388 ON THE NATURE AND CHARACTER OF THE BARK OF TREES. 
which exists in great quantities in the interior of plants. Others 
have ascribed the motion to capillary attraction; and Du Petit Tliouars 
suggests that it arises thus:— f In the spring, as soon as vegetation com¬ 
mences, the extremities of the branches and buds begin to open. 
The instant this happens, a certain quantity of sap is attracted out of 
the circumjacent tissue for the supply of these buds ; the tissue which 
is thus emptied of its sap is filled constantly by that beneath or about 
it; this is in its turn replenished by the next; and thus the mass of 
fluid is set in motion, from the extremities of the branches down to the 
roots.’ Du Petit Tliouars is, therefore, of opinion that the expansion 
of the leaves, &c., is not the effect of the motion of the sap, but, on the 
contrary, the cause of it; and that the sap begins to move at the 
extremities of the branches before it stirs at the roots. That this is 
really the fact is well known to foresters, and all persons accustomed 
to the felling or examination of timber-trees in the spring.” 
Observations on the Nature and real Character of the 
Bark of Trees. —The bark is an exterior membrane, which, after 
being exposed for some time to the atmosphere, becomes indurated into 
a dense covering, protective of the more delicate membranes within. 
The first-formed layer of bark, or cuticle, as it is sometimes called, 
receives annual additions of liber to its inner surface, and thereby 
thickening it from year to year as long as the tree lives. But this 
process proceeds with very different degrees of celerity; the liber of 
some trees is remarkably thin, appearing like a tissue of the finest 
gauze—as the beech; others like coarse lace—as the lime ; and that of 
the cork-tree so thick and fungous, that it may be stripped off the tree 
every seventh or eighth year, and of sufficient thickness for the manu¬ 
facture of corks. 
As the growth of bark takes place from within, it happens that, in 
order to make way for the internal accumulation, the outer layers 
must necessarily be so constituted as to give way somehow to permit 
the internal increase. This is effected in various ways:—the bark of 
oak, elm, and many other kinds of exogenous plants, yields to the swell¬ 
ing membranes by splitting perpendicularly, the openings every year 
becoming wider and deeper. That of the birch is split perpendicularly, 
and into very rugged pieces on the lower part of the trunk; but on the 
higher parts of the branches it is thrown off in horizontal fillets, like 
ribbons. The bark of the beech, and several other smooth-barked trees, 
is stretched horizontally, but without fracture or dismemberment of the 
exterior surface; while that of the grape-vine is thrown off in narrow 
