54 THE SYNTHESIS OF CARBOHYDRATES 



indicates not that it is the first sugar of photosynthesis, as 

 earlier workers concluded, but that it is a secondary product ; 

 the available evidence, however, does not. prove that sucrose 

 is a storage product. Priestley thinks that cane sugar is 

 produced as a secondary product by the activity of the proto- 

 plasm more especially of merismatic cells ; it, therefore, is not 

 directly concerned with the synthesis or hydrolysis of starch 

 except in so far that starch and other carbohydrate products 

 of photosynthesis are used up in the anabolic processes of the 

 cell which later produces the sucrose as a secondary product. 

 Priestley concludes that neither physiological nor chemical 

 data support the view that cane sugar has its origin in starch. 

 If starch be a glucose anhydride, it follows that if sucrose is 

 formed from starch some of the glucose must be converted 

 into levulose. There is, however, some evidence that starch 

 is converted in sucrose. Schroeder and Horn * found that the 

 cane sugar content in detached starch containing leaves of 

 Tropceolum varies with the water content, an increase occurring 

 with a falling water supply and a decrease with a rising water 

 content. It was also observed that in the detached leaves of 

 many plants the starch disappears more rapidly if the leaves 

 are allowed to wilt. Ahrens f also observed that in the wilting 

 leaf starch was converted into sucrose and hexose, presumably 

 by hydrolysis ; but from what is known of enzyme action, 

 the hydrolysis of starch should accompany an excess of 

 water whilst a lack of water should promote synthesis. 

 For this reason Priestley discounts the above work, in so far 

 as it affects the present problem ; it has, however, been sup- 

 ported by the observations of de Wolff , % who found that sucrose 

 was produced from starch grains containing not more than 

 53 per cent, of dry starch when slowly dried, but if the grains 

 contained more than the stated amount of dry starch, no 

 sucrose was formed on similar treatment. This indicates that 

 the relationship between the amount of water and starch is a 

 factor of importance, and presumably in Ahrens's wilting leaves 



* Schroeder and Horn : " Biochem. Zeit.," 1922, 130, 165. 



f Ahrens : " Bot. Arch.," 1924, 5, 234. 



\ de Wolff : " Biochem. Zeit.," 1926, 176,, 225. 



