36o 



FACTORS AFFECTING PHOTOSYNTHESIS 



gators. Some of the results obtained are of considerable theoretical impor- 

 tance, but a detailed discussion of these phenomena is out of place in an 

 introductory textbook. As one example we may mention the effects of ether 

 and chloroform. Either of these substances, even in very small concentrations, 

 will arrest photosynthesis. Respiration is much less sensitive to the effects 

 of these two anaesthetics than photosynthesis, so by exposure of a plant to 

 chloroform or ether in the proper concentration it is possible to completely 

 inhibit photosynthesis without greatly influencing the rate of respiration. 



A point of considerable theoretical interest is the fact that narcotics inhibit 

 only the light reaction, while cyanides inhibit only the dark reaction of photo- 

 synthesis. 



The Effect of Internal Factors upon the Rate of Photosynthesis. — 

 I. Chlorophyll Content. — In their extensive study of the relationship between 

 chlorophyll content and photosynthesis Willstatter and Stoll (191 8) devised 

 the photosynthetic number ("Assimilationszahl") as an index to this relation. 

 The photosynthetic number is the number of grams of carbon dioxide absorbed 

 per hour per gram of chlorophyll. These two investigators have studied the 

 relation between chlorophyll content and photosynthesis in green-leaved and 

 yellow-leaved varieties of the same species (Table 34). In this experiment 

 the leaves were exposed to strong light in an atmosphere of 5 per cent carbon 

 dioxide at 25° C. 



TABLE 34 THE RELATION BETWEEN PHOTOSYNTHESIS AND CHLOROPHYLL CONTENT IN GREEN 



AND YELLOW-LEAVED VARIETIES OF ELM AND ELDER (dATA OF WILLSTATTER AND STOLL, I918) 



As shown in this table the rate of photosynthesis in green-leaved varieties 

 is not much in excess of that in yellow-leaved varieties of the same species, 

 and when expressed in terms of the photosynthetic number the yellow-leaved 

 varieties are much more efficient per unit of chlorophyll present. Other in- 

 vestigations by the same workers also seem to point to the conclusion that 

 there is no proportional relationship between chlorophyll content and photo- 



