384 STRUCTURE OF PHANEROGAMIC PLANTS. 



ture (, a), whose thickness is often less than their own ; whilst 

 in the tangential section (Fig. 177), the cut extremities of the 

 medullary rays occupy a very large part of the area, having ap- 

 parently determined the sinuous course of the woody fibres ; 

 instead of looking, as in Fig. 172, as if they had forced their way 

 between the woody fibres, which there hold a nearly straight and 

 parallel course on either side of them. 



240. The Bark may be usually found to consist of three princi- 

 pal layers ; the external, or epiphloeum^ also termed the suberous 

 (or corky) layer ; the middle, or mesophloeum, also termed the 

 "cellular envelope;" and the internal, or endophlceum, 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 epiphlaeum is generally com- 

 posed of one or more layers of colorless 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 developed to an unusual thickness, forms Cork, a substance 

 which is by no means the product of one kind of tree exclusively, 

 but which exists in greater or less abundance in the bark of 

 every exogenous stem. The mesophloeum consists of cells, usually 

 of green color, prismatic in their form, and disposed with their 

 long diameters parallel to the axis ; it is more loosely arranged 

 than the preceding, and contains " intercellular passages," winch 

 often form a network of canals, that have been termed "latici- 

 ferous vessels ;" and although usually less developed than the 

 suberous layer, it sometimes 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 latici- 

 ferous vessels of the preceding ; and thus approaches more nearly 

 in its character to the woody layers, with which it is in close 

 proximity on its inner surface. The liber may generally be 

 found to be made up of a succession 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 additions to their interior, 

 whilst their external portions are frequently thrown off in the 

 form of thickish plates, or detach themselves in smaller and 

 thinner flakes. 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 semifluid 

 matter ; but it is really made up of cells of a very delicate tex- 

 ture, 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, w r hich encloses all those 

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



