140 PHOTOSYNTHESIS 



bearing. As a consequence much controversy and confusion has arisen 

 which could have been avoided if the points at issue had been kept more 

 clearly in mind.^*^ 



Willstatter and StoU ^** observed that there was considerable differ- 

 ence in the influence of temperature on the rate of photosynthesis between 

 leaves of high and low chlorophyll-content. The leaves of low clilorophyll- 

 content exhibit a lower acceleration with increasing temperature than do 

 the leaves of high chlorophyll-content. Thus leaves of Ulnius with low 

 chlorophyll-content showed a temperature coefificient of 1.34 and with 

 high chlorophyll-content of 1.53 for 15° to 25°. Willstatter and Stoll 

 call attention to the fact that in these experiments an increase in illumina- 

 tion intensity was without effect on the photosynthesis of leaves rich in 

 chlorophyll ; in fact the light intensity could be reduced by ^ without 

 affecting the rate. They explain this on the ground that in these leaves 

 the chlorophyll-content was relatively of much higher concentration than 

 the enzyme which they consider plays an important role in the photosyn- 

 thetic process. This enzyme is a protoplasmic (internal) factor and in- 

 duces a "dark" reaction. On the other hand, in the leaves low in chloro- 

 phyll, which have a low Qm, the enzyme ("dark reaction") is relatively 

 in excess and chlorophyll is relatively in minimum concentration. The 

 rate of the reaction is here determined by the chlorophyll-content and an 

 increase or decrease in light intensity is of decided influence on the rate 

 of photosynthesis. These observations are quite in harmony with the 

 ideas expressed above, of the influence of those factors which are rela- 

 tively in minimum concentration. Willstatter and StoU's experiments 

 are particularly valuable, because of their careful control of the chlorophyll 

 factor and their realization of the importance of the internal factor. The 

 nature of this internal factor they regard as being enzymatic. While 

 it must be admitted that virtually nothing is known of this "enzyme," 

 nevertheless the internal factor is doubtless a protoplasmic activity and 

 in so far as we largely depend upon the conception of enzyme activity 

 to account for protoplasmic activity the term seems permissible. More- 

 over, the effect of temperature on this factor corresponds to that which 

 has been observed for enzymatic reactions. Thus Willstatter and Stoll 

 point out that both types of leaves, those high and low in chlorophyll- 

 content, are equally equipped with the enzymatic factor. Temperature 

 exercises an accelerating influence on the activity of this internal factor. 

 However, this accelerating influence will result in a higher photosynthetic 

 rate only in those leaves which have a high chlorophyll-content, that is, 

 in those leaves which can make use of high intensities of light. In the 

 leaves low in chlorophyll-content the accelerating influence of increased 

 temperatures on the internal factor will not result in a higher rate of 



"'Brown and Heise, Philippine Jour, of Sci., 12, 1, 85 (1917). Brown, ibid., 13, 

 345 (1918). Smith, Ann. Bot., 33, 517 (1919). 



'^^ Willstatter and Stoll, "Untersuchungen ii. d. Kohlensaureassimilation," Berlin, 

 1918, pp. 56, 112, 143. 



