218 PHOTOSYNTHESIS 



which the latter occurs are not fully known. In a reaction which proceeds 

 by steps, the rate is determined by the step which progresses with the 

 lowest speed. The accumulation of a certain substance may tell nothing 

 concerning in which step in the reaction this substance is produced. In 

 fact, if the first product formed is very reactive, under the particular 

 conditions existing, this product may not be detected at all by means of 

 the methods of chemical analysis, or it may be found only in very small 

 amounts. 



Virtually the same point has recently been made by Priestley."' He 

 states: "All these investigators, Brown and Morris, Parkin, and Davis, 

 Daish and Sawyer, conclude that the evidence suggests that cane sugar 

 plays this role and in general upon the same ground, namely, that if the 

 sugars in the leaf are estimated at different times then the fluctuations in 

 amount, with the passage of the leaf through alternating periods of light 

 and darkness, are greater in the case of cane sugar than of the hexoses 

 present, whilst maltose is not found in some leaves (as the snowdrop) 

 and is perhaps only present in traces in other leaves. From the experi- 

 mental data, this theoretical conclusion does not seem to the present writer 

 to follow, because it must be remembered that in most green leaves sugars 

 are formed as intermediate steps in the synthesis of more complicated 

 bodies, such as starch, and it is a necessity of the case that for the prompt 

 formation of these complex anhydrides the intermediate steps must be 

 rapidly and smoothly passed through. It is to be expected then that the 

 intermediate stages in the process, including the sugars first formed in 

 photosynthesis, instead of accumulating in the light, and therefore fluctuat- 

 ing in amount should pass rapidly into other substances in the complex 

 chain of metaboHc changes so that little if any change in their concentra- 

 tion can be detected, and in any case no such phenomenon would occur 

 as a local accumulation such as is characteristic of a storage product." 



For reasons similar to those just given the view that sucrose is the 

 first sugar formed in photosynthesis has not been very generally accepted 

 by writers on the subject. Rather has the older view been retained that 

 glucose is the first carbohydrate, though it must be admitted that this has 

 little direct experimental evidence to support it. Weevers "" has obtained 

 results, which agree with those of Campbell,"^ to the effect that mono- 

 saccharides are the first sugars synthesized. Weevers worked with varie- 

 gated leaves and found that in 10 out of 12 subjects examined the 

 variegated portions contain sucrose only, while the green parts contain 

 both ^sucrose and hexoses. In the green portions monosaccharides were 

 the first sugars to appear when leaves, which had been freed by placing 

 them in the dark, were illuminated. Weevers found invertase in the varie- 

 gated as well as the green portion of the leaves. 



As was mentioned in an earlier portion of this section the weak point 



^"'Priestley, New Phytologist, 23, 255 (1924). ,1no,^ 



"* Weevers, Proc. Kon. Akad. IVctcns, Amsterdam, 27, 46 (lyzj;. 

 '■^Campbell, Jour. Agri. Sci., 4, 248 (1912). 



