4 TIMBER AND TIMBER TREES. [CHAP. 



Poplar and Cypress they are nearly erect ; the Oak, ini 

 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 desig- 

 nated 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 pos- 

 sible, in any work of construction, the outer side only 

 exposed. It is, therefore, necessary in every case to 



