EUCALYPTUS, a large genus of trees of the family Myrta ceae, indigenous, with only a few exceptions, to Australia and Tasmania. In Australia the eucalypti are commonly called "gum trees" or "stringy-bark trees." The genus, from the evidence of leaf-remains, appears to have been represented by several species in Eocene times. The leaves are leathery, hang obliquely or verti cally, and are studded with glands which contain a fragrant vola tile 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. regnans, of Victoria, one of the tallest trees known, attains a height of 326 ft. and a girth of 25 ft., thus equalling the California big tree (Sequoia gigantea) in height though not in massiveness. E. globulus is the blue gum tree of Victoria and Tas mania. The flowers are single or in clusters, and nearly sessile. 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 ob ject of lessening liability to droughts, along the line of the Cen tral Pacific railway. By aqueous distillation of the leaves a com plex straw-coloured fluid is obtained of sp. gr. o.91 o to o.930, and soluble in its own weight of alcohol. About 7o% of this fluid con sists of a volatile oil—Eucalyptol—which passes over between and 351° F., and crystallizes at 3o°F. It consists chiefly of a terpene and cymene. Eucalyptus oil also contains, after ex posure to the air, a crystallizable resin derived from eucalyptol. The dose of the oil is 3 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 phel landrene, which forms a crystalline nitrate, and is very irritating when inhaled. The oils from different species of Eucalyptus vary widely in composition.
Internally, eucalyptus oil acts as other volatile oils. Like qui nine, it arrests the normal amoeboid movements of the polymor phonuclear leucocytes, and has a definite anti-periodic action; but it is a very poor substitute for quinine in malaria. In large doses it is an irritant to the kidneys, by which it is largely excreted, and is a marked nervous depressant, abolishing the reflex func tions of the spinal cord and ultimately arresting respiration by its action on the medullary centre. An emulsion with gum solution has been used as a urethral injection, and has been given inter nally in pulmonary tuberculosis and other microbic diseases of the lungs and bronchi. The oil has acquired an extraordinary popular reputation in influenza, but there is no evidence to show that it has any marked influence upon this. It has been used as an antiseptic by surgeons, and is an ingredient of "catheter oil." Eucalyptus rostrata and other species yield eucalyptus or red gum, which must be distinguished from Botany Bay kino. Red gum is powerfully astringent and is given internally, in doses of 2 to 5 grains, in cases of diarrhoea and pharyngeal inflammation. Red gum is official in Great Britain. E. globules, E. resinifera and other species yield what is known as Botany Bay kino, an astrin gent 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. manni f era and E. viminalis is pro cured Australian manna, a hard, opaque, sweet substance, con taining melitose. On destructive distillation the leaves yield much gas, i o,000 cu. ft. being obtained from one ton. The wood is extensively used in Australia as fuel, and the timber is of re markable size, strength and durability. Maiden enumerates nearly 7o 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. globules, E. leucoxylon or ironbark, E. marginate or "jarrah" (see JARRAH WOOD), E. oblique, E. resinifera, E. siderophloia and others. The timber is often very hard, tough and durable, and useful for shipbuilding, building, fencing, 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 Muller's monograph of the genus, Eucalyptographia (Melbourne, 1879-84) ; J. H. Maiden, Useful Native Plants of Australia (5889) ; R. T. Baker and H. S. Smith, A Research on the Eucalyptus Especially in Regard to Their Essential Oils (Sydney, N.S.W., 192o) ; R. N. Parker, Eucalyptus Trials in the Simla Hills, Calcutta (1925).