THE ROOT. 



6i 



The youngest parts of the root are very much alike, but as 

 they become older they grow unlike. The just mature por- 

 tion of roots shows three characteristic regions, namely, (i) 

 an outer layer or layers, the epidermis ; (2) an inner region, 

 the stele ; (3) between these, the 

 cortex. 



72. I . The epidermis usually 

 becomes many- layered. At the 

 apex it constitutes the root-cap 

 {ep, fig. 50). On the other 

 parts of the root it sometimes 

 sloughs off entirely, exposing 

 the cells of the cortex itself, as 

 in the monocotyledons (lilies, 

 grasses, sedges, etc.) ; or, more 

 commonly, only the outer layer 

 sloughs off, leaving the inner- 

 most as the covering of the 

 cortex. It is too delicate to be 

 distinguished by the unaided 

 eye, except at the tip and 

 further back where it produces 

 root-hairs. 



73, {a) Eoot-hairs. — Those 

 cells which form the surface of 

 the root, whether they be the 

 original epidermis or cortical 

 ones which have been exposed 

 by its loss, usually develop a large number of hairs, known as 

 root-hairs (figs. 51, 52). 



Fig. 50. — Median lon^tudinal section 

 througti tlie extremity of a root of 

 Marsflia. The larger triangular cell 

 near center of figure is the apical cell. 

 The segments from the inner faces 

 may be readily traced backward ; 

 thus the dotted line ec points to the 

 fourth, c to the sixth segment from 

 the posterior right-hand face of apical 

 cell, ep, root-cap (epidermis") ; ec, 

 cortex; c, stele; en, endodermis 

 (part of cortexl ; pe, pericycle (part 

 of stele) Magniiied about loo diam. 

 — After Van Tieghem. 



These root-hairs are branches of the superficial cells (fig. 52), and may 

 be looked upon as simple extensions of them, as the finger of a glove is 

 the extension of its palm. Only one root-hair arises from a superficial 

 cell. They are usually unbranched and without transverse partitions. 



