UEV ELOPEMENT OF VEGETABLES. 75 



bark is formed by concentric layers of cortical sub- 

 stance, of which the innermost are denominated 

 the Liber; and the whole is surrounded by an outer 

 zone of cellular tissue, termed the cellular envelope. 

 The exterior surface of this envelope is called the 

 Epidermis. 



All these concentric zones may be readily dis- 

 tinguished in a horizontal section of the stem ; 

 which also presents a number of lines called Medul- 

 lary Rays, radiating from the pith to the circum- 

 ference. 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 vertical 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 minute- 

 ness, and totally imperceptible by any optical 

 means of which w^e have the command. As soon 

 as it becomes visible, and its structure can be dis- 

 tinguished, it is found to contain within itself many 

 of the parts which are to arise from it, in miniature, 

 and folded up in the smallest possible compass. 

 The portion destined to form the stem is gradually 

 expanded both in breadth and height, but princi- 

 pally the latter, so that it rises as it grows, during 

 a certain period, until the fibres acquire the soli- 

 dity and strength necessary not only for their own 

 support, but also for sustaining the parts which are 

 to be farther added. In trees this process gene- 

 rally occupies one whole season ; during which the 

 growth of the first layer of wood, with its central 

 pith, and its covering of a layer of bark, is free and 



