RATE OF CARBON ASSIMILATION 7 



increased to 50 per cent, without adverse effect,* and, accord- 

 ing to Harvey, f many species of marine algae growing in a 

 medium entirely free from oxygen, but not from carbon 

 dioxide, evolve oxygen on illumination. But since the for- 

 mation of chlorophyll is dependent upon the presence of oxy- 

 gen, the prolonged maintenance of a low oxygen pressure will 

 inhibit the development of more chlorophyll and this will in 

 turn react on carbon assimilation. 



Carbon assimilation is conditioned by various factors ; 

 wherefore the rate of the process will vary considerably ac- 

 cording to the inter-relationships of these factors. For this 

 reason it is hardly remarkable to find that different values 

 of the rate of carbon assimilation have been reported by 

 different investigators : differences in value due not only to 

 inappreciation of the conditioning factors, but also to different 

 avenues of attack. To consider a few examples : different 

 values may be expected in leaves of the same plant but 

 of different ages, and also in sun and shade leaves owing, 

 presumably, to their structural differences. Further, carbon 

 assimilation and respiration proceed side by side and are 

 antagonistic with respect to oxygen and carbon dioxide ; 

 wherefore to find the true assimilation, the respiration must 

 be ascertained ; owing to the normal variation of daylight, a 

 balance between these two processes must be of common 

 occurrence, when, owing to the low light intensity, the rate of 

 evolution of oxygen during photosynthesis is so slow that it 

 is used up in respiration as quickly as it is formed.^ Finally, 

 different methods of experiment in all probability will give 

 different values for one and the same subject. 



* Frledel : " U.S. Dept. Agric," 1901, Bull. 28. 



f Harvey : " Plant Physiol.," 1928, 3, 85. 



X This light intensity is known as the compensation point and differs 

 in different plants and in the same plant grown in different conditions ; 

 also it would appear to be lower in shade plants as the following values, 

 taken from amongst those given by Boysen-Jensen, indicate (" Bot 

 Tidsk.," 1918, 36, 219. See also Plaetzer : " Verhand phys. med. Ges.," 

 Wiirzburg, 1917, 45, 31; and Harder: " Ber. deut. bot. Ges.," 1923. 4'. 



194)- 



Sinapis alba, a sun plant . . . i-o Bunsen units X 100 



Sambucus nigra, sun leaves . . 0-7 ,, 



shade leaves . . 0-3 ,, ,, 



Oxalis acetosella, a shade plant . . 02 



