THE DIFFEEENTIATION OF THE PLANT-BODY 21 



FlG. 25. COLLENCHYMA UNDEK 

 THE EPIDERMIS OF PETIOLE. 

 x 50. 



supplemented by masses or sheaths of hardened or scleren- 



chymatous 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. There is 

 not at first a provision of an impervious membrane com- 

 posed of the outermost layer of cells ; this would be 

 inconvenient in view of the necessity for the existence of 

 root-hairs. The second layer later undergoes modification, 

 its cells becoming thickened in the peculiar manner 

 characteristic of the exodermis (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- FIG. 26. SECTION OF OUTER 



,. , . , ., , . ., ,, REGION OF BOOT, SHOW- 



tion which presents itself is the forma- ING EXODERMIS, ex. 

 tion of a system of cells and vessels for 

 the transport of water through the plant and the circula- 

 tion 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- 



