ROOTS OF PLANTS 



43 



66. Wood and bark. If we examine the root of a plant 

 freshly removed from the ground, we shall find that there is a 

 soft, easily broken outer layer covering a tougher central por- 

 tion. This central part, running lengthwise in the root, is 

 called the wood or the central cylinder. In a fleshy root 

 like a carrot or parsnip we may 



distinguish the central cylinder 

 from the bark, or cortex, in both 

 a cross section and a longitudinal 

 section. In very thin slices made 

 lengthwise through the growing 

 tip of a young rootlet we are 

 able, with the help of a micro- 

 scope, to see the character of the 

 cells (Fig. 10). The cortical, or 

 bark, layer can be distinguished 

 from the wood layer by the fact 

 that the cells of the former have 

 about the same diameter in one 

 direction as in another, whereas 

 the cells of the central cylinder 

 are considerably longer than they 

 are wide, and their long diam- 

 eter is parallel with the long 

 diameter of the root. 



67. Growing layer. The layer 

 of cells lying between the wood 

 cylinder and the bark is called 

 the growing layer. It is only 



the cells of this layer that are capable of producing new cells 

 by the process of cell division. The younger bark and wood 

 cells are capable of increasing in size, but they cannot give 

 rise to new cells. Growth in length is the result of the 

 formation of new cells by a special growing layer near the 

 tip of the root. 



e c 



FIG. 10. Diagram of root structure 



e, epidermis, or skin ; c, cortex, or 



bark ; g, cambium, or growing layer ; 



w, wood cylinder, consisting of fibers 



and vessels ; /, pith 



