THE DIFFERENTIATION OF THE PLANT-BODY 21 



FlG. 25. COLLENCHYMA UNDEB 

 THE EPIDERMIS OF PETIOLE 

 x 50. 



supplemented by masses or sheaths of hardened or scler 



enchymatous parenchyma or even by sclerenchyma itself 



In forms which are intermediate 



in requirements, such as the 



petioles of leaves, layers of 



collenchyma are developed 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 outer- 

 most layer does not at first take the form of an impervious 

 membrane ; this would be inconvenient in view of the neces- 

 sity.for the existence of root-hairs. In some cases the second 

 layer later undergoes modification, its cells becoming thick- 

 ened in a peculiar manner ; it then constitutes the exoder- 

 mis (fig. 26). Other sheathing layers 

 are also found more deeply seated, 

 while eventually the pericycle becomes 

 the place of formation of corky tissue. 

 The second prominent differentia- 

 tion which presents itself is the forma- FlG . 26 ._ SECT " IONOFOU TEK 

 tion of a system of cells and vessels for 

 the transport of water through the 

 plant and the circulation of nutritive and other materials. 

 We may speak of this as the conducting system. A little 

 reflection will show us the necessity for the development of 

 some such system as this, which must be more extensive and 

 complex as the size of the plant increases. We find that the 

 source of water on which a terrestrial plant relies is the soil 

 in which its roots are embedded. Even when it is young 

 many of its protoplasts are placed at a considerable dis- 

 tance from such a source of supply, and in the absence of 



REGION OF BOOT, SHOW- 

 ING EXODEBMIS, ex. 



