110 
PHYSIOLOGICAL VIEWS. 
with tlie 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 center of the 
stem, and from thence is conveyed in appropriate vessels to 
the buds. 
129. TXxQjperfect wood is also called the hearPwood^ or duror 
men / its color 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 
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 fibers extending longitudinally ; these 
fibers are chiefly of vascular texture, and contain sap, and the 
various secreted juices ; some contain only air. 
For illustration of the formation of wood, see Fig. 125, B, which represents a 
section of a woody stem of three years' growth ; i h, next the pith {i k), is a layer 
of the first year's growth, and the hardest part . of the wood ; h g is a layer of the 
second year's growth ; and (/ b, of the third ; the next to this is the sap-wood re- 
cently formed from the cambium. 
130. Pith. The pith (see Fig. 125, B, k i) is situated in the 
center of the trunk and branches of plants, and is a soft, spongy 
substance, analogous to the marrow of animals. It is composed 
of cellular tissue. The cells, which are very large in some 
plants, are filled with fluids when young, but in old branches 
the fluids disappear, and the cells are filled with air. In gen- 
eral, herbs and shrubs have a greater proportion of pith than 
trees ; it is also more abundant in young than old vegetables ; 
it extends from the root to the summit of the trunk or stem ol 
the plant. 
131. The MEDULLARY RAYS are lines which diverge from the 
pith toward the circumference ; they are fibrous textures in- 
terwoven in the wood, the alburnum, and the different layers 
of the bark. Tlie new buds originate from the points at which 
some 
across, as many layers of new 
129. Perfect wood — How has it been proved that wood is deposited externally 1 — Strength and ) ird» 
Bess of wood, to wiiat owing ?— III. strate the formation of wood by a reference to Fig. 12^5.-130. Pitii 
131. Medullary rays. 
