226 DISEASES OF TREES 



of expanding, and has been ruptured by the growth in thickness 

 of the stem, a new protective covering is formed beneath it, 

 which protects the inner living cortical tissues against drought. 

 This periderm on whose structure and characteristics it would 

 be out of place here to enlarge is formed from a layer of 

 phellogen (cork-cambium), which results from the tangential 

 division either of the epidermal cells while still alive, or of a 

 layer of cortical cells which is situated at a greater or less 

 distance beneath the epidermis. The radially arranged cells, 

 which are being constantly formed by division, die and become 

 converted into cork, and thus a protecting envelope, more or less 

 thick, is formed on the outside of the living tissues. By division 

 of the phellogen-layer the envelope is constantly being renewed 

 on its inner surface, whereas the oldest cork-cells on the outside 

 are being lost by the exfoliation or detachment of compact 

 layers of cork-cells. In the case of most trees bark is formed 

 sooner or later, owing to the older layers of the cortex and bast 

 losing their power of expansion.* When this occurs new cork- 

 layers form in the interior of the cortex, and these separate the 

 inner layers from the outer layers of cortex, immediately before 

 the latter die, dry up, and rupture. 



It is evident that an injury to the dead periderm or bark 

 is unaccompanied by any prejudicial .results. The only way in 

 which it can affect the growth of the tree is that by diminishing 

 the pressure it stimulates the cambium to increased activity. 

 Where the dead bark has been mostly removed in a broad 

 zone from pines, for the purpose of laying on a ring of tar 

 with the object of intercepting caterpillars, the trees during suc- 

 ceeding years grow distinctly faster at the barked region than 

 either above or below. In the event of the layer of living 

 phellogen being injured, a new zone of phellogen and cork, 

 which is continuous with the cork layer along the edge of 

 the wound, is formed from the uninjured cells which are situated 

 deeper in the cortex or phelloderm. 



The cortical parenchyma (Fig. 132, b c] which lies beneath the 



*[" Bark " is, therefore, all the dead tissue situated outside the phellogen: 

 it may be represented by the corky layer of the periderm only, or may include 

 this and dead tissues of the cortex, which the periderm has cut out as well. 

 ED.] 



