708 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



had forced their way between these cells, which there hold a nearly 

 straight and parallel course on either side of them. The medullarv 

 rays maintain a connection between the external and the internal 

 parts of the cellular tissue or fundamental parenchyme (also called 

 ' ground-tissue ') of the stem, which have been separated by the 

 interposition of the wood. 



The bark is usually found to consist of three principal layers : 

 the external or epiphlceum, which includes the suherous (or corky) 

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

 and the internal, or endophlceum, which is more commonly known 

 as the librr. 1 The two outer layers are entirely cellular, and are 

 chiefly distinguished by the form, size, and direction of their cells. 

 The epiphlceu/m is generally composed of one or more layers of colour- 

 less 01 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 110 means the product of one 

 kind of tree exclusively, but exists in greater or less abundance in 

 the bark of every exogenous stem. The mesophloeum consists of 

 cells, usually containing more or less chlorophyll, prismatic in tlieir 

 form, and disposed with their long diameters parallel to the axis ; it 

 is more loosely arranged than the preceding, and contains inter- 

 cellular passages, which often form a network of canals which have 

 the character of laticiferous vessels ; and. although usually less 

 developed than the suberous layers, 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 laticiferous canals of the preceding ; and thus approaches more 

 nearly in its character to the woody layers, with which it is in C!OM- 

 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 either 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 por- 

 tions 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 layer seems to con- 

 sist of mucilaginous semi-fluid matter ; but it is really made up of 

 cells of a very delicate texture, which gradually undergo transfor- 

 mation, whereby they are for the most part converted into tracheids, 

 ducts, spiral vessels, &c. These materials are so arranged as to 

 augment the fibro-vascuiar 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 



Tlif term ' lilit-r' is also sometimes applied to the 'phloem-portion' of a fibro- 

 luimlle. Ei>.l 



