CHAPTER VI 



The Structure of Roots 



Ix the mature plant the different types of elements are grouped 

 together to form tissues subserving diverse functions. Thus, for 

 instance, one tissue, which is always superficial and has the general 

 function of protecting the underh-ing parts, constitutes the 

 epidermis. The vascular tissues, comprising the wood or xylem 

 and the bast or phloem, form the respective conducting sj'stems 

 for water and mineral salts, and for elaborated food-substances, 

 whilst others — largeh' parenchymatous in character — go to form 

 the general matrix or ground tissue. It must not be supposed, 

 however, that, because there is one general function, the com- 

 ponent elements of these and other tissues are necessarily uniform 

 in character, a fact that will be best reahsed by the study of 

 concrete examples. 



In every j'oung root we can distinguish a number of regions, 

 beginning \\ith the root-cap, covering the growing point (Figs. 9 

 and 30, r.c), and followed successively by the zone of elongation, 

 the zone bearing root-hairs, and the older part where the root- 

 hairs have withered and the lateral roots (Fig. 30, l.r.) are seen 

 emerging through the surface-layers, commonly in four or five 

 vertical rows. In a longitudinal section through the tip of the 

 root the cap is found to consist of a number of concentric layers 

 of thin-waUed parenchj-matous cells, whose arrangement becomes 

 less regular towards the outside (Fig. 9). The outermost cells 

 gradually become mucilaginous and break down, whilst the 

 substance of the cap is constantly renewed from the underlying 

 meristem. A Uttle way behind the gro',\ing point (p. 20) the 

 middle of the root is occupied by a continuous, rather denser 

 strand of elongated elements, which develop into the vascular 

 tissue, and which offer a marked contrast to the thin-waUed 

 parench\-matous cortex around. • 



5 ' 65 



