The more either one of these factors is relatively 

 in minimum concentration, the greater will be the resulting augmentation 

 in the photosynthetic rate when this factor is increased. Under natural 

 conditions sunlight is available in excess, while the relatively minimum 

 factors are carbon dioxide and chlorophyll content. 



Recently Miss M. Henrici ^* obtained curves of the rate of photo- 

 synthesis with increasing light intensity which show two maxima. At low 

 illumination intensities the rate rises sharply, with increasing intensity 



"Lundegardh, Der Kreislaiif der Kohlensaure in der Natur. Jena (1924), p. 74. 

 "Henrici, Verhand. Naturf. Ges. Basel, 32, 107 (1921). 



