SECT. xiv. THE BARK CAMBIUM. 407 



The bark is divided into three regions, or zones. The 

 external coat, lying immediately under the skin, is 

 formed of one or many layers of cubical or oblong cells, 

 elongated horizontally. They are transparent and 

 colourless at first, but become brown and opaque with 

 the colouring matter of cork as they grow older. On 

 that account it is called the suberous zone, and some- 

 times acquires great thickness, as in the Quercus Suber, 

 or cork tree. 



The green cellular envelope comes next to the corky 

 layer, and consists of prismatic cells and laticiferous 

 tubes, which form an irregular wide-meshed network, 

 elongated in the direction of the axis of the tree, and 

 sometimes constituting the chief thickness of the bark. 

 This zone, as well as the succeeding, increases impercep- 

 tibly by new layers added to its interior, while the ex- 

 terior coats of the bark perish annually. In some trees 

 they are annually cast off in plates and large flakes, as 

 in the Oriental plane, whose stem and branches look as 

 if they had been peeled in autumn. 



The liber, which is the third and innermost zone of 

 the bark, generally consists of several layers of cellular 

 tissue, traversed longitudinally by bundles of woody 

 fibre and laticiferous tubes. 



The generating layer of cambium, in which all the 

 phenomena of growth takes place, is a semi-fluid mu- 

 cilaginous substance, which comes between the liber 

 and the wood. It is most abundant in the spring, and 

 is the origin of all horizontal growth. This mucilage 

 is really made up of a vast multitude of cells, with cell- 

 walls as delicate as those of a soap bubble, which 

 gradually undergo transformation into woody fibre, lati- 

 ciferous ducts, spirals, &c., thence called the cambium 

 zone. The whole of this matter spontaneously divides 

 into two parts : one forms a new layer of liber on the 

 interior of all those which precede it, and the other a 



