STRUCTURE OF DICOTYLEDONOUS STEM, 413 



parts of the Cellular basis of the stem, which have been separated 

 by the interposition of the Wood. 



303. The Bark may be usually found to consist of three prin- 

 cipal layers ; the external, or Epiphlozum, also termed the Suberous 

 (or Corky) layer ; the middle, or Mesophlceum, also termed the 

 Cellular Envelope ; and the internal, or Endophloeum, which is 

 more commonly known as the Liber. The two outer layers are 

 entirely Cellular ; and are chiefly distinguished by the form, size, 

 and direction of their cells. The Epipklceum is generally composed 

 of one or more layers of colourless or brownish cells, which usually 

 present a cubical or tabular form, and are arranged with their long 

 diameters in the horizontal direction ; it is this which, when de- 

 veloped to an unusual thickness, forms Cork, a substance which is 

 by no means the product of one kind of Tree exclusively, but 

 exists in greater or less abundance in the Bark of every Exogenous 

 stem. The Mesophlceum consists of Cells, usually of green colour, 

 prismatic in their form, and disposed with their long diameters 

 parallel to the axis ; it is more loosely arranged than the pre- 

 ceding, and contains Intercellular Passages, which often form a 

 network of canals that have been termed Laticiferous Vessels ; and 

 although usually less developed than the suberous layer, it some- 

 times constitutes the chief thickness of the bark. The Liber or 

 Inner Bark, on the other hand, usually contains Woody Fibre in 

 addition to the Cellular tissue and Laticiferous Vessels of the pre- 

 ceding ; and thus approaches more nearly in its character to the 

 Woody layers, with which it is in close proximity on its inner sur- 

 face. The Liber may generally be found to be made up of a suc- 

 cession of thin layers, equalling in number those of the Wood, the 

 innermost being the last formed ; but no such succession can be 

 distinctly traced in the Cellular Envelope, or in the Suberous layer ; 

 although it is certain that they too augment in thickness by addi- 

 tions to their interior, whilst their external portions are frequently 

 thrown-off in the form of thickish plates, or detach themselves in 

 smaller and thinner lamina?. — The bark is always separated from 

 the wood by the Cambium- Layer, which is the part wherein all 

 new growth takes place : this seems to consist of mucilaginous 

 semi-fluid matter ; but it is really made-up of Cells of a very 

 delicate texture, which gradually undergo transformations, whereby 

 they are for the most part converted into Woody Tissue, Ducts, 

 Spiral Vessels, &c. These materials are so arranged as to augment 

 the Fibro-Vascular bundles of the Wood on their external surface, 

 thus forming a new layer of Alburnum which encloses all those 

 that preceded it ; whilst they also form a new layer of Liber, on 

 the interior of all those which preceded it : they also extend the 

 Medullary Rays, which still maintain a continuous connection 

 between the Pith and the Bark; and a portion remains unconverted, 

 so as always to keep apart the Liber and Alburnum. — This type of 

 Stem-structure is termed Exogenous; a designation which applies 



