W. J. V. OStERHOUT AND A. R. C. HAAS 11 



It might also be suggested that the rate becomes constant through 

 the operation of a "Hmiting factor" such as lack of light, carbon 

 dioxide, or of temperature. But it is evident that the effect of such a 

 factor would be fully felt at the very start of the reaction and that it 

 could not cause a gradual falling off in the increase of speed. 



This puts clearly before us a fundamental difficulty. The fact that 

 the rate increases most rapidly at first and then more slowly shows that 

 photosynthesis is not an autocatalytic reaction in the usual sense of the 

 word, for in such a reaction^^ the rate would increase slowly at first, 

 then more and more rapidly as time goes on. We must therefore con- 

 clude that photosynthesis belongs in a different category. 



T 



Fig. 4. Curve showing the behavior of the catalyst on the assumption that it is 

 produced by the monomolecular reaction A — >C (C, catalyst; T, time). 



The key to the situation is furnished by the figures in the second 

 column of Table I, which show that if the reaction is catalyzed by a 

 substance, it must be produced more rapidly at first and then more and 

 more slowly. It is also evident that this substance must be limited 

 in amount and that when its production ceases the rate of photosyn- 

 thesis becomes constant. We may assume that the rate of photo- 

 synthesis is proportional to the amount of the catalyst, which we will 

 call C. The figures suggest that this substance may be produced in 

 the manner characteristic of a monomolecular reaction as shown in 

 Fig. 4. We may therefore assume that C is produced by a substance 

 A, under the influence of sunlight, according to the monomolecular 

 reaction: A—^C. 



^^ I.e., under the conditions of the present experiment, where the reacting sub- 

 stances are kept approximately constant in composition. 



