338 



THE WOODY STEM 



cells (Fig. 56, a), i.e. immediately below the epidermis, 

 sometimes from the epidermis itself, occasionally in 

 a deeper layer of the cortex, and fairly often in the 

 pericycle. In roots the phellogen is nearly always 

 formed in the pericycle. 



Each phellogen cell arises as the result of two rapidly 

 following divisions of the living tissue cell in which it 

 is formed, the two new parallel walls being tangentially 



cork 



CTT) 



Fig. 56. — Cork formation : a, b. Beginning of phellogen in the outer- 

 most layer of cortex ; ep., epidermis ; phel., phellogen. c. Seven 

 layers of cork formed by phellogen below epidermis, d. Diagram 

 of two cork cells showing the three layers of the wall (see text). 



directed and cutting out the phellogen cell between them 

 (Fig. 56, b, phel.). The phellogen cell then continues to 

 divide,cutting off a series of daughter cells on the outside, 

 i.e. towards the surface of the stem, forming a regular 

 radial row, and the walls of these are thickened, and be- 

 come partially converted into corky substance (Fig. 56, c). 

 The middle lamellae remain unchanged, the next layers 



