Source, Etc

The bark that is commonly known as ' wild cherry bark,' is obtained from Prunus serotina, Ehrhart (N.O. Rosaceoe), the black cherry, a tree widely distributed over North America, especially throughout the northern and central States. The wood is highly valued for cabinet work, whilst the bark is employed medicinally for which purpose it is collected from the branches in the autumn, experiments proving that such bark is the most active.

Description

Wild cherry bark varies considerably in appearance. It occurs usually in flattened, curved, or recurved pieces, attaining 12 cm. in length and 5 cm. in width, but generally smaller, and about 2 mm. in thickness. Young bark is frequently covered with a thin, smooth, often glossy, reddish brown cork, much interrupted by whitish lenticels which are strongly tangentially elongated; it can easily be peeled off in thin, membranous tangential strips, disclosing a smooth greenish brown cortex. Old bark is darker and rougher. Much of the commercial drug has been deprived of its cork, and then the smooth greenish brown cortex, bearing scars corresponding to the lenticels, constitutes the outer layer. Sometimes even this has been removed, and the exposed part is then the outer layer of bast, which has a rough or rasped appearance and is of a uniform, dark cinnamon-brown colour; examined under the lens such bark exhibits pale longitudinal strands (sclerenchymatous cells) alternating with darker parenchymatous tissue (medullary rays).

The inner surface of the bark is of a cinnamon-brown colour and is finely longitudinally striated or rough, with reticulately anastomosing pale strands, the interstices of which are only partially filled with the brown (parenchymatous) tissue of the medullary rays.

The fracture is short and granular; the fractured surface has a reddish grey colour, and usually exhibits numerous tortuous, pale red medullary rays alternating with bast rays containing much sclerenchymatous tissue, the latter projecting beyond the medullary rays, owing to their contracting less on drying.

The bark has a slight odour of bitter almonds, which becomes much more apparent when it is moistened; the taste is astringent, aromatic, and bitter, resembling that of bitter almonds.

The students should observe

(a) The reddish brown cork with numerous lenticels,

(b) The irregularly reticulated or fissured inner surface,

(c) The granular fracture,

(d) The taste of bitter almonds when chewed.

Constituents

Wild cherry bark yields, when moistened with water, hydrocyanic acid and benzaldehyde. This reaction has been shown by Power and Moore (1909) to be due to laevo-mandelonitrile glucoside, C14H1706N, which is isomeric, but not identical with prulaurasin and sambunigrin (compare p. 37); it is hydrolysed by an enzyme also contained in the bark, yielding hydrocyanic acid, benzaldehyde and dextrose.

Experiments have shown that thin green bark collected in the autumn from trees of moderate size yields most hydrocyanic acid (0.12 to 0.16 per cent.); the bark of the root is said to be more active than that from the stem or branches (Stevens, 1896).

The bark contains also a brown resin, trimethylgallic acid, para-cumaric acid and traces of benzoic acid and volatile oil; a green resin yielding by acid hydrolysis β-methylaesculetin is also present.

Fig. 128.   Virginian Prune bark, showing transverse lenticels, land papery cork peeling off. Natural size.

Fig. 128. - Virginian Prune bark, showing transverse lenticels, land papery cork peeling off. Natural size.

The commercial drug afforded Power and Moore 0.075 per cent. of hydrocyanic acid.

Uses

The bark has mild tonic and sedative properties; it is frequently given for coughs and chest complaints.

Substitutes

The bark of other North American species of Prunus is occasionally substituted for the official. The latter is well characterised by its short granular fracture and distinctive taste, as well as by the presence of abundant sclerenchymatous cells and absence of bast fibres. Spurious barks may be fibrous or more astringent or almost devoid of taste. Old (trunk) bark is characterised by the numerous depressions on the outer surface and the absence of lenticels.