SECTION 16.] 



ANATOMY OP STEMS. 



141 



2. The Gbjeen Base or Middle Bark. This consists of cellnlar tissue 

 only, and contains the same green matter (chlorophyll, 417) as the leaves. 

 In woody stems, before the season's growth is completed, it becomes cov- 

 ered by 



3. The Cokey Layeb or Outer Bark, the cells of which contain no 

 chlorophyll, and are of the nature of cork. Common cork is 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, — light gray in the Ash, purple in the Red Maple, red in several 

 Dogwoods, etc. 



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

 sided empty cells, which may be considered to be the outermost layer, or 

 in most herbaceous stems the only layer, of cork-cells. 



432. The green layer of bark seldom grows much after the first season. 

 Sometimes the corky layer grows and forms new layers, inside of the old, 

 for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the 

 Paper Birch. But it all dies after a while ; and the continual enlargement 

 of the wood within finally stretches it more than it can bear, and sooner or 

 later cracks and rends it, while the weather acts powerfully upon its sur- 

 face ; so the older bark perishes and falls away piecemeal year by year. 



433. So on old trunks only the inner bark remains. This is renewed 

 every year from within and so kept alive, while the older and outer layers 

 die, are fissured and rent by the distending trunk, weathered and worn, and 

 thrown off in fragments, — in some trees slowly, so that the bark of old 

 trunks may acquire great thickness ; in others, more rapidly. In Honey- 

 suckles and Grape- Vines, the layers of liber loosen and die when only a 

 year or two old. The annual layers of liber are sometimes as distinct as 

 those of the wood, but often not so. 



Pig. 481. Magnified view of surface of a bit of young Maple wood from which 

 She bark has been torn away, showing the wood-cells and the bark-ends of medul- 

 lary rays. 



Pig. 482. Section in the opposite direction, from bark (on the left) to beginning 

 »f pith (on the right), and a medullary ray extending from one to the other. 



