Source, Etc

The willow bark of commerce is generally referred to the white or common willow, Salix alba, Linne (N.O. Salicineoe), a common tree on river-banks and marshy ground in England and throughout central and southern Europe. Its astringent properties were well known to Dioscorides, but the bark appears to have fallen into disuse until the latter end of the last century, when it was recommended as a remedy for ague. For this purpose, however, salicin, the principal active constituent of willow bark, has entirely taken the place of the bark, which is now seldom used medicinally. Salicin is usually obtained from the bark of other species of Salix, which contain it in greater abundance than S. alba. Very frequently, too, the structure of commercial willow bark shows that it is not derived from S. alba.

Description

The willow bark of commerce usually occurs in channelled pieces several inches in length and 10 to 20 cm. in width; they are usually of a greenish or greyish brown colour.

The outer surface is in young bark a smooth, sometimes glossy cork; in older barks it is dull, slightly longitudinally wrinkled, and usually of a greenish brown colour. The inner surface has a pale reddish colour, and, in the younger pieces at least, appears smooth to the naked eye, but is seen under the lens to be finely striated longitudinally. Older pieces are more coarsely striated. The fracture is short in the outer but fibrous in the inner portion.

The section examined under a lens shows numerous minute tangen-tially arranged groups of bast fibres. Under the microscope the cork is seen to consist of but few rows of cells, the outer wall of which is strongly thickened and bulges outwards; one such row of cork cells is formed each year. The bast contains no groups of sclerenchymatous cells. These characters sharply distinguish willow barks from poplar and other barks.

Description 218Fig. 142.   Willow bark, showing, A, outer surface'; B, inner surface. Natural size.

Fig. 142. - Willow bark, showing, A, outer surface'; B, inner surface. Natural size.

Willow bark often has a slight agreeable odour, and an astringent, slightly bitter and aromatic taste.

Constituents

The principal constituent of commercial willow bark is tannin together with a little salicin.

Salicin, C13H1807, is a crystalline bitter glusoside present in the bark of various species of Salix and Populus. Emulsin hydrolyses it in aqueous solution to saligenin,

Constituents 220

(orthohydroxy-benzyl alcohol), and dextrose. Boiled with dilute sulphuric acid it yields saliretin and dextrose. Cautiously heated with dilute sulphuric acid and potassium bichromate, salicylic aldehyde with an odour of meadow sweet is evolved.

The student should observe

(a) The greenish or greyish brown colour;

(b) The smooth or wrinkled outer surface;

(c) The pale reddish inner surface; and should compare this bark with

(i) Oak bark, which has a silvery cork and dark brown inner surface; (ii) Witch-hazel bark, which has a pale greyish cork and reddish pink inner surface.

Uses

Willow bark is astringent and has been used in rheumatism and ague; it has, however, been completely replaced by salicin.

Allied Species

Salix discolor, Muehlenberg, and S. nigra, Marsh, yield black willow bark which occurs in long, thin, tough, fibrous strips; brownish or greenish brown externally; taste bitter, astringent, somewhat aromatic; contains tannin (about 4 per cent.) and salinigrin (about 1 per cent.) a colourless, crystalline glucoside.

Different species of the genus Salix are known as willows, osiers, and sallows. The name willow is generally applied to those species that form trees, osiers to those that form long slender shoots with few. lateral branches, and sallows to those that have a shrubby growth.

The chief willows are S. alba, the wood of which is used for making cricket-bats, chip-boxes, etc, S. triandra, which is often pollarded close to the ground to give long shoots (also called osiers) for white basket work, and S. fragilis, Linne, the shoots from which have a reddish colour. S. fragilis is largely grown in Belgium, and the bark peeled from the shoots constitutes the ' rood scorce ' (i.e. red bark) from which salicin is made. Rood scorce is largely imported into this country for that purpose, and contains about 3 per cent, of salicin.

The chief osiers are S. viminalis, which is used for hampers, 8. vitellina, which is employed for binding, and S. purpurea, which is used for fine basket work. The bark of the last-named species is especially rich in salicin, containing 6 or 7 per cent. of salicin. The bulk of the salicin employed medicinally appears, however, to be made from S. fragilis.