374 THE MICROSCOPE AND ITS REVELATIONS. 



posed with their long diameters parallel to the axis; it is more loosely 

 arranged than the preceding, 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 layers, 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 canals of the preceding; and thus ap- 

 proaches 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 sue- , 

 cession 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 portions are frequently 

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

 and thinner laminae. 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 

 transformation, whereby they are for the most part converted into woody 

 fibres, ducts, spiral vessels, etc. 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 incloses 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 the alburnum. This type of stem-structure is termed exogenous; 

 a, designation which applies very correctly to the mode of increase of the 

 woody layers, although (as just shown) the liber is formed upon a truly 

 endogenous plan. 



373. Numerous departures from the normal type are found in partic- 

 ular tribes of Dicotyledons. Thus in some the wood is not marked by 

 concentric circles, their growth not being interrupted by any seasonal 

 change. In other cases, again, each woody zone is separated from the 

 next by the interposition of a thick layer of cellular substance. Some- 

 times wood is formed in the bark (as in Calycanthus), so that several 

 woody columns are produced, which are quite independent of the princi- 

 pal woody axis, but cluster around it. Occasionally the woody stem is 

 divided into distinct segments by the peculiar thickness of certain of the 

 medullary rays; and in the stem of which Fig. 266 represents a trans- 

 verse section, these cellular plates form four large segments, disposed in 

 the manner of a Maltese cross, and alternating with the four woody seg- 

 ments, which they equal in size. 



374. The Exogenous stem, like the (so-called) Endogenous, consists, 

 in its first-developed state, of cellular tissue only; but after the leaves 

 have been actively performing their functions for a short time, we find a 

 circle of fibro-vascular bundles, as represented in Fig. 252, interposed 

 between the central (or medullary) and the peripheral (or cortical) por- 

 tions of the cellular matrix; these fibro-vascular bundles being themselves 

 separated from each other by plates of cellular tissue, which still remain 

 to connect the central and the peripheral portions of that matrix. This 

 first stage in the formation of the Exogenous axis, in which its principal 

 parts the pith, wood, bark, and medullary rays are marked-out, is seen 



