140 BARK 



strips of cambium giving rise to the lenticels subsequently become 

 continuous with the cork-cambium (Fig. 68). 



Where the cork is deep-seated, the tissues external to it are 

 practically cut off from all sources of food and all connection 

 with the interior, and therefore die away. These dead tissues, 

 on the outside of the cork, are shed sooner or later, leaving the 

 latter exposed as bark. In some plants [e.g. Beech, Oak) the 

 same cork-cambium continues to divide year after year, although 

 inactive during the winter, so that a thick mass of cork is formed. 

 This is also the case in the Cork Oak (Quercus suber), which is 

 the main source of the commercial article. The first cork, which 



nv 



FIG. 68. Transverse section through a lenticel (/.) of the Elder (Sambucus}. 

 ex., cork-cambium ; m, mechanical tissue. 



here arises subepidermally, is, however, of no value, being re- 

 moved when the tree is ten to fifteen years old, the cortex thus 

 exposed forming a new phellogen which gives rise to the 

 thin-walled cork of commerce. This is peeled off every eight to 

 twelve years. 



In most woody plants the first-formed phellogen ceases to 

 divide, and indeed itself becomes changed into a layer of cork, 

 at a comparatively early stage. A new cambium then arises at 

 a deeper level in the cortex, produces a fresh zone of cork, and 

 then in its turn passes out of action, to be succeeded by another 

 situated still deeper. The bark formed in this way consists of 

 alternating layers of cork and dead cortex, and comprises all 

 the tissues beyond the most recently established phellogen. When 



