WOOD. 
115 
tutes 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 with the 
principle of life. As the liber is formed annually from the cambium 
or descending sap, new layers of alburnum are supposed to have 
the same origin, and to be formed daring the same intervals of time. 
Most of the sap ascends through the alburnum, though some passes 
through the perfect wood. The sap which nourishes the buds, 
passes through the centre of the stem, and from thence is conveyed 
in appropriate vessels to the buds. 
Fig. 118. 
The perfect wood, is sometimes called the heart; its colour is 
usually darker than that of the sap-wood, and its tex^ture is firmer 
and more compact ; it is also more durable for timber, it is formed 
by the gradual concentration and hardening of the alburnum. The 
v/ood constitutes the greater part of the bulk of trees and shrubs ; 
v/hen cut across, it is found to consist of numerous concentric 
layers. It is supposed that one of these circular layers is formed 
every year. To prove that the wood is deposited externally from 
the cambium, pieces of metal have been introduced under the bark 
of trees that were growing, and the wounds carefully bound up ; 
after some 3^ears, on cutting them across, as many layers of new 
wood have been found on the outside of the metal, as years had 
elapsed since its insertion. 
The strength and hardness of wood, is owing to woody fibres 
extending longitudinally ; these fibres are chiefly of vascular tex- 
ture, and contain sap, and the various secreted juices ; some con- 
tain only air. 
For illustration of the formation of wood, see Fig. 118, B, which 
represents a section of a woody stem of three years' growth ; i h, 
next the pith, is a layer of the first year's growth, and the hardest 
part of the v/ood ; h g, is a layer of the second year's growth ; and 
g 6, of the third ; the last is the sap-wood recently formed from the 
cambium. 
Pith. The pith (see Fig. 118, j5, k i) is situated in the centre of 
the trunk and branches of plants, and is a soft, spongy substance, 
analogous to the marrow of animals. It is composed of cellular 
texture. The cells, which are very large in the elder and some other 
Perfect wood— How has it been proved that wood is deposited externally ?— Strength 
and hardness of wood—Illustrate the foj-mation of wood by a reference to Fig. 118— 
Pith. 
