560 Journal of Agricultural Research voi. v. no. 13 



higher temperatures approach an end point very rapidly, so that we find 

 at first a rapid transformation and after a few days almost a cessation 

 of the processes. At 5° the rates of the reactions are greately reduced, 

 but the processes continue over a much longer period of time, and the 

 starch conversion and sugar accumulation are much more extensive. At 

 this temperature the course of the reactions becomes clear. The con- 

 version of starch results in the formation of reducing sugar. As the 

 concentration of reducing sugar increases, the rate of formation of cane 

 sugar rises, but at first there is a lag in the production of cane sugar. 



There remains to be considered the influence of the vines on the car- 

 bohydrate transformations of the sweet potato. From work formerly 

 reported it appears that the conversion of starch to sugar does not take 

 place to any marked extent in the growing potato, and that the inau- 

 guration of this process is probably associated with the cessation of the 

 flow of materials from the vines. The data of the third series of experi- 

 ments confirm this suggestion and show that when the vines are de- 

 stroyed, even if the roots are left untouched in the ground, the carbo- 

 hydrate transformations begin. In the third series of experiments carried 

 out with sweet potatoes which were left in the ground for some time after 

 their vines had been cut, there is evidence which has been set forth in 

 the description of the experiments that the carbohydrate transformations 

 were well under way when the potatoes were dug. It is therefore safe to 

 conclude that the activity of the vines inhibits the conversion of starch 

 to sugar in the growing sweet potato. 



CONCLUSIONS 



From the data given in this paper it appears that in the carbohydrate 

 transformations in stored sweet potatoes starch is first converted to 

 reducing sugar and cane sugar is synthesized from the reducing sugar. 

 The rates of starch hydrolysis and of sugar synthesis in a general way 

 conform to the Van't Hoff temperature rule for rates of chemical reac- 

 tions. At high temperatures the reactions are rapid at first, but soon 

 become slower and approach an end point. At low temperatures the 

 rates are slower and the end point is so shifted as to permit a greater 

 concentration of sugar. The reactions are continuous. 



In the growing sweet potato the concentration of sugar remains com- 

 paratively low. The extensive conversion of starch into sugar appears 

 to be inhibited by the activity of the vines. When the vines are de- 

 stroyed and the flow of materials to the roots is thus interrupted, the 

 carbohydrate transformations characteristic of stored sweet potatoes are 

 begun, even if the roots are left in the ground 



