114 BOTANY. 
many of the herbaceous plants, are exogenous. In herbaceous stems, the 
medullary rays are generally large, and the more solid part of the stem 
Fy 
consists of vascular bundles, arranged around the pith, and these bundles, 
when cut across, appear somewhat wedge-shaped, becoming broader _ 
outwards. The arrangement is beautifully regular. In trees and shrubs, 
the medullary rays generally become very fine as the wood hardens, so_ 
that they do not much affect its solidity. 
The pith consists of cellular tissue—The wood consists of vascular 
tissue, and chiefly of long vessels, closed at the ends, which, as they 
become old, are gradually filled up with solid matter. The wood nearest 
the pith is harder than the younger wood near the bark, and in some 
-trees is also very different in colour. The matured wood is called 
heart-wood or duramen;1 and the soft wood, which forms the outer 
part of the stem, is called sap-wood or alburnum.?—Next to the wood, 
on the outside, is a layer of soft cells, called the cambium layer, 
formed in a mucilaginous fluid, called cambiwm,? which may be 
best observed in young shoots, and in spring when vegetation is most 
active. From the cambium layer, the new layer of wood is formed, and 
the necessary increase is also made to the bark.—The bark, in young stems 
or branches, consists of mere cellular tissue, but vessels are afterwards 
formed in the inner portion of it, which often become long and strong 
fibres. The valuable fibres of flax and hemp belong to the inner bark of 
these plants ; and bast mats are made of the inner bark of the linden or 
lime tree. There is an Indian tree of the bark of which sacks are made 
by merely stripping the bark from the wood, which is done by beating it 
till it comes off readily, as boys make ‘ plane-tree’ whistles. The znner 
bark is also called the bast. Its Latin name is liber, which is also the Latin 
for ‘a book,’ this bark having been used for writing on before the use of 
paper or parchment was known. The outer part of the bark is entirely 
cellular, but consists of two layers, easily to be distinguished. The 
outermost of these is that which, in the cork-tree, increases to a great 
thickness, affording us the substance known as cork. The bark itself 
is surrounded by an integument, called the epidermis, the use of which 
seems merely to be the protection of the whole: in old stems it is thrown 
off, although it is always to be found in young and tender shoots. 
Endogenous Stems.— Endogenous stems have no pith and no bark. The 
outer part of the stem may indeed be somewhat different from the rest, 
and have the appearance of a bark, but it cannot be separated from the 
wood like the bark of an exogenous tree. As an endogenous plant grows, 
the stem increases in thickness, not by the addition of layers on the 
1 From Latin durus, hard. 3 Low Latin, ‘nutriment,’ from Latin cambio, to change. 
2 From Latin albus, white. ; 
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