CHAP. II. 
EXOGENOUS STEMS. 
69 
vascular system sometimes developing more rapidly than itself, 
it occasionally happens that it is either torn or divided into 
irregular cavities, as in the Horse Chestnut, the Rice-paper 
plant, and many others ; or that it is so much lacerated as to 
lose all resemblance to its original state, and to remain in the 
shape of ragged fragments adhering to the inside of the vas- 
cular system : this is what happens in Umbelliferous and other 
fistular-stemmed plants. 
Sometimes the pith is much more compact at the nodes than 
in the internodes, as in the Ash; whence an idea has arisen 
that it is actually interrupted at those places : this is, however, 
an obvious mistake; there is no interruption of continuity, but 
a mere alteration in compactness. 
It very seldom happens that any part of the vascular system 
intermixes with the pith, which is almost always composed of 
cellular tissue exclusively ; but in Ferula and the Marvel of 
Peru, it has been proved by Mirbel and De Candolle, that 
bundles of woody fibre are intermixed; and in Nepenthes 
there is a considerable quantity of spjral vessels scattered 
among the cellular tissue of the same part. 
The Bark is the external coating of the stem, lying imme- 
diately over the wood, to which it forms a sort of sheath, and 
from which it is always distinctly separable. When but one 
year old, it consists of an exterior coating of cellular substance, 
called the cellular integument or the epidermis, and of an in- 
terior lining of woody tissue, called the liher or inner bark : if 
more than one year old, then it is composed of as many layers 
of cellular integument and woody tissue as it is years old, the 
former being invariably external, and the latter internal, in 
each layer; and every layer being formed beneath the previous 
one, and therefore next the wood. In consequence of the 
new bark being continually generated within that of the pre- 
vious year, it is necessary that the latter, which is pushed 
outwards, should be extensible ; and in many plants this ex- 
tensibility takes place to a considerable degree. In the Apple, 
several successive zones of bark are formed without any 
appearance of a dislocation or disruption of the tissue of 
the outside; and in the Daphne Lagetto, the fibres of the 
liber are so tenacious that, instead of being ruptured by 
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