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 
heart-wood, and alburnum 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 bark (cortex) lies external to the wood, and like it, consists of several 
layers. At first it is cellular, hike pith; subsequently it becomes more or 
less altered by secondary deposits. While composed of a cellular and vascular 
system, 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 epidermis, already described, then one of 
epiphloeum, within which is the mesophloeum ; the vascular portion of the 
internal layer is called liber, endophloewm, or true bark. 
The endophloewm, 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- 
tainmg chlorophylle, sometimes raphides. The epiphloewm is the outer 
covering of the bark, the epidermis excepted, which is often absent, and 
consists of cubical or tabular cells, without 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. 
13 
