MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 373 



371. Iii another fossil wood, whose transverse section is shown in Fig. 

 262, and its tangential section in Fig. 263, the medullary rays are seen 

 to occupy a much larger part of the substance of the stem: being shown 

 in the transverse section as broad bands (a a, a a) intervening between 

 the closely-set woody fibres, among which some large ducts are scattered; 

 whilst in the tangential, they are observed to be not only deeper than the 

 preceding from above downwards, but also to have a much greater thick- 

 ness. This section also gives an excellent view of the ducts, b b, b b, 

 which are here plainly seen to be formed by the coalescence of large 

 cylindrical cells, lying end-to-end. In another fossil wood in the Author's 

 possession, the medullary Kays constitute a still larger proportion of the 

 stem; for in the transverse section (Fig. 261), they are seen as very broad 

 bands (#, #), alternating with plates of woody structure (#, a), whose 

 thickness is often less than their own; whilst in the tangential section 

 (Fig. 265) the cut extremities of the medullary rays occupy a very large 

 part of the area, having apparently determined the sinuous course of the 



Transverse and Vertical Sections of a Fossil Wood; showing the separation of the Woody 

 plates, a a, a a, by the very large Medullary Rays, b 6, 6 &. 



woody fibres; instead of looking (as in Fig. 260) 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. The medullary rays main- 

 tain a connection between the external and the internal parts of the cel- 

 lular basis of the stem, which have been separated by the interposition of 

 the wood. 



372. The lark may be usually found to consist of three principal 

 layers; the external, or epiphlaum, also termed the suberous (or corky) 

 layer; the middle, or mesophloeum, also termed the cellular envelope; and 

 the internal, or endopJilmim, 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 epiphlosum is gener- 

 ally composed 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 devel- 

 oped 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 color, prismatic in their form, and dis- 



