Poplar and Cypress they are nearly erect; the Oak, in open and exposed situations, taking a wide-spreading form, its branches assuming every imaginable curve; while in the Cedar they are nearly at a right angle.
The stem is constructed upon the principle of a cone, and consists of a series of perfected layers designated heart-wood, or duramen, with, commonly outside these, some layers that are imperfect, which are known as the sap-wood, or alburnum; while the exterior is composed of the bark, or liber. The main portion of the stem is broadest at the base, and somewhat bell-shaped near the root, but gradually diminishes upwards to the part where the first branches are thrown out, and from this point there is again a still further diminution, until it is finally lost in the extremity of the branchlets.
The central and first-formed part of an exogenous stem, namely, the pith, is composed of cellular tissues, the cells being very numerous and varying considerably in size, but generally diminishing towards the outer edge. The pith is somewhat large and full of fluid in the young plant, but does not increase in bulk as the tree grows older; on the contrary, it appears rather to diminish than otherwise, by the fluid drying out. It retains, however, its place in the oldest trees, in the form of a dry consistent powder, although it is scarcely noticeable in some species on their arrival at maturity.
In the employment of timber in carpentry, due regard must always be had to the position of the pith, since there is an outside and an inside to every board and piece of scantling; and the careful workman is so well aware of this, that he will study to leave, if possible, in any work of construction, the outer side only exposed. It is, therefore, necessary in every case to
