120 PHOTOSYNTHESIS 



percentage of carbon dioxide which the plant was capable of utiHzing. 

 The results of different workers varied greatly ; in some cases 4 per 

 cent carbon dioxide produced injurious effects, in others 10 per cent 

 produced optimal activity. As is now evident the cause for these con- 

 tradictions lay primarily in the fact that the other factors which influence 

 photosynthetic activity were not taken into consideration, that is, the 

 various investigators were using different light intensities, temperatures, 

 plants with unknown chlorophyll-content, etc., and while focussing their 

 attention on the influence of carbon dioxide-concentration, became en- 

 tangled in complications resulting from the interaction of these other 

 factors. Treboux,^"^ in his study of the effects of a variety of substances 

 on photosynthesis, established a direct proportionality between this activ- 

 ity and carbon dioxide-concentration and concluded that with low light 

 intensity the carbon dioxide optimum shifts with the light intensity. 



It was Blackman who first broke away from the conception of a 

 single optimum factor and developed the theory of the interaction of 

 various factors. Blackman and Smith ^°^ studied the effect of varying 

 concentrations of carbon dioxide on the photosynthetic activity of the 

 aquatic plants, Fontinalis antipyretica and Elodca. They obtained curves 

 which are of the nature of those shown in the diagram in Fig. 7. Of 

 these they state : 'Tn the weaker solutions of CO, the curve shows steadily 

 increasing assimilation proportional to the increase of COg-supply. Here 

 the light and temperature are in excess, but, at a certain point, sharply 

 defined, increase of CO, is no longer followed by further increase of 

 assimilation but the value of the assimilation remains at a fixed level. . . . 

 This part of the curve is due undoubtedly to the limiting action of either 

 illumination or the temperature. . . . 



"Had a more intense light and higher temperature been fixed upon, 

 then the ascending part of the curve would have been prolonged further 

 and a fixed (but higher) level only attained with a greater concentra- 

 tion of COg. With less light the limiting value would have been arrived 

 at sooner." 



The outstanding feature of Blackman and Smith's conclusions on the 

 relation of the rate of photosynthesis to the environmental factors, COo- 

 supply, temperature and light intensity is that photosynthesis "in every 

 combination of these factors is determined by one or the other of them 

 acting as a limiting factor. 



"The identification of the particular limiting factor in any definite 

 case is carried out by applying experimentally the following general 

 principle. When the magnitude of a function is lifiiited by one of a set 

 of possible factors, increase of that factor, and that alone, ivill be found 

 to bring about an increase of the magnitude of the function." 



Blackman and Smith also give an interpretation of the results of 



""•Treboux, Flora., 92, 63 (1903). 



""Blackman and Smith, Proc. Roy. Soc, 83 B, 389 (1911). 



