340 Jountal of Agricultural Research voi. lu. N0.4 



to show that there is an actual transformation of sugar to starch during 

 this period. In a general way the course of the carbohydrate transforma- 

 tions in sweet potatoes seems to be correlated with the seasonal varia- 

 tion in the temperature of the storage room. That the temperature may 

 be the controlling factor in determining the direction of the carbohydrate 

 transformation is shown by the continuous transformation of starch into 

 sugar in the sweet potato as well as in other storage organs of plants at 

 low temperatures and the reversion of the process at higher temperatures. 

 Further experimentation is necessary, however, in order to determine 

 whether temperature is the sole controlling factor. 



In some respects the behavior of the sweet potato is in marked contrast 

 to the behavior of resting storage organs of plants of temperate regions. 

 In general, it has been found that the accumulation of sugar as a result 

 of starch transformation ceases at temperatures on!}' a few degrees above 

 0° C. Thus, Fischer (1891} found that in the cortex of trees the regenera- 

 tion of starch takes place at a temperature a few degrees above 0° C, 

 while Miiller-Thurgau (1882) found that in the common potato the accu- 

 mulation of sugar practically ceases at 8° C. In the sweet potato a rapid 

 transformation of starch into sugar in excess of the quantity used for 

 respiration takes place in freshly dug potatoes at temperatures as high as 

 30° C. At later periods a marked accumulation of sugar takes place in 

 the sweet potato at temperatures much higher than those at which the 

 accumulation of sugar ordinarily ceases in resting storage organs.* 



The sweet-potato roots exhibit a further peculiarity with respect to 

 the quantitative relations of the substances formed by the conversion 

 of starch. With the exception of soft-wooded trees, where oil results 

 from the conversion of starch, reducing sugars have been obsers^ed as 

 the most usual and most abundant products resulting from starch 

 transformation in resting storage organs. In the common potato 

 Miiller-Thurgau (1877) found that cane sugar is present together with 

 glucose in the proportion of i part of cane sugar to 2.5 parts of glucose, 

 while Appleman (1914) reports in potatoes kept at a temperature around 

 0° C. for 2>2 months 3.94 per cent of total sugar and 2.40 per cent of 

 reducing sugar. In the sweet potato cane sugar is the principal product 

 formed by the conversion of starch, while the quantity of reducing 

 sugar is small. In warm storage the cane-sugar content of the Big 

 Stem sweet potatoes reached 6.96 per cent and that of the Southern 

 Queen 4.05 per cent, while the maximum reducing sugar contents were, 

 respectively, 1.67 and 0.77 per cent. In cold storage the cane-sugar 

 content of the two types rose to 8.74 and 6.94 per cent, respectively, 

 while the maximum reducing sugar content in the two cases was 2.44 



1 The transitory solution initiating the process of translocation of starch in leaves and in the storage 

 organs of plants about to resume active growth, of course, takes place at higher temiieratures. This process 

 seenis to be somewhat different in its nature from the solution of starch in resting storage organs as a result 

 of exposure to low temijeratures. 



