WOOD AND PITH. 
137 
The alburnum is so called (from albus white) on account of 
the paleness of its colour. This is the most newly formed wood 
and constitutes 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 annual- 
ly 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. 
The perfect icood, is sometimes called the heart ; its colour is 
usually darker than that of the sap wood, and its texture is firm- 
er and more compact, it is also more durable for timber. It is 
formed by the gradual concentration and hardening of the al- 
burnum. The wood constitutes the greater part of the bulk of 
trees and shrubs ; when cut across, it is found to consist of nu- 
merous 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 cam- 
bium, pieces of metal have been introduced under the barks of 
trees that were growing, the wounds carefully bound up, and 
after some years on cutting them across, the layers of new 
wood have been found on the outside of the metal. 
The strength and hardness of wood, is owing to woody fibres 
extending longitudinally ; these fibres are chiefly of vascular 
texture, and contain sap, and the various secreted juices; some 
contain only air. 
PITH. 
The pith ( medulla ) 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, are filled with flu- 
ids when young, but in old branches the fluids disappear, and 
the cells are filled with air. In general, herbs and shrubs have 
a greater proportion of pith than trees. It is also more abun- 
dant in young than old vegetables; it extends from the root to 
the summit of the trunk or stem of the plant. 
The medullary rays are lines which diverge from the pith 
towards the circumference ; they are fibrous textures interwo- 
ven in the wood, the alburnum, and the different layers of the 
Alburnum — Perfect wood — Strength and hardness of wood. 
Pith — Medullary rays — 
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