EXOGENOUS STRUCTURE. 119 



the bark. The inner portion, next the wood, has woody tissue 

 formed in it, and becomes 



198. The Liber, or Fibrous Inner Bark (Fig. 156, /). These 

 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 are deposited as detach- 

 ed bundles, or in bands separated by extensions of the medullary 

 rays, one accordingly corresponding to each of the woody plates 

 or wedges, or sometimes (as in Negundo, Fig. 159, 160) they are 

 confluent into an unbroken circle round the whole circumference. 

 The liber has received the technical name of ENDOPHL^EUM (liter- 

 ally inner lark). The exterior part of the bark, in which no 

 woody tissue occurs, is early distinguishable, in most stems, into 

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



199. The Cellular Envelope, or Green Layer (Fig. 156, g),-also 

 called, from its intermediate position, the MESOPHL^UM. This is 

 composed of loose parenchyma, with thin walls, much like the 

 green pulp of leaves (which last is, indeed, an outlying part of the 

 same system), and containing an equal abundance of chlorophyll. 

 It is the only part of the bark that retains a green color. In 

 woody stems this is covered with 



200. The Corky Envelope, or EPIPHLJEUM (Fig. 156, ), which 

 gives to the twigs of trees and shrubs the hue peculiar to each spe- 

 cies, generally some shade of ash-color or brown, or occasionally 

 of much more vivid tints. It is rarely colored green, as in Ne- 

 gundo, where the inner cells contain chlorophyll. It is this tissue, 

 which, taking an unusual development, forms the cork of the Cork- 

 Oak, and those corky expansions of the bark which are so con- 

 spicuous on the branches of the Sweet Gum (Liquidambar), of 

 some of our Elms (Ulmus alata and racemosa), &c. It also forms 

 the paper-like exfoliating layers of Birch-bark. It is composed of 

 laterally flattened parenchymatous cells, much like those of the 

 EPIDERMIS (Fig. 156, t), which directly overlies it, and forms the 

 skin or external surface of the stem. 



201. To recapitulate the elements which compose the fabric of 

 an exogenous stem of a year old, especially in a woody plant, and 

 at the same time to exhibit them in an accurately drawn, more 

 magnified view, we have, proceeding from the centre towards the 

 circumference, 



