THE NATURE OF PHOTOSYNTHESIS 153 



synthetic activity much greater than that of the yellow varieties. Hence 

 the latter are constituted as well in regard to the activity of the proto- 

 plasmic factor as the former. Increase in temperature augments the activ- 

 ity of this internal protoplasmic factor. But the activity of this internal 

 factor is only one step in a series of reactions. An increase in its activity 

 will result in an augmentation of the rate of the total reaction only if 

 the other steps are proceeding at a higher rate, i.e. if the rate of the re- 

 action of the internal factor is determining the total rate. Therefore, an 

 increase in temperature will become effectual and result in an augmented 

 photosynthetic rate only in those leaves which have enough chlorophyll 

 to absorb the radiant energy necessary to maintain this increased rate. In 

 leaves poor in chlorophyll an increase in temperature results in but a 

 slight rise of photosynthetic rate, because here the reactions depending 

 upon the absorption of radiant energy by the chlorophyll are determining 

 the total reaction. 



In normal leaves which usually have more light available than they 

 can utilize in photosynthesis, the internal factor is relatively in minimum 

 concentration and is determining the rate of the process, at least under 

 experimental conditions with ample carbon dioxide supplied. In the 

 leaves poor in chlorophyll the amount of absorbed radiant energy is de- 

 termining the rate of the process and the internal factor is relatively in 

 excess. Further discussion of the role of the internal factor will be found 

 in another section. 



The work of Willstatter and Stoll clearly establishes the dispropor- 

 tionality between chlorophyll-content and photosynthetic rate. Tliese 

 studies also demonstrate the complexity of the reactions constituting the 

 photosynthetic process. They emphasize the importance of agents about 

 which we know as yet practically nothing beyond the fact that they exist, 

 namely the protoplasmic or enzymatic factors. These investigations, more- 

 over, make evident the fruitfulness of the quantitative chemical treatment 

 of physiological problems and the value of chemical and physical concep- 

 tions in interpreting such complex phenomena. 



f. Water Supply. 



That water enters chemically into the photosynthetic reaction was 

 clearly established by de Saussure. The eft'ect of the water-content of the 

 plant and of the water vapor in the atmosphere on the stomatal oi>enings 

 through which the carbon dioxide passes to the centers of photosynthetic 

 activity has also been recognized for a long time. Thus water is an essen- 

 tial compound in the chemical reactions comprising the photosynthetic 

 process and through its action on the stomata indirectly influences the rate 

 of this important function of the plant. The latter fact has been brought 

 out by Kreusler's ^^' investigations. Moreover, the fact that the water- 

 content of a leaf influences the carbohydrate ratio thereof, that is, the 



^"'Kreusler, Landw. Jalirb., 14, 913 (1885). 



