THE NATURE OF PHOTOSYNTHESIS 123 



concentration surrounding the plant and that of the centers of photo- 

 synthetic activity. In the case of the higher plants there would thus be 

 introduced the factor of diflfusion in determining the rate of photo- 

 synthesis for any one condition of carbon dioxide-concentration and con- 

 sequently be more complex than under the conditions studied by Warburg. 



There is no rational basis upon which the photosynthetic activity of 

 two plants of widely different structure can be compared. Such meas- 

 ures as area, fresh weight, chlorophyll-content and number of chloro- 

 plasts, which have occasionally been used, are either not universally 

 applicable or can be determined only for certain types of plants. A quanti- 

 tative comparison of two such widely different plants as a unicellular 

 alga and the leaf of a sunflower is impossible as yet. 



Harder ^^^ has made a rather extensive study of the principle of 

 limiting factors. He was particularly concerned with the form of the 

 curves of photosynthetic activity under varying external conditions and 

 subjects Blackman's theory to rather severe criticism. In order to test 

 this theory Harder sets the following conditions : 



1. The course of the rate of photosynthesis must be followed when 

 all factors but one are maintained constant. This single factor must be 

 gradually increased. Under these conditions it will be disclosed whether 

 the photosynthetic rate curve rises in a straight line and then abruptly 

 becomes horizontal or whether the transition to the horizontal is gradual. 



2. The course of the rate of photosynthesis must be followed when 

 more than one factor is increased. Under these conditions it can be 

 determined whether the rate of photosynthesis is actually affected only 

 by the one factor which is the limiting one, or whether the other factors 

 which are changed, also influence the rate. 



As experimental material Fontinalis antipyretica, CincUdotus aqiicatlles, 

 and two species of Cladophora were employed ; the rates of photosynthesis 

 were determined by analysis of the water for oxygen, and metal filament 

 lamps used as the source of light. 



Harder found that with all of the plants used, when carbon dioxide- 

 concentration or light intensity alone are altered, the photosynthetic rate 

 curve does not exhibit an abrupt change, but changes gradually from the 

 almost vertical to a line approaching the horizontal. The form of the 

 curves varies with different plants ; an example is given in Figs. 10 and 11. 



So Harder concludes that when only one factor is changed the curve 

 representing the rate of photosynthesis does not exhibit an abrupt turn ; 

 it is rather a logarithmic curve. In other words the point at which one 

 factor ceases to be the limiting one and some other factor becomes such 

 is not a sharply defined one, but the transiticMi is a gradual one. Harder 

 furthermore, attempted to apply the mathematical formula which Baule ^^^ 



"'Harder, R., Jahrh. u-tss. Bat., 60, 531 (1921). 

 "'Baule, Landw. Jahrb., 51, 361 (1918). 



