THE THEE ITS BOOT-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 

 phloem 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 phloem, 



