PRODUCTS OF CARBON ASSIMILATION 55 



there was a sufficiency of water for the reaction to take place. 

 Priestley's thesis is examined by Parkin * and by Chapman f 

 whose papers may be consulted in the original. Attention 

 may be drawn to two facts : that all sorts of mutations, 

 sometimes chemically obscure, obtain between various car- 

 bohydrates both in the plant and in the animal ; and that the 

 methods of chemical analysis are in the main too gross for the 

 identification of the subtle changes which rapidly occur in 

 the green leaf in which in the light anabolic and katabolic 

 changes are taking place together with others concerned in 

 translocation. For these reasons it is not illogical to assume 

 that the results so far obtained have no significant bearing 

 on the point at issue. As Spoehr $ justly remarks, " In a 

 complex system such as that with which we are dealing here, 

 in which the synthesis and hydrolysis of complex compounds 

 are affected by a variety of conditions, the quantity of any 

 particular substance gives little information regarding the 

 role of that substance in a series of reactions. All of the car- 

 bohydrates, monosaccharides, disaccharides, and starch are 

 capable of conversion into each other quite independently 

 of the photosynthetic process, and even the identity of the 

 hexoses is not fixed, but they may undergo stereoisomeric 

 change. Unquestionably it would be very helpful if it were 

 known just what the first sugar is that is produced in photo- 

 synthesis. In the present state of our knowledge glucose 

 fits the theoretical requirements most adequately. Yet the 

 fact cannot be entirely disregarded that the demonstration of 

 glucose actually being the first sugar formed is still wanting." 



Fats sometimes have been described as direct products of 

 carbon assimilation, more especially in plants, such as Van- 

 cheria, which have an abundance of fat-like substance in their 

 green tissues. The evidence for the contention is anything 

 but satisfactory, especially in view of the transmutation of 

 carbohydrate into fat in various plants and under the influence 



* Parkin : " New Phyt.," 1925. 24, 57- 



f Chapman : id., 1925, 25, 30S. 



t Spoehr : '* Photosynthesis," New York, 1926, p. 219. 



