io THE SYNTHESIS OF CARBOHYDRATES 



is conceivable, and is indeed probable, that when two factors 

 are close to the limiting value a change in the one not limiting 

 may have some appreciable effect in assimilation. This will 

 show itself about the inflexion of the curve where the limiting 

 factor is changing. For example, when carbon dioxide is 

 limiting, increase of temperature may cause a small increase 

 of assimilation by increasing the rate of diffusion of the carbon 

 dioxide. But all minor details like these apart, the hypothesis 

 of limiting factors rests broadly on the possibility by its means 

 of interpreting simply and logically the greatest number of 

 known facts about the rate of carbon assimilation." * 



Boysen- Jensen f studied the carbon assimilation of light 

 and shade plants : on plotting the rate of photosynthesis 

 in unit time for unit area against light intensity, he found 

 that the graph approximates to a logarithmic curve ; at first, 

 when light is limiting, the course is more or less linear ; at 

 last, when a factor other than light, temperature or carbon 

 dioxide for example, is limiting the course is again linear and 

 more or less parallel to the abscissa axis. Between these 

 extremes the curve is continuous and not abrupt since it is 

 the resultant, not of a single factor, but of two or more which 

 come into play when their intensity is more or less equivalent 

 to that of the original limiting factor. Harder J investigated 

 the problem, using, for the most part, the aquatic moss 

 Fontinalis antipyretica which can be continually used in a 

 series of experiments so that the ill-defined factor of variation 

 in material, inevitable when detached leaves are used, is 

 avoided. Using the three factors, light intensity, carbon 

 dioxide supply, and temperature, he found that when two 

 factors were kept constant and the third varied, the curve 

 obtained was approximately logarithmic. Further, he varied 

 light intensity and carbon dioxide supply, using potassium 

 hydrogen carbonate for its source. The following table sets 

 forth some of his results : — ■ 



* Smith : " Aim. Hot.," 1919, 33, 535. See also Maskell : " Proc. 

 Roy. Soc," B, 1928, 102, 48S. 



f Boysen-Jensen : '* Bot. Tidsk.," 1918, 36, 219. 

 \ Harder : " Jahrb. wiss. Bot.," 1921, 60, 531. 



