THE STEM. 



23 



does not take root. If a circular band of liber 

 be removed from the trunk of a tree, in such a 

 manner as to leave tlie woody body exposed, not 

 only will all the parts of the tree above this 

 band cease to be developed the following year, 

 but the entire tree wiU ultimately perish. This 

 process, called girdling, the Americans have re- 

 course to in clearing their forests. 



The liber is hardened each year, and new 

 layers are formed at its inner surface, by means 

 of the cambium. 



Alburnum or false wood. The outermost 

 woody layers, or those which are in contact 

 with the liber, constitute the alburnum. This 

 substance is not distinct from the wood properly 

 so called, the layers of which are situated be- 

 neath it. It is wood, but wood in a young 

 state, and not yet possessed of all the hardness 

 and tenacity which it is ultimately to present. 

 Accordingly, the alburnum exhibits precisely 

 the same structure as the wood, although its 

 tissue is formed of fibres that are weaker, more 

 distant from each other, and generally of a paler 

 tint. 



The difference of colour between the wood 

 and alburnum is very remarkable in trees whose 

 wood is very hard and compact, and especially 

 in those in which it is more or less of a deep 

 colour. Thus in ebony and logwood, the wood 

 properly so called is black or deep red, while the 

 layers of alburnum are of very light grayish 

 tint. But in trees which have white and coarse- 

 grained wood, the difference between the woody 

 layers and alburnum is very slight. 



The wood derives its origin fi'om the inner- 

 most layers of the alburnum, which become 

 successively harder, and are ultimately converted 

 into true wood. The latter is therefore com- 

 posed of all the circular layers situated between 

 the alburnum and the medullary tube. At a 

 certain period in the life of the vegetable, there 

 are formed each year a layer of wood and a layer 

 of alburnum; in other words, the innermost 

 layer of thfe alburnum is converted into wood in 

 proportion as a new layer of alburnum is pro- 

 duced at the outside, so that every year a new 

 concentric band is added to those previously 

 existing. 



The wood is generally the hardest part of the 

 trunk ; but its hardness is not the same in all 

 the layers of which it is composed. In dicotyle- 

 . donous trees, the innennost layers, which are 

 also the oldest, are more solid and more compact 

 than the outer, which generally approach the 

 alburnum in these respects. The transition from 

 wood to alburnum is, in most cases, hardly per- 

 ceptible, their colour being commonly the same; 

 but sometimes the difference is very decided, as 

 we have already remarked with reference to 

 ebony and logwood. 



A not less remarkable difference between wood 



and alburnum exists in the circumstance that 

 the latter is entirely destitute of vessels, while 

 they are distinctly perceived in the wood. The 

 vessels of the wood are false tracheae and porous 

 vessels, but never true tracheae or true spiral 

 vessels. By means of these tubes, which are 

 sometimes dispersed without order in the sub- 

 stance of the wood, and sometimes collected into 

 bundles, the sap is conveyed into the substance 

 of the trunk. But a period arrives when, through 

 the progress of age, the walls of these vessels be- 

 come thickened, their cavity diminishes, and at 

 length even disappears, and the course of the 

 fluids is for ever arrested in the woody sub- 

 stance.. 



Du Hamel very clearly demonstrated the trans- 

 formation of the alburnum into wood. He 

 passed a silver wire into the layers of alburnum, 

 brought its two extremities out, and tied them. 

 Some years after, having cut the branch, and 

 examined the wires which he had passed into 

 the alburnum, he found them embedded in the 

 wood, wliich proved that the alburnum had 

 been converted into wood. 



The medullari/ tiibe oeciipies the centre of the 

 stem, lining the innermost layer of the wood, 

 and containing the pith. Its walls are formed 

 of very long parallel vessels, longitudinally dis- 

 posed. These vessels are trachese, false tracheae, 

 and porous vessels. The form of this tube varies 

 considerably in different plants, being frequently 

 roundish in its transverse section, sometimes 

 oval, compressed, or with three, four, five, 

 or more angles. Its form appears to be deter- 

 mined by the position of the leaves upon the 

 branches ; thus, when the leaves are opposite, 

 the transverse section of the medullary canal is 

 elliptical, as in the ash ; if they are verticiUate 

 in threes, it is triangular, as in the rose-bay, and 

 so forth. This law, however, presents numerous 

 exceptions ; as the hortensia, which with op- 

 posite leaves, has a regular hexagonal medullary 

 tube. 



The medullary tube, once formed, never 

 changes its figure and dimensions, but remains 

 the same during the whole life of the vegetable. 

 It is therefore eri'oneous to say that it gradually 

 contracts upon itself, and at length disappears, 

 as the plant grows old. 



The pith or inner medulla, is the loose, trans- 

 parent, light, and spongy substance, formed al- 

 most entirely of cellular tissue, in its most simple 

 state, which fiUs the medullary tube. A few 

 vessels seem to run through it in the longitudinal 

 direction. Its cellules are generally very regular. 

 Like those of the cellular tissue in other parts, 

 they all communicate with each other. Some- 

 times, and especially in young branches and 

 herbaceous plants, the cellular tissue of the pith 

 is abundantly supplied with fluids, and filled 

 with granulations of a green colour, as may bs 



