80 OEGANOGRAIPHY. 



Envelope, Green Layer, orMesopMosum {figs. 168, c, and 163, g) ; 

 and 3. Suberous, Corky Lager, or Epiphloeum (figs. 168, b, and 

 163, k). We shall describe these layers in the order in which 

 they are here placed. 



a. The Liber, Inner Bark, or EndopMaeum. {Figs. 168, d, and 

 163,/.) — This is composed of that kind of woody tissue which 

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

 some laticiferous vessels and parenchymatous cells. Some au- 

 thors 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 of the bark which is situated between the cambium-layer 

 on the inside and the cellular envelope on the outside. It is 

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

 under the head of Woody Tissue of the Liber, fully described 

 the general characters and nature of the liber-cells (see p. 39). 



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

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

 the Horsechestnut ; but far more frequently they present a wavy 

 outline, and only touch one another at certain points, so that nu- 

 merous 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 commonly presents a netted 

 appearance, and such is especially the case in that of the Lace- 

 bark tree {Lagetta lintearia) of Jamaica, and of other plants be- 

 longing to the same natural order. 



b. The Celhdar Envelope, Green Layer, or Mesophlosum. {Figs. 

 168, c, and 163,^.) — This layer lies between the liber and epi- 

 phlcBum, and hence the name Mesophloeum, signifying middle 

 layer, which is applied to it. It is connected on its inner sur- 

 face with the medullary rays. It consists of thin-sided, usually 

 angular or prismatic, parenchymatous cells ; these are loosely 

 connected, and thus leave between their walls a number of inter- 

 cellular 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 com- 

 monly distinguished. This is the only part of the bark which 

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

 liber, we generally find some laticiferous vessels. 



c. Suberous, Corky Layer, or Epiphl(Xiim. {Figs. 168, h, and 

 163, h.) — This is the outer layer of the bark proper, and is in- 

 vested by the epidermis {fig. 168, a). It has also received the 

 name of periderm ; this term is, however, sometimes used to 

 indicate the dead portion of the bark, or that which has ceased 

 to perform any active part in the life of the plant ; which is com- 

 monly the case, as we shall presently see, in a few years with 

 the two outer layers. In this sense the periderm may consist 

 of epiphlceum alone, or of mesophlceum chiefly, or of portions 

 of both, or even in some cases of a portion of the liber. Those 



