Jan. 7. 1918 Behavior of Sweet Potatoes in the Ground 1 7 



of time the decrease in starch and the increase in reducing sugar precede 

 somewhat the increase in cane sugar. It appears, therefore, that reducing 

 sugar is formed first as an intermediate step in the change from starch 

 to cane sugar. The loss caused by respiration, which is considerable 

 during the curing process and in storage, is apparently slight in sweet 

 potatoes in the ground. Appreciable destruction of carbohydrates 

 appears not to occur under these conditions imtil late in the season when 

 the roots have been injured by frosts. 



The changes here described have a practical bearing on the question 

 of maturation of sweet potatoes and on the choice of the time of harvest. 

 Since the carbohydrate relations of the roots in the ground remain practi- 

 cally unchanged while the vines are uninjured, the roots can not be said 

 to undergo a definite process of ripening, in the sense of a progressive 

 transformation of one reserve substance into another, such as the change 

 of insoluble pectin into soluble pectin derivatives in the peach, or the 

 transformation of starch into cane sugar and invert sugar in the apple. 

 Under ordinary conditions the potatoes continue to grow until frost 

 without reaching any definite state of maturity recognizable by pro- 

 gressive changes in the reserve materials which they contain. The 

 changes in storage, which may perhaps be regarded as a process of matura- 

 tion, do not come in for consideration here, since statements relating to 

 the degree of maturity of sweet potatoes always refer to the growing 

 roots. It is evident from these considerations that the choice of time of 

 harvest is not a matter of maturity of the roots, but is governed by other 

 factors. The potatoes may safely be kept in the ground until the leaves 

 have been injured by frost. 



Of the changes which occur after the destruction of the leaves, 

 the accumulation of water in the roots deserves foremost considera- 

 tion. It can scarcely be doubted that this increased water con- 

 tent is detrimental to the successful storage of the roots, and causes 

 them to be more subject to decay than roots of normal water content. 

 One of the objects of the rather expensive operation of curing is to elimi- 

 nate a part of the water contained in the roots. As a rule observers agree 

 that cured sweet potatoes keep better than uncured ones. Only occasion- 

 ally a statement to the contrary is found. It may therefore safely be 

 assumed that the increase in the relative proportion of water in the roots 

 will be detrimental to storage. On this account it is of utmost importance 

 that the harvesting of sweet potatoes be not long delayed after the leaves 

 have been killed by frost. The other changes occurring in sweet potatoes 

 in the ground are essentially the same as the changes occurring in storage. 

 These changes are therefore in no way detrimental to the crop, since no 

 appreciable loss of carbohydrates occurs until the roots have been so 

 severely injured that they have lost their market value. 

 27804°— 18 2 



