1. The Liber or Fibrous Bark, the Inner Bark. This contains some wood-cells, or their equivalent, commonly in the form of bast or bast-cells ([411], Fig. [444]), such as those of Basswood or Linden, and among herbs those of flax and hemp, which are spun and woven or made into cordage. It also contains cells which are named sieve-cells, on account of numerous slits and pores in their walls, by which the protoplasm of contiguous cells communicates. In woody stems, whenever a new layer of wood is formed, some new liber or inner bark is also formed outside of it.
2. The Green Bark or Middle Bark. This consists of cellular 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 covered by
3. The Corky Layer 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.
4. 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.
Fig. 481. Magnified view of surface of a bit of young Maple wood from which the bark has been torn away, showing the wood-cells and the bark-ends of medullary rays.
Fig. 482. Section in the opposite direction, from bark (on the left) to beginning of pith (on the right), and a medullary ray extending from one to the other.
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 surface; so the older bark perishes and falls away piecemeal year by year.