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CHARACTERISTICS OF THE GENUS EUCALYPTUS.
The Eucalyptus derives its name from two Greek words which may be translated as "well-covered"—a name applied to the little cap which protects the unopened flower, and one which aptly describes what is perhaps the leading feature of the genus. The most noticeable feature about a Eucalyptus flower is the absence of both sepals and petals, and the presence of the operculum or bud-cap which protects the stamens in the bud stage. The operculum usually falls off entirely as the flower expands, but sometimes remains hinged on to the calyx after the flower opens. These characteristics, together with the presence of the inferior ovary and the conspicuous stamens of indefinite number, serve to distinguish the genus Eucalyptus from all other flowering plants. The Eucalypts are closely allied to the genus Angophora, which is found only in the Eastern States, and in which there are petals which soon fall after expansion, and small but distinct calyx-teeth. The calyx of Eucalyptus, although generally without lobes or teeth at all, has, in a few instances, small teeth, four in number, which are situated at or near the top of the calyx, and which appear to be the rudiments of sepals. The operculum of Eucalyptus takes the place of petals as regards their protective functions, but the filaments are the most conspicuous part of the flower, and, being attractive, serve as petals in this respect. If, as is generally supposed, the Angophoras are the ancestors of the Eucalypts, the operculum may have, at one time, consisted of free petals. In some species of Eucalyptus the operculum is double—the outer one falling off before the inner.
The Eucalypts are all evergreen trees or shrubs, with the exception of one tropical tree which is deciduous, and another from the same latitude which is partially deciduous. Eucalypts have simple leaves which usually have leaf stalks. The leaves are generally of a lance or egg shape. The venation consists of a midrib connected by fine secondary veins with a vein which runs close to the margin of the leaf (intramarginal). With few exceptions, the bracts and bracteoles so characteristic of other flowering shrubs and trees are absent from Eucalyptus, and, when present, are either rudimentary or fall before the flowering period.
Eucalypts are divided, as regards their habit, into Trees, Shrubs, Mallees and Marlocks. Trees are distinctive in habit, in that, they possess a well defined trunk. Shrubs branch from the base, or close to the base. Mallees have a bulbous root-stock, either subterranean or half above the soil, from which arise stems (usually 4 to 8 in number), which are all of about the same height. Typical Mallees have a large woody stock. Marlocks, which may be called "sand-plain Mallees," have a smaller reduced stock, or become true shrubs. It is often difficult to differentiate between a Marlock and a shrub, as intermediate forms occur, which may be one or the other. Trees and Mallees, on the other hand, are quite distinctive forms of vegetation. For the sake of convenience, the Mallees, Marlocks and true shrubs, have been placed in the same general key.
