Source, Etc

Eucalyptus leaves are obtained from Eucalyptus Globulus, Labillardier (N.O. Myrtaceœ), the ordinary 'blue gum' tree of Victoria and Tasmania. This tree, which is one of the largest known, attains a height of over 100 metres; it is cultivated in Italy, Spain, southern France, Portugal, Algeria, and other warm countries. It grows rapidly, and is largely planted in unhealthy marshy districts, such as the Campagna near Rome, upon which it exercises a beneficial influence.

The tree is remarkable for the dimorphism of its leaves. On young plants these are opposite, ovate, cordate at the base, and sessile, and they grow with one surface directed upwards and one downwards. On the upper parts of older trees, longer, scimitar-shaped leaves are produced, the short petioles of which are twisted so that the leaves grow with one margin directed upwards and the other downwards; these alone are employed in making preparations of eucalyptus leaves. They are collected in southern Europe and dried. Both forms of the leaf are used fresh for the distillation of the volatile oil.

Fig. 27.   Eucalyptus leaf. Reduced in size. (Maisch.)

Fig. 27. - Eucalyptus leaf. Reduced in size. (Maisch).

Description

Eucalyptus leaves are well characterised by their great length (up to 30 cm.) and narrow, ensiform outline. They taper gradually towards the apex, but narrow rather abruptly at the base into a short, twisted petiole. The lamina is thick, coriaceous, and, when quite dry, brittle. The margin is entire and somewhat thickened; the midrib is not prominent on either surface, and the lateral veins, most of which leave the midrib at an acute angle, anastomose near the margin to a continuous line. The leaves are quite glabrous, but distinctly punctate from the presence of numerous oil-glands situated in the mesophyll; these are best observed by examining the surface with a lens whilst the leaf is held against a strong light. The surfaces are frequently marked with a number of minute warty brown spots (groups of cork cells).

The odour of fresh eucalyptus leaves is strong, camphoraceous, and characteristic; in the dry leaves it is less perceptible until they are crushed. The taste of the dry leaves is aromatic, pungent, and slightly bitter.

The student should observe

(a) The ensiform outline,

(b) The coriaceous texture,

(c) The similarity between the two surfaces.

Constituents

Eucalyptus leaves contain, when fresh from 3 to 5 per cent, of volatile oil containing 50 or more per cent, of cineol (eucalyptol, cajeputol). They also contain tannin, a bitter principle which has not yet been investigated, and several resins, one of which is crystalline.

Uses

Eucalyptus leaves are used as an astringent; they have also been employed in the form of a cigarette for asthma. The volatile oil has antiseptic properties.

Note

The official eucalyptus oil may be obtained from E. Globulus, E. dumosa, A. Cunningham, and other species. Much is imported from Australia. Citron-scented eucalyptus oil is obtained from E. citriodora, Hooker (Queensland).