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 witn 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 during 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. 



n ^^- 



A 



mMMt. _ _ 



t h T h d c a 



The p.jftct irood, is soraetmies called the hiart , its colour is 

 usually darker than that of the sap-wood, and its texture 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 

 wood constitutes the greater part of the bulk of trees and shrubs ; 

 when 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 years, o"n 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, 5, 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 wood ; /«, g-, is a layer of the second year's growth ; and 

 g b, of the third ; the last is the sap-wood recently formed from the 

 cambium. 



Pith. The pith (see Fig. 118, B, 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 1— Strength 

 and hardness of wood— Illustrate the formation of wood by a reference to Fig. 118— 

 Vith. 



