TRITICUM 



157 



level where maturation is complete, there are three well-defined 

 regions: (i) the epidermis; (x) the cortical zone limited cen- 

 tripetally by the endodermis; and (3) the polyarch stele limited 

 externally by the pericycle. (Fig. 67.) 



pel 



-I mx) 



!> emrck 



Fig. 67. Transection of primary root of the wheat seedling : co, cortex ; en, endodermis ; 

 e/>, epidermis ; »?x, metaxylem ; pc/, pericycle ; /»A, phloem; /ix, protoxylem. (After Avery.) 



The epidermis consists of thin-walled cells which are elongated 

 in their axial dimension. Many of them develop as root hairs by 

 an extension of a portion of the outer tangential wall, usually 

 that part of it nearest the root tip; and they may reach a length 

 of I to i^ mm. Ultimately they become desiccated, and the epider- 

 mis, with much of the cortical tissue, may disintegrate and die. 

 The cortex consists of four to eight layers of rather compact 



