THE NATURE OF PHOTOSYNTHESIS 103 



TABLE 18 

 Per Cent Production of Dry AIatter of Different Plants. (Boysen-Jensen.) 



Number of Per Cent Production 



Plant Days of Dry Matter 



Triticum vulgare ; . . . . 35 6.5 



77 5.9 



Raphanus satious 19 16.8 



Pisum sativum 17 5.9 



Tropaeolum majus 11 11.2 



30 13.4 



Salsola kali 24 10.1 



results of Warburg and Negelein *^* are of direct bearing on this question 

 and of interest in this relation. They studied the photosynthetic activity 

 of the unicellular alga Chlorella and observed great variations in the 

 efficiency of these plants. They found that when the plants are raised 

 under conditions of high light intensity, the amount of radiant energy 

 utilized is low. That is, plants raised under these conditions convert but 

 a small fraction of the absorbed radiant energy into chemical energy. 

 While plants grown under low light intensity convert a larger portion of 

 the absorbed energy into chemical energy. In other words, Warburg and 

 Negelein consider that they can at will produce "light" and "shade" plants 

 and that the photosynthetic efficiency of the two types of plants is different. 

 It would be interesting to determine whether shade plants such as the 

 Oxalis used by Boysen-Jensen, while they may in total produce a smaller 

 amount of dry matter, do not utilize a greater proportion of the light ab- 

 sorbed than plants growing in the direct sunlight. 



In a previous section some discussion has already been devoted to the 

 interaction of the different factors which influence photosynthesis. Ac- 

 cording to the original theory of Blackman on "limiting factors" "the 

 rate of photosynthesis is determined by the intensity of the weakest 

 factor." If the intensity of this latter factor is increased, the rate of 

 photosynthesis increases until some other factor becomes relatively the 

 weakest or limiting one. Further increase in the intensity of the first 

 factor does not result in an augmented photosynthetic rate because this 

 factor is no longer the limiting one. These conditions have been described 

 graphically under the sections devoted to the "Principle of Limiting Fac- 

 tors" and the "Partial Pressure of Carbon Dioxide," where the recent in- 

 vestigations of Warburg and of Harder are also discussed. The latter 

 has followed the rate of photosynthesis when one factor only is changed 

 as well as when two factors (light intensity and COa-concentration) are 

 altered. 



Harder's results appear to necessitate a modification of Blackman's 

 original conception of the influence of various factors, in the sense that 

 the effect of the factor which is varied is not the same throughout the 



''Warburg and Negelein, Zdt. physik. Chem., 102, 246 (1922). 



