BOTANY. 13 



two years in its formation. The rule is liable to fewer exceptions in the 

 trees of temperate climates, where there are well defined periods of heat and 

 cold alternating once in the year. Not only the size but the texture of the 

 woody layers varies in different parts of the same cross-section. The vessels, 

 at first open and admitting a ready passage to the juices of the plant, ulti- 

 mately become thickened and possibly entirely filled by the deposit of hard 

 matter. It is this which constitutes the distinction between duramen or 

 heai't-ti-ood, and alhurmnn or sap-wood, the latter being exterior to the 

 former, lighter colored, and less compact. In some trees, as Tilia, the chestnut, 

 and others, no such distinction is readily evident. The thicker the tree the 

 greater is the proportion of heart-wood in the cross section. It is the heart- 

 wood that constitutes the most useful portion of timber, owing to its greater 

 strength and less tendency to decay. 



The cambium is a layer of semifluid matter which marks the separation 

 between the wood and the bark. This is an organizable mucilage, and from 

 it new elementary organs are formed, whether these consist of vascular or of 

 cellular tissue. 



The hark {cortex) lies external to the wood, and like it, consists of several 

 layers. At first it is cellular, like pith ; subsequently it becomes more or 

 less altered by secondary deposits. While composed of a cellular and vascular 

 S3^stem, like the wood, the position and relative proportion of the elements 

 vary in the two. In the bark the cellular system is external and much 

 developed ; in the wood it is internal and restricted. The cellular portion 

 consists of an external layer of epide?'??iis, already described, then one of 

 epiphloeum, within which is the m,esophlocum : the vascular portion of the 

 internal layer is called liber, endophloeutn, or true bark. 



The endophloeujn, or liber, is composed of pleurenchyma, mixed with 

 laticiferous vessels and cellular tissue, resting on the alburnum. The tubes 

 of the pleurenchyma are often thickened by deposits of secondary matter in 

 concentric cylinders, thus acquiring a considerable degree of tenacity, as in 

 the Lace tree, the Linden, the Paper mulberry, &c. The mesophloeum lies 

 immediately outside of the liber, and consists of polyhedral cells, usually con- 

 taining chlorophylle, sometimes raphides. The epiphloeum is the outer 

 covering of the bark, the epidermis excepted, which is often absent, and 

 consists of cubical or tabular cells, Avithout chlorophylle ; the elongation of 

 these cells is horizontal, thus differing from the cells of mesophloeum. Usually 

 of a single layer of cells, epiphloeum sometimes exhibits several, as in the 

 bark of the cork tree, or the cork of commerce. 



The increase of bark takes place in a manner directly opposite to that of 

 the wood. In the latter, new layers are developed on the outside of the old 

 ones ; in the former, on the inside of the several portions. Thus the outer 

 layers of bark become distended, and if elastic, retain their continuity, as in 

 the beech ; if not elastic, they either become fissured and crumbled off, or 

 they exfoliate in patches, as in some species of Hickory, Birch, and 

 Buttonwood. An incision in the wood of a tree is deepened with increasing 

 age ; if in the bark, it gradually becomes shallower and shallower, finally 

 disappearing. 



IS* 



