. Introduction to structural and systematic botany, and vegetable physiology. Botany. THE BARK. 127 • it grows at all. The green layer does not increase at all after the first year; the opaque corky layer soon excludes it from the hght; and it gradually perishes, never to be renewed. The corky layer commonly increases for a few years only, by the formation of new tabular cells : occasionally it takes a remarkable development, form- ing the substance called Gorlc, as in the Cork Oak. A similar growth occurs on the bark of several species of Elm, of our Liquid- ambar or Sweet-Gum, &c., pro-
. Introduction to structural and systematic botany, and vegetable physiology. Botany. THE BARK. 127 • it grows at all. The green layer does not increase at all after the first year; the opaque corky layer soon excludes it from the hght; and it gradually perishes, never to be renewed. The corky layer commonly increases for a few years only, by the formation of new tabular cells : occasionally it takes a remarkable development, form- ing the substance called Gorlc, as in the Cork Oak. A similar growth occurs on the bark of several species of Elm, of our Liquid- ambar or Sweet-Gum, &c., pro- ducing thick corky plates on the branches. In the White and Pa- per Birch, thin layers, of a very durable nature, are formed for a great number of years; each layer of tabular and firmly cohe- rent cells (Fig. 200, a) alternates with a thinner stratum of delicate, somewhat cubical and less compact cells (5), which break up into a fine powder when disturbed, and allow the thin, paper-like plates to exfoliate. 227. The liber, or inner bark (215), continues to grow through- out the life of the tree, by an annual addition from the cambium- layer applied to its inner surface. Sometimes the growth is plainly distinguishable into layers, corresponding with or more numerous than the annual layers of the wood: often, there is scarcely any trace of such layers to be discerned. In composition and appearance the liber varies greatly in different plants,* especially in trees and shrubs. That of Bass-wood or Linden, and of other plants with a similar fibrous bark, may be taken as best representing the liber. Here it consists of strata of very thick-walled cells alternating with thin-walled cells. The thick-walled cells are bast-ceUs (55, Fig. 49, 53), are much elongated vertically, and form the fibrous portion of * The best account of the libcr that has yet been given is that by Mohl, in the Botanische Zeitunij, Yol. 13, p. 873 (1855), of which a French translation is published in the Anna
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