216 



A TEXTBOOK OF BOTANY 



[Ch. V, 



1 ' 



tive enzymes, and partly by mechanical rupture, as a later 

 picture illustrates (Fig. 164). Thismethodof origin contrasts 

 greatly with that of leaves, which arise as surface swellings 

 in the bud, while the origin of branches is intermechate in 



nature. The vein, or 



fibro-vascular, system 

 of roots is in perfect 

 continuity with the 

 systems in stems and 

 leaves. The separate 

 fibro-vascular bundles 

 of young roots, clearly 

 visible in sections by 

 aid of a hand lens, 

 differ little from those 

 of the stem, although 

 the fibro-vascular sys- 

 tem of roots as a whole 

 is more strongh' con- 

 densed towards the 

 center, often obliterat- 

 ing the pith (Fig. 158). 

 Thus, while stems ap- 

 proximate, as we have 

 seen (page US'), to the 

 hollow-column princi- 

 ple of construction, 

 roots are built rather 

 on the plan of cords or 

 cables. The difference 

 is obviously correlated with the different kinds of strains 

 the two parts have to bear; for, while stems are exposed 

 to great lateral strains from the winds (and, on the non- 

 vertical parts, from their weight), against which the hollow 

 column is most effective, the roots are exposed only to 

 pulling strains, in resistance to which the soUd cable is best. 



Fig. 159. — A typical root tip, of Radish ; 

 magnified. 



