THE ROOTS 49 



ing used for the storage of sugar. The sweet potato 

 family deposit the reserve food in the form of starch 

 in the roots, which become extremely large, as may 

 be seen in the cultivated form. One of this family, 

 the " man of the earth " {Ipomea pandurata), enlarges 

 the main root and deposits starch and oil in it until 

 it reaches a length of several feet, a diameter of six 

 to eight inches, and weighs as much as twenty 

 pounds. It is to be borne in mind that not all parts 

 of the plant found imderground are roots. Many 

 species have stems which live very much like roots 

 and resemble them in general appearance. They 

 may be separated from them, however, by the 

 absence of the root-cap, and by the fact that they 

 show joints, or nodes, and produce huds, although 

 the roots of the sweet potato are also capable of 

 giving rise to buds. 



54. Metliod of growth of roots. — Another fea- 

 ture which distinguishes roots from stems is the 

 method of growth. Germinate a pea or bean, as 

 shown in § 34, until the main root is two or three 

 inches long, then mark it off in sections a quarter of 

 an inch long by means of lines of India-ink applied 

 by means of a thread or a thin splinter of wood. 



