110 



GENERAL BOTANY 



oot hair 



.Endodermis 

 Phloem 

 -Xylem 

 Cortex 

 Epidermis 



ROOTS 



Structure. Roots resemble stems in their general structure, 

 so that little need be repeated here concerning the cell elements 

 of the mam tissue areas of roots. The essential differences in 

 the arrangement of the tissues in roots and stems is indicated in 

 the following summary and figure relating to root structure. 



The epidermis of roots (Fig. 58) is often termed the piliferous 

 layer, for the reason that its cells may elongate in the younger 



portion of the root to 

 form so-called root hairs, 

 which are the most im- 

 portant absorbing area 

 of roots. It is estimated 

 that as many as three 

 hundred hairs may grow 

 on one square millimeter 

 of root surface, and that 

 the absorptive surface of 

 this portion of the root 

 is thereby increased from 

 five to ten times that of 

 an equal naked root sur- 

 face, or even more than 

 that. These hairs cover 

 a limited surface of the 

 root back of the elongating zorje. As the root grows older the 

 hairs disappear and new hairs are formed nearer the tip. 



The cortex of a typical root resembles that already described 

 for the herbaceous stem, except that there is no strengthening 

 layer in roots. The inner layer of the cortex, called the endo- 

 dermis, is, however, much better developed than it is in most 

 stems. This endodermis in roots, like the outer epidermis of 

 stems, is practically impervious to water (except in certain places 

 where the cells are not of the usual impervious type), thus keep- 

 ing the water and food streams of the phloem and xylem confined 

 and hindering them from flowing laterally into the cortex. 



FIG. 58. A transverse section of a root 



Note the lack of pith, the central xylem, and the 



radially arranged phloem masses, characteristic 



of roots 



