THE TREE ITS ROOT-SYSTEM. 93 



verse of that of the primary xylem ; there are also no 

 spiral vessels formed now. In fact, the structure of the 

 vascular bundles of the root has now changed its char- 

 acter, and from this point forward 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 thick ens the root ; were this not the case, 

 the layer of cork would soon be ruptured as the root in- 

 creases 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 pri- 

 mary 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 cylin- 

 der 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, jiist as the present 

 outer layers are doing. 



