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