86 ORGANOGRAPHY. 



which is called bast tissue or woody tissue of the liber, mixed 

 with some laticiferous vessels and parenchymatous cells. Some 

 authors confine the term liber to that part of the inner bark 

 which contains liber-cells, but it is best to extend this name to 

 all that portion which is situated between the cambium-layer on 

 its inside and the green layer of the bark on its outside. It 

 is used in the latter sense in this volume. We have already, 

 under the head of Woodi/ Tissue of the Liber, fully described the 

 characters of the liber-cells and their value for manufacturing 

 purposes (see p. 30). 



The liber-cells are sometimes placed side by side in a parallel 

 direction, and thus form by their union a continuous layer as in 

 Negundo, Horsechestnut, &c. ; more frequently however they 

 present a wavy outline, and hence only touch each other at 

 certain points, so that numerous interspaces are left between 

 them, in which the medullary rays connecting the bark and 

 the pith may be observed. From this circumstance the inner 

 bark presents frequently a netted appearance, which is especially 

 remarkable in that of the Lace-bark tree {Lagetta lintearid) 

 of Jamaica, and of other plants belonging to the same natural 

 order. 



b. The Cellular Envelope or Green Layer^ or Mesophlaum, 

 (Figs. 176, c, and 171, g.) — This layer lies between the liber and 

 epiphlceum, and hence tlie name Mesophlceum which is applied 

 to it. It is connected on its inner surface Avith the medullary 

 rays. It consists of thin-sided, usually angular or prismatic, 

 parenchymatous cells, which are loosely connected, and thus 

 leave between their sides a number of intercellular cavities. 

 The cells of which it is composed contain an abundance of 

 chlorophyll, which gives the green colour to young bark, and 

 hence the name of green layer, by which it is commonly distin- 

 guished. This is the only part of the bark which usually 

 possesses a green colour. In this layer also, as in the liber, we 

 find generally some laticiferous vessels. 



c. The Corky Envelope, or Epiphlceum. (Figs. 176, b, and 171, A.) 

 This is the outer layer of the bark proper, and is invested by the 

 epidermis {Jig. 176, a). It has also received the names of /jA/ce?/7» 

 and periderm. Tills term periderm is, however, sometimes used, 

 as by Weddell, to indicate the dead portion of the bark, or that 

 which has ceased to perform any active part in the life of the 

 plant; wliicli is commonly the case, as we shall presently see, in 

 a few years with the two outer layers. Hence in tliis sense the 

 periderm may consist of epiphlanim alone, or of mesophlceum 

 chieHy, or of portions of both, or even in some cases of a portion of 

 the liber. Those botanists who adopt this nomenclature, in such 

 cases, apply the term derm to the inner living portion of the bark. 



Tlie epiphlceum consists of one or more layers of flattened 

 tabular or cubical cells, which are generally elongated more or 



