120 THE STEM. 



the mucli more magnified Fig. 192,- the section is made so as to 

 show the surface of one of these plates, and one of the Medullary 

 Eays passing horizontally across it, connecting the pith (p) with 

 the bark (b). These medullary rays form the silver-grain, (as it 

 is termed,) which is so conspicuous in the Maple, Oak, &c., and 

 , which gives the glimmering lustre to many kinds of wood when cut 

 in this direction. But a section made as a tangent to the circum- 

 ference, and therefore perpendicular to the medullary rays, brings 

 their ends to view, as in Fig. 193 ; much as they appear when seen 

 on the surface of a piece of wood from which the bark is stripped. 

 They are here seen to be composed of parenchyma, and to represent 

 the horizontal system of the wood, or the woof, into which the ver- 

 tical woody fibre, &c., or warp, is interwoven. The inspection of a 

 piece of oak or maple wood at once shows the pertinency of this 

 illustration. 



214. The Bark, in a stem of a year old, must next be considered. 

 At first it consists of simple parenchyma, undistinguishable from 

 that of the pith, except that it assumes a green color when exposed 

 to the hght, from the production of chlorophyll (92) in its cells. But 

 during the formation of the wood of the season, an analogous forma- 

 tion occurs in the bark. The inner portion, next the wood, has 



' woody tissue formed in it, and becomes 



215. The liber, or Inner Bark (Fig. 191,/). The fibre-like cells, 

 which give to the inner bark of those plants that largely contain 

 them its principal strength and toughness, are of the kind already 

 described under the name of bast-cells or bast-tissue (55). They are 

 remarkable for their length, flexibility, and the great thickness of 

 their walls. They form in bundles, or in bands separated by exten- 

 sions of the medullary rays, one accordingly corresponding to each 

 of the woody plates or wedges ; or sometimes (as in Negundo, Fig. 

 194, 195) they are confluent into an unbroken circle round the 

 whole circumference. Complete and well-developed liber, like that of 

 the Basswood, consists of three elements, viz.: 1. bast-cells or fibres ; 

 2. large and more or less elongated cells, with thinner walls variously 

 marked with transparent spots, appearing like perforations, and 

 usually traversed by an exceedingly minute net-work ; and 3. cells 

 of parenchyma. The liber has received the technical name of 

 Endophlceum (literally inner bark). In most woody stems the 

 exterior part of the bark, in which no woody tissue occurs, is early 

 distinguishable into two parts, an inner and an outer. The former is 



