The most striking feature in an Australian landscape is the Gumtree. It is the typical tree of Australia, and makes up the bulk of our forests. There are about a hundred and fifty species, comprising all sizes, from shrubs to some of the loftiest trees in the world. They are equally variable in distribution, occurring from Tasmania to the Northern Territory, from the extreme east to the farthest west, and from sea-level to near the top of mountains. With all this wide range it is singular that only two or three species are found outside Australia, and they only extend to the Indian Archipelago. Although Gums have such a wide range, most are very sensitive to environment. Different altitudes support different species. The tree of a 2000-feet situation will not, as a rule, be found much above or below it. Hardly a Gum native of Tasmania is found as far north as Sydney, and equally few are common to both East and West Australia. They constitute the genus Eucalyptus, which again belongs to the Myrtle family. The name is composed of two roots: eu signifies complete or typical; and calyptus, a hood. The name is very appropriate, as it marks the peculiar cap that falls off when the flower expands. The distribution, varied forms, and relation to its immediate allies lead us to conclude that Eucalyptus is a very modern genus; that is, it is of comparatively recent development. Of course, when we speak of recent in geology or botany we mean something more than a few thousand years. Such a period, though much to man, is as nothing when we are dealing with changes of the earth's surface or the evolution of animal or plant life.
In order to understand more of a Gumtree, let us take what we may term an intermediate form, and in studying that compare it with other types. We will examine Manna-gum, also called White-gum, or, botanically, Eucalyptus viminalis, Lab. The seed is a little dark object of irregular shape. It has a thick outer coat and a thin inner one, within which is a minute embryo plant with a miniature root and shoot, and two leaves which are closely wrapped round the other parts. Here the little being will lie for months, even years, nearly dormant, awaiting a favourable opportunity to break into activity. If the favourable conditions do not occur within a period, varying with the species, but probably never extending beyond a few years, changes will have taken place within the embryo which will render it incapable of responding. It will be dead. But should the seed before it has lost vitality find itself on soil that is damp and not too cold, it will absorb moisture, the root will grow out of the coats down into the earth, and in about 10 days the leaves, with the shoot between them, will expand into the air. It should be noted that there are a pair of first, or seed, leaves; that they are equal and opposite to one another. This is a marked feature of the largest of the two divisions of flowering plants. Very soon the shoots grow up into a stem, and branches on its way to attain the dimensions of a tree. With this, as with most Eucalypts, the leaves of the younger parts are without stalks, and placed in opposite pairs; also, they are broader than the later ones, and do not hang down like them. Soon the branches grow the mature foliage, which is different; the leaves are narrow, stalked, alternately placed, and hang down, so as not to expose their surface at right angles to the sun. Some few Gums bear but little juvenile foliage; others retain it for life; while most, in response in injury, will revert to it. It is a common thing for a bunch of twigs with juvenile foliage to spring from a spot where the tree has been injured. Probably the ancestors of our Gums had opposite stalkless leaves, and the other state is an adaptation better suited to our conditions.
When a Gumtree is a few years old it commences to bear flowers. The size of these, their arrangement, and some other details vary with the species. The flowers of Manna-gum are small; the solid part, that is, not taking the spread of the stamens into consideration, seldom equals three lines diameter; in some dwarf Peppermints they are still smaller; while in Blue-gum they approach an inch. They are arranged three together. A short stalk grows in the axil of a very young leaf. As the parts grow older three lesser stalks, each bearing a flower appear on the end of this. In Blue-gum the flower is single, and closely placed in the axil; in Stringy-bark, Peppermint, Weeping-gum, and some others they are many, but all arising from the apex of the common flower-stalk. The bud is oblong, and it is easy to see it is divided horizontally about the centre into two different halves. On maturity the upper part falls off, and looks very like a mitre or hood, and the enclosed stamens spread out in a circle. There do not appear to be sepals or petals. In Blue-gum and one or two more of our forms there is an outer hood, which is thrown off rather early. Now, comparison with closely-related genera, as, for instance, the Australian genus Ango-phora, leads to the conclusion that the outer hood is a much-altered calyx, and the inner one a modified corolla. Even if we could not directly detect the change, we should come to the same conclusion, for we find new structures seldom appear; what look like such are nearly always simply old structures changed in detail.
The stamens in Eucalyptus are of exceptional interest, as the forms of their anthers are of great use in sorting it into groups. They are numerous, free from one another, arranged in a circle upon the edge of the floral tube, and in the bud the long filaments are doubled, so that the anthers are tucked down towards the centre of the flower. In some West Australian species the stamens are straight from the first. When that is so the hood is like a long horn. The anthers are small, and in Tasmania assume two forms, which split our Gums into two natural sections. In Manna, Blue, Cider, and a few others the two halves of the anthers are arranged parallel to one another. In Stringy. Drooping, Peppermint, and Swamp the two halves touch above, but diverge below, assuming the shape of a kidney. In both cases each half opens by splitting down the centre. There is a third form not represented in Tasmania, where the anther opens by a pore instead of splitting. The pistil differs in no important detail from the form described in last chapter. The fruit is a capsule closely combined with the tube. In Manna-gum it protrudes, at least its valves do, giving the fruit a spherical appearance. In Cider, and more so in Urn Gum, it is deeply sunk, the tube much exceeding it.