Most Gums take time in maturing their flowers. From the first appearance of the bud to the bursting of the lid in Blue-gum generally takes two years; also, except those growing at a high altitude, they appear indifferent to the period of the season. Specimens of our lowland forms may be found in flower at any period of the year. When many of a species are in flower at the same time it is the result of a climatic condition, as dryness, that occurred some time before causing the change in the tree's condition that is responsible for the laying down of flowers. A Gum does not flower in a desultory manner; however long it takes for the blooms to come to maturity, they all burst out at the same time. This, though a common habit amongst plants, is not universal. In cases where flowering is sea sonal, the advantage is readily understood; but where, as in Gum, there appears an indifference, it can only be an advantage in that it enables the masses of their small flowers to be seen far off. Gums depend for cross-fertilisation principally on honey-feeding birds. It is an advantage to the birds that a succession should continue all through the year, and to both that a flowering tree may be picked out in the forest at a great distance.
These trees seem in no more hurry to disperse their seeds than they are in flowering. Seed is seldom ripe before a year, and generally longer; and when ripe they remain in the capsules sometimes for years, till those bodies are quite dead and dry. Gather old capsules that have been on the boughs for many seasons, and leave them to dry in paper; they will open their valves and let good fertile seed be shaken out. This condition of fruit and seed is of great value to trees growing in countries subject to fire. Any large Gum tree in our forests will be found to have on its branches thousands of capsules in a condition of maturity, but that have never opened to permit the seed to escape. If a fire occurs all the finer portions of the tree are killed, and the ground below is cleared of all undergrowth. The scorched capsules dry and open, and the seed falls on land quite prepared for it. This adaptation to fire conditions is very common in capsular Myrtles. Some Pines have the same habit, retaining seed in the cones for an indefinite period until a fire sweeps the forest away, when the resistant, but scorched, cones deliver up their seed to build a forest anew.
A smooth-barked Eucalypt is readily killed by fire, that is, all that is above ground; but the roots in many instances respond by sending up suckers. In some cases the result of ringing trees is that the land is soon covered with a denser growth of Gum trees than it bore before. But all of them have not this resource, and can only produce shoots at the base of the dead stem, or not at all. The smooth condition of bark is produced by the tree shedding its outer bark as soon as dried. Gums do this at no stated period. If a tree is developing girth rapidly the bark is shed at short intervals; while the same species, growing under less favourable conditions, will shed it at longer intervals. Some trees, like Stringy-bark and Black Peppermint, have persistent, thick fibrous bark. This is a great protection against fire. It takes a very severe scorch to kill it. It is common to see such a tree, after fire has killed all its smaller branches and burnt the bark black, break out into bunches of fresh vegetation all along the stem
If you cut a large piece of bark right down to the wood it can easily be torn away; then both the wood and bark will be covered with a colourless slimy substance. This appears in such a condition because its structure has been destroyed in pulling off the piece of bark. In its natural condition in consists of a few layers of very delicate cells lying between bark and wood. These cells are in a condition of active growth, constantly forming layers of bark cells or fibres on the outside and wood on the inner side. This layer of delicate cells, which is called the cambium, is responsible for forming the whole of the bark and wood. It may go on with its work all the year round, or may rest during the cold of the winter. Its activity may also be much reduced when lack of rain causes a dryness of soil. Trees growing in places where there is a marked but constant change of summer heat and winter cold, but never irregular periods of soil dryness, will form regular yearly rings of softer spring wood and denser autumn wood. This is very well marked in trees that shed their leaves in winter, in Pines, and in Gums growing at a considerable altitude. A Eucalypt placed at a low elevation grows all the year round, though it slows down somewhat in the winter; but if in summer its water-supply is reduced beyond its maximum requirement by a period of drought, development will be checked and the forming wood will be thin and hard, but will again be looser on return of moisture. This is why in Gum timber the rings are not always to be relied upon as indicating the age of the tree, and has given rise to the statement that our trees form two layers of wood per annum. The same tree may lay down one to three layers according to the condition of the season.
We have in Tasmania only one native, a Beech, of our westerly mountains, that sheds its leaves in winter; all the rest are evergreens. A tree growing at a high altitude, where there is inefficient or no sunlight during some months, will benefit by shedding its foliage; also, trees growing in districts subject to regular periods of great summer drought may benefit in the same way. But with us neither condition obtains. We can only think our deciduous Beech an immigrant from a more southern land that has not the ability to change its character.