THE VEGETABLE KINGDOM. 379 



it lies. The suberous layer, generally unseen on account 

 of its thinness, but which attains to a thickness of several 

 centimetres (a centimetre is -39,371 of an inch) in some 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 re- 

 moved, 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 



205. Section of the Trunk of a Cork-tree. Zones of Suber and Liber; Concentric Zones and 



Mednllai-y Layers. 



envelope completely it dies ; we could not effect several suc- 

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



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

 acterized at the first glance by its little cells filled with 

 granules, usually green, the colouring of which is seen 

 through the ejDidermis. 



The layers of the liber are found still deeper. They 

 form thin membranes composed of elongated fibres, and 

 often of a beautiful white. These layers are superimposed 

 like the leaves of a book, and can sometimes be separated 

 with facility, whence they have acquired the name of liber, 



