_Oak Bark_; F. _Ecorce de Chêne_; G. _Eichenrinde_.
=Botanical Origin=—_Quercus Robur_ L., a tree, native of almost the
whole of Europe, from Portugal and the Greek Peninsula as far north as
58° N. lat. in Scotland, 62° in Norway, and 56° in the Ural Mountains.
There are two remarkable forms of this tree which are regarded by
many botanists as distinct species, but which are classed by De
Candolle[2225] as sub-species.
Sub-species I. _pedunculata_—with leaves sessile or shortly stalked,
and acorns borne on a long peduncle, and acorns either sessile or
growing on a short peduncle.
Sub-species II. _sessiliflora_—with leafstalks more or less elongated.
Both forms occur in Britain. The first is the common oak of the greater
part of England and the lowlands of Scotland. The second is frequently
scattered in woods in which the first variety prevails, but it rarely
constitutes the mass of the oak woods in the south of England. In North
Wales however, in the hilly parts of the north of England, and in
Scotland, it is the commoner of the two forms (Bentham).
[2225] _Prodromus_, xvi. (1864) sect. 2. fasc. 1.
=History=—The astringent properties of all parts of the oak[2226] were
well known to Dioscorides, who recommends a decoction of the inner
bark in colic, dysentery and spitting of blood. Yet oak bark seems at
no time to have been held in great esteem as a medicine, probably on
account of its commonness; and it is now almost superseded by other
astringents. For tanning leather it has always been largely employed.
[2226] Probably not _Q. Robur_ L.
=Description=—For medicinal use the bark of the younger stems or
branches is collected in the early spring. It varies somewhat in
appearance according to the age of the wood from which it has been
taken: that usually supplied to English druggists is in channelled
pieces of variable length and a tenth of an inch or less in thickness,
smooth, of a shining silvery grey, variegated with brown, dotted
over with little scars. The inner surface is light rusty-brown,
longitudinally striated. The fracture is tough and fibrous. A
transverse section shows a thin, greenish cork-layer, within which
is the brown parenchyme, marked with numerous rows of translucent
colourless spots. The smell of dry oak bark is very faint; but when
the bark is moistened the odour of tan becomes evident. The taste is
astringent and in old barks slightly bitter.
=Microscopic Structure=—The outer layer of young oak bark consists of
small flat cork-cells; the middle layer of larger thick-walled cells
slightly extended in a tangential direction, and containing brown
grains and chlorophyll. This tissue passes gradually into the softer
narrower parenchyme of the inner bark, which is irregularly traversed
by narrow medullary rays. It exhibits moreover a ring, but slightly
interrupted, of thick-walled cells (sclerenchyme) and isolated shining
bundles of liber-fibres.
Groups of crystals of calcium oxalate are frequent in the middle and
inner bark, but the chief constituents of the cells are brown granules
of colouring matter and tannin. As the thickness of the bark increases
the liber is pushed more to the outside, the middle cortical layer
being partly thrown off by secondary cork-formation (rhytidoma, see pp.
354 and 538). Hence the younger barks, which alone are medicinal, are
widely different from the older in structure and appearance.
=Chemical Composition=—The most interesting constituent is a peculiar
kind of tannin. Stenhouse pointed out in 1843 that the tannic acid of
oak bark is not identical with that of nutgalls; and such many years
afterwards was proved to be the case.
The first-named substance, now called _Querci-tannic Acid_, yields
by destructive distillation pyrocatechin, and according to Johanson
(1875) very little pyrogallol. By boiling it with dilute sulphuric
acid querci-tannic acid is split up into a red derivative and sugar.
A solution of gelatine is precipitated by querci-tannic acid as well
as by gallo-tannic acid; yet the compound formed with the latter is
very liable to putrefaction, whereas the tannin of oak bark, which is
accompanied by a large amount of extractive matter, furnishes a stable
compound, and is capable of forming good leather.
As querci-tannic acid has not yet been isolated in a pure state, the
exact estimation of the strength of the tanning principle in oak
bark has not been accomplished, although it is important from an
economic as well as from a scientific point of view. The method of
Neubauer (1873) depends upon the amount of permanganate of potassium
decomposable by the extract of a given weight of oak bark. Neubauer
found in the bark of young stems, as grown for tanning purposes, from 7
to 10 per cent. of querci-tannic acid, soluble in cold water.
Braconnot (1849) extracted from the seeds of the oaks under notice
a crystallized sugar, which was shown in 1851 by Dessaignes to be
a peculiar substance, which he termed _Quercite_. Prunier proved
(1877-1878) that it agrees with the formula C₆H₇(OH)₅ + 4 OH₂, and is
closely allied to kinic acid, C₆H₇(OH)₄COOH (see page 363). Quercite
gives off water at 100°, melts at 225° C., and again losing water
yields a crystallized anhydride. In the oak bark extremely small
quantities of quercite appear also to be present, as pointed out by
Johanson.
A colourless, crystallizable, bitter substance, soluble in water, but
not in absolute alcohol or ether, was extracted from oak bark in 1843
by Gerber, and named _Quercin_. It requires further examination: Eckert
(1864) could not detect its existence in young oak bark.
=Uses=—Occasionally employed as an astringent, chiefly for external
application.