OCCURRENCE OF SUGAR IN THE STEM 79 



It is evident that in the potato we have to do with a very 

 highly modified form of stem. The corky layer of the bark is 

 well represented, and the loose cellular layer beneath is much 

 developed ; ' wood is almost lacking, but the pith is greatly 

 developed and constitutes the principal bulk of the tuber. 

 All this is readily understood if we consider that the tuber, 

 buried in and supported by the earth, does not need the kinds 

 of tissue which give strength, but only those which are well 

 adapted to store the requisite amount of food. 



93. Occurrence of sugar in the stem. Grape sugar is an 

 important substance among those used for food by the plant. It 

 received its name from the fact that it was formerly obtained 

 for chemical examination from grapes. Old dry raisins usually 

 show little masses of whitish material scattered over the skin 

 which are nearly pure grape sugar. Commercially it is now 

 manufactured on an enormous scale from starch by boiling with 

 diluted sulphuric acid. In the plant it is made from starch 

 by processes as yet imperfectly understood, and another sugar, 

 called maltose, is made from starch in the seed during germina- 

 tion. Sugar is not as well adapted for reserve deposits as starch, 

 since it ferments easily and may escape by osmosis from tissues 

 which contain it. In the onion bulb it is stored in considerable 

 quantities and may be detected by a simple chemical test. 



