254 



ELEMENTARY STUDIES IN BOTANY 



Roots differ from subterranean stems even in external 

 appearance, since they do not bear leaves or scales and do 

 not have nodes. It should be remembered that roots are 

 produced not only by the hypocot}d ( 113, p. 180), but also 

 by the nodes of stems. It is convenient to distinguish these 

 two origins by calling the roots developed by the embryo 

 primary roots, and those not developed by the embryo 



secondary roots. It has been ob- 

 served ( 136, p. 248) how sub- 

 terranean stems, prostrate stems, 

 and even erect stems " strike 

 root " from the nodes, and such 



FIG. 220. Fleshy roots; A, radish with fleshy tap-root; B, dahlia with cluster of 



fleshy roots. 



secondary roots are the only roots of many plants. For 

 example, when a new plant is developed by " layering " a 

 raspberry ( 131, p. 232), or planting a " slip " or " cutting " 

 from a grape-vine, or planting slices of a potato tuber, 

 secondary roots are the only kind possible, for the new 

 plants are not produced by seeds. 



The primary root developed by the hypocotyl may con- 

 tinue as a conspicuous, vertically descending axis that gives 

 off small branches (Fig. 219, A) ; or it may break up at once 



