56 o 
Journal of Agricultural Research 
Vol. 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 0 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. 
