HOW ROOTS STORE UP STARCH. 245 



crops is more satisfactorily and reasonably explained, as 

 will be shown in a succeeding chapter. 



More as a matter of special than general interest, another 

 function of the roots is mentioned in passing. It is the 

 power of storing up matter sometimes differing from any 

 kind found in the plant, and at other times having a close 

 similarity with it. Thus we have roots which contain sub- 

 stances entirely different from any found in other parts of 

 the plant but which must necessarily have passed through 

 the plant and returned to the root. The various medicinal 

 roots; the root of the cotton plant; and of rhubarb; are ex- 

 amples of this kind. Of the other kind are those roots, as 

 the turnip, carrot, and parsnip, in which a very large quan- 

 tity of starch, sugar, etc., is stored up; and also those tu- 

 bers, as the sweet potato, and the common potato, which con- 

 tain much starch. All this matter has been elaborated in 

 the plant and returned to the roots. 



