DEVELOPMENT OF VEGETABLES. 73 



cular impression round the stem. The age of the tree may 

 accordingly be estimated by the number of these circles, or 

 knots, which appear along its stem. The successive knots 

 which are seen in the stems of other endogenous plants, as 

 may be observed in growing corn, and also in various grasses, 

 may be traced to a similar origin. 



The structure of exogenous trees is more complicated: 

 for, when fully grown, they are composed of two principal 

 parts, the toood and the hark. The woody portion ex- 

 hibits a farther division into the j?;///?, which occupies the 

 centre, and consists of large vesicles, not cohering very 

 closely, but forming a light and spongy texture, readily per- 

 meable to liquids and to air; the harder luood, which sur- 

 rounds the pith in concentric rings, or layers; and the 

 softer wood, or alhiirnum, which is also disposed in con- 

 centric la)^ers on the outer side of the former. Each of these 

 concentric layers of wood and of alburnum may be farther 

 distinguished 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 b}^ con- 

 centric 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 en- 

 velope. 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 num- 

 ber 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 di- 

 ameter of the tree, and composing by their union conti- 

 nuous 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 origi- 

 nally of inconceivable minuteness, and totally imperceptible 

 by any optical means of which we have the command. As 



Vol. I. 10 



