W. J. V. bSTERHOUT AND A. R. C. HAAS 13 



question then arises whether it is a natural one. It would seem very 

 probable that the light produces a substance which accelerates the 

 reaction and unless this substance is produced in unlimited amount 

 there must come a time when the rate will become steady (or fall off) . 

 The assumption therefore seems to be reasonable. 



It is attractive to form a hypothesis as to the nature of the catalyst. 

 One might be tempted to suppose that it is chlorophyll but for the 

 fact that some plants which are deep green may not photosynthesize 

 as rapidly as those which possess less chlorophyll.'^ It is of course 

 possible that the less active plants are deficient in some essential 

 factor other than chlorophyll. On the other hand it may be necessary 

 for chlorophyll to be transformed by the light from an inactive into 

 an active form,'° so that the rate of photosynthesis depends on the 

 amount of "active chlorophyll" present. This would be analogous to 

 the well known activation of enzymes by various means. 



An equally satisfactory quantitative explanation is obtained if we 

 suppose the amount of photosynthesis to correspond to the amount 

 of a substance P, produced (under the influence of light) by the 

 monomolecular reactions 



in which .S represents a constant source {i.e., a substance which does 

 not appreciably diminish during the experiment). 



Let us suppose that in the morning, before the frond is exposed to the 

 light, 5 alone is present. On exposure to light the formation of M 

 and P begins. The amount of M. will then increase until it reaches a 

 constant value (when its rate of formation is equal to its rate gf decom- 

 position) but the value of P will continually increase, since it does not 

 undergo decomposition. When M has reached a constant value we 

 find (putting K as the velocity constant of the reaction M -^ P) 

 that the amount of M decomposed in 1 minute (unit time) is KM; 

 this is also the amount of P which is formed in 1 minute, and since the 

 reaction S -^ M produces just enough of M to balance the loss of M 

 (by transformation into P) the amount of M produced each minute 



^^ Aquatic plants taken directly from ice-covered ponds in winter are found to 

 possess but feeble photosynthetic power, though of a deep green color. 

 ^^ Such activation of substances by light is well known in photochemistry. 



