330 
fore has either a thicker or otherwise a more 
protective structure in exogens than in endo- 
gens, in trees of the temperate zone than in those 
of the tropics, and in trees of alpine or other very 
cold districts than in those of moderately warm 
plains.—Exogenous bark serves also the grand 
| purpose of conveying down for secretion, in its 
own layers, in the wood, and in the roots, certain 
| fluids which have been elaborated by the leaves 
out of the ascending sap of the plant and the 
gases of the atmosphere. It is penetrated by the 
medullary rays as fully as the wood, and is con- 
nected with them by myriads of points through- 
out its interior, and, in consequence, possesses 
countless and pervading lateral or horizontal 
communications between its own exterior and 
the very pith of the stem; the cellular and re- 
ticulated confirmation of its living liber affords 
a ready descending percolation, in constant con- 
tact with the medullary processes, of the portions 
of fluid designed to be conducted through these 
processes for secretion in the hgneous portions 
| of the stem; its woody tubes permit an easy de- 
| scent, and assist a facile secretion, of the portion 
of fluid designed for the formation of such chemi- 
| cal substances as peculiarly impregnate the bark; 
_ and its loose texture readily permits and aids the 
| formation and filling of the fistular cysts which 
| are designed to be the stores of resinous secre- 
_ tions. Yet the chief volume of descending fluid 
_ In a tree, or that which is assimilated into the 
| new alburnum of the wood and the new liber of 
| the bark, must be viewed in a different light, 
and will be noticed, at its alphabetical place, un- 
der its proper name of Camprum.—lIn succulent 
_ plants, such as stapelias and cacti, where no 
| proper leaves exist, the bark appears to effect 
the processes of exhalation, inhalation, and ela- 
boration of sap with gases, which in other plants 
are effected by the leaves. See the article Lear. 
In consequence partly of the outer or dead por- 
tion of old exogenous bark being excrementiticus, 
| and partly of the inner or living portion being 
the seat of the secretion of cortical juices and 
the medium of the conveyance of some juices of 
the woody secretions, it very widely differs from 
the wood in chemical composition, sometimes 
containing substances of which the wood is 
nearly or altogether destitute, and often con- 
taining a larger proportion than the wood does 
of even the latter’s peculiar chemical character. 
The bark of the oak, the larch, and other tan- 
niniferous trees contains the peculiar principle 
| which serves the purposes of tanning leather; 
the bark of the cinchona tree, the cinnamon tree, 
and many other similar plants, contains their 
febrifugal and aromatic principles ; and the bark 
of the acacias, the pines, and the many other gum- 
miferous, balsamiferous, and resin-bearing trees 
contains all their gum, their balsam, their turpen- 
| tine, and their rosin. ‘“ When,” says Liebig, “‘ we 
compare the barks of the fir, pine, beech, or oak, 
with their sap and wood, we find that they differ 
——_ ae eee = 
BARK. 
essentially from each other, both in their composi- 
tion and characters. True wood yields only one- 
fourth to two per cent. of ashes, while the bark 
of the oak, fir, willow, and beech, gives six, ten, 
to fifteen times more. The ashes of wood and of 
the bark have a very different composition. The 
inorganic ingredients of the bark are obviously 
inorganic substances expelled by the living or- 
ganism. The same reasoning holds good in the 
case of the organic substances as it does in the 
case of the bark. The bark of the cork-tree con- 
tains nearly half its weight of fat or of fatty sub- 
stances, which we also find present, although in 
smaller proportion, in the bark of firs and pines. 
The solid material (insoluble in alcohol or ether) 
of these barks is entirely different from woody 
fibre. The barks of firs and pines are completely 
soluble in potash leys, forming a liquid of a dark 
brown colour, which yields, on the addition of an 
acid, a precipitate strongly resembling the sub- 
stance called humic acid. But wood is not at- 
tacked by potash ley.” The strictly excrementi- 
tious nature of the dead portion of bark, appears 
in at once its exterior and decaying position on 
the plant, its total destitution of organism, its 
possession of inorganic substances which the 
living portions of the plant either totally want or 
possess in a smaller degree, and its playing no 
part whatever in either the plant’s growth or in 
any other of its functional processes. The re- 
gular organic effort of trees and shrubs to throw 
off the exterior portions of their bark, either in 
annual peelings or in occasional masses, is, in the 
strictest sense, excrementition. Hence the well- 
known circumstance that all the dead portions 
of bark, such as the peelings of the birch and the 
snowberry, the irregular plates of the plane-tree 
and the pines, and the thick and massive layers 
of the cork-tree, may be removed without occa- 
sioning the slightest injury to the plants. The 
excrementition, moreover, is far from being con- 
fined to the old bark of stems, but may be readily 
observed in the embrowning of water by the ve- 
getation in it of the smallest living twig, or the 
growth in it of the radical fibres of a hyacinth 
or any other bulb, and may be presumed or al- 
most proved to go on in the bark of every por- 
tion of a plant from the extremity of every root- 
let to the point of every little branch. The whole 
of the excrementition, quite analogously to that 
of animals, consists in the rejection of inorganic 
substances which the vital processes cannot as- 
similate; and so far as this goes on from the 
roots or beneath the surface of the soil, it makes 
deposits and effects chemical changes in the 
ground, which afford the grand explanation of 
the well-known phenomena of rotation of crops. 
See the article Excrerions or Piants. 
A wound through the cortical layers of a young 
plant is healed without a scar; but a wound 
through those of an old plant is altogether in- 
curable. When a wound in a middle-aged tree 
is small, the upper lip gradually grows downward 
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