WOOD. 



109 



leaves of a book. This substance, by its developnieut, pro- 

 duces new roots, branches, leaves, flowers, and fruits. 



a. It is composed of a kind of net-work, which has been compared to cloth ; tho 

 elongated fibers representing the warp, and the cellular texture the filling up. As 

 the cambium descends between the liber and the wood, and a layer of new liber 

 and of new wood are every year formed, the old layer is pushed outward, and at 

 length, losing its vital principle, it becomes a hfeless crust. The natives of Ota 

 heite manufacture garments from the liber of the paper mulberry. The liber of 

 flax is, by a more refined process, converted into fine linen. This part of the bark 

 is important to the life of vegetables ; the outer bark may be peeled off without 

 injury to them, but the destruction of the liber is generally fatal. The operation 

 of girdling trees, which is often practiced in new countries, consists in making, with 

 an axe, one or more complete circles through the outer bark and the liber of the 

 trunk. Trees seldom survive this operation, especially if it be performed early in 

 the spring, before the first flow of the sap from the root toward the extremities. 



b. During the repose of vegetation, that part of the liber most recently organ- 

 ized, and which of course retains its vital power, remains inactive between the 

 wood and the outer layers of the bark, until the warmth of spring causes the ascent 

 of the sap. After promoting the development of buds, and the growth of new 

 wood and bark, the liber, like that of the preceding year, hardens and loses its 

 vital energy. 



^' i h fj b d ca 



Fig. 125, at A, represents a young dicotyledonous stem, cut transversely; the 



inner circle surrounds the pith ; the wood extends to the bark, which at a appi^ara 



darkly shaded. 



At B, is a section of the same stem magnified ; a & is the bark, b i the wood, 



and i k the pith. 



The divisions of the bark may be seen as follows : a c represents the cuticle, or 



the dry, disorganized part ; at c d is the cellular integument ; at d b is the cortex, 



the extreme part of which, at b, is the liber. 



^ 128. Wood. The wood (ligmtm) of exogenous stems con- 

 sists of two parts, alburnum or sajp-wood^ and perfect wood. 

 The alburmim is so called from aJhts^ white, on account of the 

 paleness of its color. This is the newly formed wood, and con- 

 stitutes the outer part of the woody substance of the plant. It 

 is at first soft and tender, and in this state appears to be active 



a. Liber annually renewed— Girdling— J, What ultimately becomes of the liber ?— 128. Wood— 

 ^Iburnoim. 



