_Common Laurel or Cherry-laurel Leaves_; F. _Feuilles de
Laurier-cerise_; G. _Kirschlorbeerblätter_.
=Botanical Origin=—_Prunus Lauro-cerasus_ L., a handsome evergreen
shrub, growing to the height of 18 or more feet, is a native of the
Caucasian provinces of Russia (Mingrelia, Imeritia, Guriel), of the
valleys of North-western Asia Minor, and Northern Persia. It has been
introduced as a plant of ornament into all the more temperate regions
of Europe, and flourishes well in England and other parts, where the
winter is not severe and the summer not excessively hot and dry.
=History=—Pierre Belon, the French naturalist, who travelled in
the East between 1546 and 1550, is stated by Clusius[970] to have
discovered the cherry-laurel in the neighbourhood of Trebizond. Thirty
years later, Clusius himself obtained the plant through the Imperial
ambassador at Constantinople, and distributed it from Vienna to the
gardens of Germany. Since it is mentioned by Gerarde[971] as a choice
garden shrub, it must have been cultivated in England prior to 1597.
Ray,[972] who like Gerarde calls the plant _Cherry-bay_, states that it
is not known to possess medicinal properties.
[969] _Pharm. Journ._ xviii (1852) 109.
[970] _Rariorum Plantarum Historia_, 1601. 4.
[971] _Herball_ (1636) 1603.
[972] _Hist. Plant._ ii. (1693) 1549.
In 1731, Madden of Dublin drew the attention of the Royal Society of
London[973] to some cases of poisoning that had occurred by the use of
a distilled water of the leaves. This water he states had been for many
years in frequent use in Ireland among cooks, for flavouring puddings
and creams, and also much in vogue with dram drinkers as an addition to
brandy, without any ill effects from it having been noticed. The fatal
cases thus brought forward occasioned much investigation, but the true
nature of the poison was not understood till pointed out by Schrader in
1803 (see art. Amygdalæ amaræ, p. 248, note 2). Cherry-laurel water,
though long used on the Continent, has never been much prescribed in
Great Britain, and had no place in any British Pharmacopœia till 1839.
=Description=—The leaves are alternate, simple, of leathery texture
and shining upper surface, 5 to 6 inches long by 1¾ to 2 inches wide,
oblong or slightly obovate, attenuated towards either end. The thick
leafstalk, scarcely half an inch in length, is prolonged as a stout
midrib to the recurved apex. The margin, which is also recurved, is
provided with sharp but very short serratures, and glandular teeth,
which become more distant towards the base. The under side, which is of
a paler colour and dull surface, is marked by 8 or 10 lateral veins,
anastomosing towards the edge. Below the lower of these and close to
the midrib, are from two to four shallow depressions or _glands_, which
in spring exude a saccharine matter, and soon assume a brownish colour.
By the glands with which the teeth of the serratures are provided, a
rather resinous substance is secreted.[974]
The fresh leaves are inodorous until they are bruised or torn, when
they instantly emit the smell of bitter almond oil and hydrocyanic
acid. When chewed they taste rough, aromatic and bitter.
=Microscopic Structure=—The upper surface of the leaf is constituted of
thin cuticle and the epidermis made up of large, nearly cubic cells.
The middle layer of the interior tissue exhibits densely packed small
cells, whereas the prevailing part of the whole tissue is formed of
larger, loose cells. Most of them are loaded with chlorophyll; some
enclose crystals of oxalate of calcium.
=Chemical Composition=—The leaves when cut to pieces and submitted to
distillation with water, yield _Bitter Almond Oil_ and _Hydrocyanic
Acid_, produced by the decomposition of _Laurocerasin_. This is
an amorphous yellowish substance isolated by Lehmann (1874) in
Dragendorff’s laboratory. He extracted the leaves with boiling alcohol,
and purified the liquid by gently warming it with hydroxide of lead.
From the liquid, crude laurocerasin was precipitated on addition of
ether; it was again dissolved repeatedly in alcohol and precipitated
by ether. The yield of the leaves is about 1⅓ per cent. Laurocerasin
is readily soluble in water, the solution deviates the plane of
polarization to the left, yet not to the same amount as amygdalin. The
molecule of laurocerasin, C₄₀H₆₇NO₃₀, would appear to include those of
amygdalin, C₂₀H₂₇NO₁₁, amygdalic acid, C₂₀H₂₆O₁₂ and 7 OH₂.
[973] _Phil. Trans._ xxxvii. (for 1731-32) 84.
[974] Reinke, in Pringsheim’s _Jahrbücher für wissenschaftliche
Botanik_, x. (1875) 129.
The proportion of hydrocyanic acid in the distilled water of the leaves
has been the subject of many researches. Among the later are those of
Broeker (1867), who distilled a given weight of the leaves grown in
Holland under precisely similar circumstances, in each month of the
year. The results proved that the product obtained during the winter
and early spring was weaker in the acid in the proportion of 17 to
24, 28, or 30, the strongest water being that distilled in July and
August. This chemist found that a stronger product was got when the
leaves were chopped fine, than when they were used whole. According
to Christison,[975] the buds and very young leaves yield ten times as
much essential oil as the leaves one year old. We have ascertained that
leaves collected in January when they were thoroughly frozen yielded a
distillate containing about ten times less of hydrocyanic acid than in
summer. The product obtained from the leaves collected in January, but
previously dried for several days at 100° C. (212° F.), still proved to
contain both essential oil and hydrocyanic acid.
The unwounded leaves of the cherry-laurel in vigorous vegetation have
been shown by our friend Prof. Schaer, not to evolve naturally a trace
of hydrocyanic acid, though they yield it on the slightest puncture.
We are ignorant of the mode of distribution in the living tissue of
the laurocerasin, and of the substances causing its decomposition, and
how these two bodies are packed so as to prevent the slightest mutual
reaction. The leaves may be even dried at 100° C. and powdered without
the evolution of any odour of hydrocyanic acid, but the latter is at
once developed by the addition of a little water; on distilling its
presence is proved by means of all the usual tests in the first drops
of the product.
Besides the substances concerned in the production of the essential
oil, the leaves contain sugar which reduces cupric oxide in the cold,
a small quantity of an iron-greening tannin, and a fatty or waxy
substance.
Schoonbroodt (1868) treated the aqueous extract of the fresh leaves
with alcoholic ether, which yielded ¼ per mille of bitter, acicular
crystals; these quickly reduced cupric oxide, losing their bitterness.
Bougarel (1877) isolated from the leaves under notice and several
others, _Phyllinic acid_, a crystalline powder melting at 170° C.
=Uses=—The leaves are only employed for making cherry-laurel water
(_Aqua Lauro-cerasi_), the use of which in England is generally
superseded by that of the more definite hydrocyanic acid.