152 ANATOMY OF THE [LESSON 25. 



ing plates, giving a peculiar appearance to Oak, Maple, and other 

 wood with large medullary rays. 



427. The Bark covers and protects the wood. At first it is all 

 cellular, like the pith ; but soon some slender woody fibres, called 

 bast-cells (Fig. 342), generally appear in it, next the wood, forming 



The Liber, or Fibrous Bark, the inner bark ; to which belongs the 

 fine fibrous bast or bass of Basswood, and the tough and slender fibres 

 of flax and hemp, which are spun and woven, or made into cordage. 

 In the Birch and Beech the inner bark has few if any bast-cells in 

 its composition. 



The Cellular or Outer Bark consists of cellular tissue only. It is 

 distinguished into two parts, an inner and an outer, viz. : 



The Green Bark, or Green Layer, which consists of tender cells, 

 containing the same green matter as the leaves, and serving the 

 same purpose. In the course of the first season, in woody stems, this 

 becomes covered with 



The Corky Layer, so named because it is the same substance as 

 cork ; common cork being the thick corky layer of the bark of the 

 Cork-Oak, of Spain. It is this which gives to the stems or twigs of 

 shrubs and trees the aspect and the color peculiar to each ; namely, 

 light gray in the Ash, purple in the Red Maple, red in several Dog- 

 woods, &c. Lastly, 



The Epidermis, or skin of the plant, consisting of a layer of thick- 

 sided empty cells, covers the whole. 



428. Growth of the Stem year after year. So much for an exogenous 



stem only one year old. The stems of herbs perish at the end of the 

 season. But those of shrubs and trees make a new growth every 

 year. It is from their mode of growth in diameter that they take the 

 name of exogenous, i. e. outside-growing. The second year, such a 

 tern forms a second layer of wood outside of the first ; the third year, 

 nother outside of that ; and so on, as long as the tree lives. So that 

 the trunk of an exogenous tree, when cut off at the base, exhibits as 

 many concentric rings of wood as it is years old. Over twelve hun- 

 dred layers have actually been counted on the stump of an aged tree, 

 such as the Giant Cedar or Redwood of California; and there are 

 doubtless some trees now standing in various parts of the world which 

 were already in existence at the beginning of the Christian era. 



429. As to the bark, the green layer seldom grows much after the 

 first season. Sometimes the corky layer grows and forms new 

 layers, inside of the old, for a good many years, as in the Cork-Oak, 



