80 THE MECHANICAL FUNCTIONS. 



guished into an inner and an outer portion ; the 

 former being of less density than the latter, and 

 consisting of a lighter cellular tissue : while the 

 outer portion is composed of the denser woody 

 fibres resulting from the union of numerous 

 vessels with a cellular envelope. The bark is 

 formed by concentric layers of cortical substance, 

 of which the innermost are denominated the 

 Liber ; and the whole is surrounded by an outer 

 zone of cellular tissue, termed the cellular enve- 

 lope. Of this envelope the exterior surface is 

 called the Epidermis. 



All these concentric zones may be readily 

 distinguished in a horizontal section of the 

 stem ; which also presents a number of lines 

 called Medullary Rays, radiating from the pith 

 to the circumference. They are composed 

 chiefly of large cells, extending transversely, 

 or in the direction of the diameter of the tree, 

 and composing by their union continuous ver- 

 tical planes the whole length of the trunk. 



Every vegetable stem, and also every branch 

 which arises from it, is developed from a germ, 

 or bud, which is originally of inconceivable mi- 

 nuteness, and totally imperceptible by any op- 

 tical means of which we have the command. 

 As soon as it becomes visible, and its structure 

 can be distinguished, it is found to contain 

 within itself the parts which are to arise from it, 

 in miniature, and folded up in the smallest possi- 



