42 



For a time the endodermis of these roots is composed of thin-walled 

 cells not very clearly differentiated from the adjacent cortex and pericycle 

 cells ; later, the inner walls become extensively thickened except at points 



T 



a 



FIG. 31. Median longitudinal section of the root-tip of a seminal root (xzio). a, 

 Initial cells of cortex and piliferous layer ; r, meristem of root-cap ; pr, periblem ; 

 o, outer layer of periblem with mucilaginous cell walls (c) ; n, endodermal layer ; 

 s, pericycle layer ; x, central column of cells which develop into vessels. 



opposite the xylem, where the walls remain thin, allowing the movement 

 of water from the vascular cylinder into the cortex for a longer period 



than is the case in the more slender roots. 



The pericycle at first consists of thin- 

 walled cells which become thickened later. 



At the growing-point of the roots of wheat 

 are found the usual three layers of meristem 

 which give rise to the vascular cylinder, the 

 cortex, and the root-cap respectively. 



The plerome or meristem of the vascular 

 cylinder at the apex consists of a number of 

 cells somewhat irregularly arranged ; its boundary, however, can be 

 distinguished without much difficulty from the external periblem. The 

 latter, from which the cortical tissues are developed, originates from 

 one or two initial cells (Fig. 31). 



FIG. 32. Recently exfoliated 

 cells of root-cap ( x 210). 



