350 THE UNIVERSE. 



The bark, which is the outermost, is formed of tolerably 

 numerous layers. The parts most deserving of remark in 

 it are the epidermis, or outermost layer, a fine, transparent 

 membrane, which generally allows us to see the tissue over 

 which it lies ; and under the epidermis, the suberous layer, 

 generally unnoticed on account of its thinness, but which 

 exceptionally attains to a thickness of an inch or so in some 



a. 



161. Section of Cork-Tree, a, Layers of cork or suber; b, inner bark and suber; c, concentric 



rings of wood and medullary rays. 



trees, particularly in the cork-tree. This layer constitutes 

 the cork of which we make such great use for our domestic 

 wants. In the south of Europe and in Africa it is taken 

 from the trees, and as this tissue grows after being removed 

 a new crop can be gathered every seven or eight years. 

 The cork then is not the bark, but merely its superficial 

 layer, for when we strip a trunk of its cortical envelope 

 completely it dies ; we could not effect several successive 

 removals of it ; the trees would be killed if we did. 



Beneath the suberous layer is seen the inner bark, char- 

 acterized at the first glance by its little cells filled with gran- 

 ules, usually green, the coloring of which is seen through 

 the epidermis. 



