98 PERIDERM AND BARK [CH. 



derm is deeply situated from the first, and a new set of 

 events supervenes owing to the fact that the surface of 

 the branch is now covered by something more than true 

 periderm ; namely by all those parts of the original cortex 

 which have been cut out by the most deeply situated 

 cork-cambium as well as its own cork-layer. 



Bearing in mind that, owing to the water-tight nature 

 of this cork everything outside it dies, we recognise that 

 the covering thus produced consists of various kinds of 

 dead cells derived from the cortex, the outer periderms, 

 and, in the case of very deeply situated periderm, of 

 parts of the inner cortex, phloem, &c. This congeries 

 of dead tissues cut out by the deepest cork-cambium is 

 known as Bark, and, as we have seen, it may be a 

 relatively simple or a very complex mass of layers. 

 Moreover, even when it consists for the most part of 

 cork only, which is an extensible and elastic tissue, the 

 continued groAvth in length and thickness of the twig or 

 branch soon brings about its rupture, especially by means 

 of longitudinal fissures, but also, and particularly in those 

 cases where it is developed at an early period on young 

 twigs, by transverse cracks; and as the pressure of the 

 enlarging branch extends these fissures and cracks, the 

 older bark peels or flakes or strips off in successive layers, 

 patches or strips, and exposes newer periderms or bark 

 lying deeper in the tissues. The latter then subsequently 

 suffers a similar fate. 



It is owing to these phenomena that the very various 

 characteristics of bark, in the widest sense, are due. 

 Strictly speaking the perennially renewed epidermis of 

 the Mistletoe, and the superficial periderm of Beech, 

 Hornbeam, Hazel, Birch, BarbeiTy, Privet, Ailanthus, 

 are not true bark, though such may be formed after 

 many years i.e. on old stems as deeper periderms are 



