ROOTS AND THEIR FUNCTIONS 73 



attachment to the soil. They are only efficient 

 so long as they are uninjured, and perhaps 

 this helps to explain why the hairy zone is 

 such a short one on any one root, for the hairs 

 do not grow again when once they are injured 

 or worn out. The underground system as a 

 whole, however, repairs this defect by forming 

 a mass of branching roots, each one of which 

 may repeat the form and the four stages 

 indicated above. 



In order to understand how the root-hair, 

 and the root as a whole, play their respective 

 parts in the absorption of water, some ac- 

 quaintance with the cellular structures con- 

 cerned is necessary. 



We can ascertain this by examining under 

 the microscope sections of roots cut in various 

 directions. The annexed illustration (Fig. 10) 

 represents, rather diagrammatically, a trans- 

 verse section of a root. The hairy outgrowths 

 are the root- hairs. They consist of an outer 

 cell wall enclosing the living protoplasm which 

 lines the interior of the wall, though it does 

 not fill the entire space, for its own interior 

 is occupied by a " vacuole " of watery sap. 

 Passing inwards from the superficial root-hair 

 layer we notice a band of " cortical " cells 

 consisting of several layers forming the rind. 

 Still more interiorly we arrive at a starlike 

 arrangement of certain cell groups. This 

 inner cylinder is the vascular strand of the 

 root, and it consists of wood (xylem) and bast 

 (phloem), just as in the strands of the leaf or 



