THE BARK. 127 



tabular cells : occasionally it takes a remarkable development, 

 the cells swell out into polyhedral shapes, and multiply with un- 

 usual rapidity and in great quantities, forming the substance called 

 Cork, as in the Cork-Oak. A similar growth occurs on the bark of 

 several species of Elm, of our Liquidambar or Sweet-Gum, &c., 

 producing thick corky plates on the branches. In the Birch, thin 

 annual layers, of very durable nature, are formed for a great num- 

 ber of years : each layer of tabular, firmly coherent cells (Fig. 

 165, a) alternates with a thinner stratum of delicate, somewhat cu- 

 bical and less compact cells (b), which separate into a fine powder 

 when disturbed, and allow the thin, paper-like plates to exfoliate. 



214. The liber, or inner bark (198), 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 plain- 

 ly distinguishable into layers, corresponding with the annual layers 

 of the wood : often, there is scarcely any trace of such layers to 

 be discerned. The liber of the Bass-wood or Linden, and of 

 other trees and shrubs with an evidently fibrous bark, consists of 

 alternate strata of bast-cells * (or of parenchyma abounding with 

 bundles of bast-cells) and of parenchyma alone. In the Sugar 

 Maple, only a small proportion of bast-cells are formed after the 

 first year. In Negundo there is a fine deposit of bast-cells the 

 first year (Fig. 159, well distinguished by their opaline appearance 

 in the magnified cross-section), but they are not again repeated, 

 and the liber ever after consists of parenchyma alone, or with 

 some thin and short prosenchymatous cells intermixed. The brit- 

 tle liber of the Beech is nearly destitute of bast-cells. So is 

 that of the Birch ; but it abounds with clusters of solidified cells, 

 which take their place, and exactly imitate ordinary bast-cells on 

 the cross-section (Fig. 18) ; but a longitudinal section exhibits the 

 same appearance, showing that they are globular in shape. In 

 the first year's growth of the stem of Menispermum Canadense, 

 there is a broad arc of bast-cells immediately before each wedge 

 of wood ; in a stem of two or three years this is carried away from 

 the wood by the development of purely cellular bark from the in- 



* The name, liber, is applied, even by the same author, sometimes to the 

 whole inner bark, of whatever structure, sometimes to its bast-cells alone. It 

 is applied in this work to the inner bark which grows year after year from the 

 cambium-layer, (that is, to all within the green layer,) whether it continues to 

 produce bast-cells or not. 



