EUCALYPTUS, a large genus of trees of the natural order Myrtaceae, indigenous, with a few exceptions, to Australia and Tasmania. In Australia the Eucalypti are commonly called "gum-trees" or "stringy-bark trees," from their gummy or resinous products, or fibrous bark. The genus, from the evidence of leaf-remains, appears to have been represented by several species in Eocene times. The leaves are leathery in texture, hang obliquely or vertically, and are studded with glands which contain a fragrant volatile oil. The petals cohere to form a cap  which is discarded when the flower expands. The fruit is surrounded by a woody cup-shaped receptacle and contains very numerous minute seeds. The Eucalypti are rapid in growth, and many species are of great height, E. amygdalina, the tallest known tree, attaining to as much as 480 ft., exceeding in height the Californian big-tree (Sequoia gigantea), with a diameter of 81 ft. E. globulus, so called from the rounded form of its cap-like corolla, is the blue gum tree of Victoria and Tasmania. The leaves of trees from three to five years of age are large, sessile and of a glaucous-white colour, and grow horizontally; those of older trees are ensiform, 6-12 in. long, and bluish-green in hue, and are directed downwards. The flowers are single or in clusters, and nearly sessile. This species is one of the largest trees in the world, and attains a height of 375 ft. Since 1854 it has been successfully introduced into the south of Europe, Algeria, Egypt, Tahiti, New Caledonia, Natal and India, and has been extensively planted in California, and, with the object of lessening liability to droughts, along the line of the Central Pacific railway. It would probably thrive in any situation having a mean annual temperature not below 60° F., but it will not endure a temperature of less than 27° F. Its supposed property of reducing the amount of malaria in marshy districts is attributable to the drainage effected by its roots, rather than to the antiseptic exhalations of its leaves. To the same cause also is ascribed the gradual disappearance of mosquitoes in the neighbourhood of plantations of this tree, as at Lake Fezara, in Algeria. Since about 1870, when the tree was planted in its cloisters, the monastery of St Paolo a la trè Fontana has become habitable throughout the year, although situated in one of the most fever-stricken districts of the Roman Campagna. An essential oil is obtained by aqueous distillation of the leaves of this and other species of Eucalyptus, which is a colourless or straw-coloured fluid when freshly prepared, with a characteristic odour and taste, of sp. gr. 0.910 to 0.930, and soluble in its own weight of alcohol. This consists of many different bodies, the most important of which is eucalyptol, a volatile oil, which constitutes about 70%. This is the portion of eucalyptus oil which passes over between 347° and 351° F., and crystallizes at 30° F. It consists chiefly of a terpene and cymene. Eucalyptus oil also contains, after exposure to the air, a crystallizable resin derived from eucalyptol. The dose of the oil is to 3 minims. Eucalyptol may be given in similar doses, and is preferable for purposes of inhalation. The oil derived from E. amygdalina contains a large quantity of phellandrene, which forms a crystalline nitrate, and is very irritating when inhaled. The oils from different species of Eucalyptus vary widely in composition.
Eucalyptus oil is probably the most powerful antiseptic of its class, especially when it is old, as ozone is formed in it on exposure to air. Internally it has the typical actions of a volatile oil in marked degree. Like quinine, it arrests the normal amoeboid movements of the polymorphonuclear leucocytes, and has a definite antiperiodic action; but it is a very poor substitute for quinine in malaria. In large doses it acts as an irritant to the kidneys, by which it is largely excreted, and as a marked nervous depressant, abolishing the reflex functions of the spinal cord and ultimately arresting respiration by its action on the medullary centre. An emulsion, made by shaking up equal parts of the oil and powdered gum-arabic with water, has been used as a urethral injection, and has also been given internally in drachm doses in pulmonary tuberculosis and other microbic diseases of the lungs and bronchi. The oil has somehow acquired an extraordinary popular reputation in influenza, but there is no evidence to show that it has any marked influence upon this disease or that its use tends to lessen the chances of infection. It has been used as an antiseptic by surgeons, and is an ingredient of "catheter oil," used for sterilizing and lubricating urethral catheters, now that carbolic oil, formerly employed, has been shown to be practically worthless as an antiseptic. Eucalyptus rostrata and other species yield eucalyptus or red gum, which must be distinguished from Botany Bay kino. Red gum is very powerfully astringent and is given internally, in doses of 2 to 5 grains, in cases of diarrhoea and pharyngeal inflammation. It is prepared by the pharmacist in the form of tinctures, insufflations, syrups, lozenges, etc. Red gum is official in Great Britain. E. globulus, E. resinifera, and other species, yield what is known as Botany Bay kino, an astringent dark-reddish amorphous resin, which is obtained in a semi-fluid state by making incisions in the trunks of the trees. The kino of E. gigantea contains a notable proportion of gum. J.H. Maiden enumerates more than thirty species as kino-yielding. From the leaves and young bark of E. mannifera and E. viminalis is procured Australian manna, a hard, opaque, sweet substance, containing melitose. On destructive distillation the leaves yield much gas, 10,000 cub. ft. being obtained from one ton. The wood is extensively used in Australia as fuel, and the timber is of remarkable size, strength and durability. Maiden enumerates nearly 70 species as timber-yielding trees including E. amygdalina, the wood of which splits with remarkable facility, E. botryoides, hard, tough and durable and one of the finest timbers for shipbuilding, E. diversicolor or "karri," E. globulus, E. leucoxylon or ironbark, E. marginata or "jarrah" (see Jarrah Wood), E. obliqua, E. resinifera, E. siderophloia and others. The timber is often very hard, tough and durable, and useful for shipbuilding, building, fencing, planks, etc. The bark of different species of Eucalyptus has been used in paper-making and tanning, and in medicine as a febrifuge.
For further details see Baron von Müller's monograph of the genus, Eucalyptographia (Melbourne, 1879-1884); J.H. Maiden, Useful Native Plants of Australia (1889).
 Whence the name (Gr., well-covered) given by L'Héritier, 1788.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)