THE TREE—ITS ROOT-SYSTEM 93 
formed now. In fact, the structure of the vascular 
bundles of the root has now changed its character, and 
from this point forwards the root increases in thickness 
exactly as the stem does, whence I refer the reader to 
the following chapter for further details. 
The development of the layers of cork which now 
surround the thickening axis-cylinder go on forming 
year after year, as the cambium forms more xylem and 
phloém and so thickens the root ; were this not the case 
the layer of cork would soon be ruptured as the root 
increases in diameter. Such rupture, in fact, does occur, 
but the cork-forming tissue in the pericycle goes on 
growing and acts as a cork-cambium, and repeatedly 
develops more cork to make good the layers which are 
being split and worn off in the soil. 
From what has been said it will be understood that 
a transverse section of an old root differs entirely in 
structure from that of a young one, although all the 
changes in the former can be correlated with the 
primary structures of the latter. In the first place, 
such a section shows no piliferous layer or cortex, both 
having been sloughed off long ago; the protective 
function of these layers is now assumed by the cork 
jacket (often called periderm) developed by the cork- 
cambium cylinder in the pericycle, and even this will 
not show all the cork that the cambium has developed, 
because many outer layers will have flaked away, just 
as the present outer layers are doing. 
Then, inside this periderm we shall find the phloém, 
