THE DIFFEKENTIATION OF THE PLANT-BODY 25 



FlQ. 26. COLLENCHYMA TINDER 



THE EPIDERMIS OP PETIOLE. 

 X 50. 



latter is worn away. Certain cells become specialised and 

 form layers of cork (fig. 23), which arise successively at 

 gradually increasing distances from the exterior, and in the 

 case of trees finally lead to the construction of a bark (fig. 24). 

 The corky formations are supple- 

 mented by masses or sheaths of 

 hardened or sclerenchymatous 

 parenchyma, or even by scleren- 

 chyma itself. In forms which 

 are intermediate in requirements, 

 such as the petioles of leaves, 

 layers of collenchyma are de- 

 veloped below the epidermis 

 (fig. 25). 



Sometimes sheaths or layers 

 of sclerenchyma are developed 

 instead of cork ; this condition 

 occurs especially among the stouter Monocotyledons. 



The protective mechanisms developed by roots also show 

 a good deal of variety. The outermost layer does not at first 

 take the form of an impervious membrane ; this would be 

 inconvenient in view of the necessity for the existence of 

 root-hairs. In some cases the second 

 layer undergoes modification, its cells 

 fit closely together, and the radial 

 walls become cuticularised where they 

 are in contact with each other (fig. 

 26, ex) ; it then constitutes the 

 exodermis. Later the corky change 

 extends to all the cell-walls of this 



layer. Other sheathing layers are also found more deeply 

 seated, while eventually the pericycle becomes the place of 

 formation of corky tissue. 



The second prominent differentiation which presents itself 

 is the formation of a system of cells and vessels for the trans- 

 port of water through the plant and the circulation of nutri- 

 tive and other materials. We may speak of this as the 



Fia.26. SECTION OF OUTER 

 REGION OF ROOT, SHOW- 

 ING EXODERMIS, ex. 



