ENDODERMIS AND VASCULAR CYLINDER 293 



each cell of which is capable under favourable conditions 

 of growing out to form a root hair (r.h.). Within 

 is the cortex (cor.), composed of thin- walled parenchyma 

 interpenetrated by intercellular spaces. In the centre 

 is the vascular cylinder, containing the conducting 

 (vascular) system. 



The vascular cylinder is separated from the cortex 

 by a single layer of cells, the endodermis ^ (Fig. 48, e). 

 The lateral and transverse cell walls, i.e. those walls which 

 are common to adjacent cells of this layer, or the central 

 strips of these walls (Fig. 49, B), are composed of cutin 

 (see p. 284) : the outer and inner walls of the endodermis, 

 i.e. those abutting on the cortex and the tissues of the 

 cylinder respectively, are cellulose. Thus the endodermis 

 is stiffened and united into a more or less rigid layer 

 whose cells cannot be separated (Fig. 49, C), and there 

 is a free passage for water and solutes from cortex 

 to cylinder only through the inner and outer walls and 

 through the bodies of the endodermal cells. 



Immediately below the endodermis is a layer of 

 parenchymatous cells, the pericycle (Fig. 48, per.),^ 

 generally only one cell thick. Below and immediately 

 abutting on the pericycle come the vascular (conducting) 

 elements proper, arranged in alternating strands of 

 tracheids and vessels — xylem 3 and sieve tubes {s.t.) 

 with companion cells (ex.) — phloemA The xylem and 

 phloem strands are separated from one another by 



" Greek evdov, within, ddp/j.a, skin, i.e. the inner skin, the skin of 

 the vascular cylinder. This is not to be verbally confused, as is often 

 done by beginners in biology, with the " endoderm," the lining of 

 the primitive gut of animals. The terms are both derived from the 

 same Greek words, but have a very different technical meaning. It 

 is therefore important to keep the distinctive ending. 



' Greek TTepC, around, and k4kXoq, a ring, ' ' the cell layer surround- 

 ing the cylinder." 



3 Greek ^vXov, wood — the wood of a tree is secondary xylem, see p. 331. 



4 Greek, ^Xoi6q, bark — the inner bark of a tree is secondary phloem 

 (P- 34°)- 



