Pruning is an important operation in the management of planta tions, but it is often improperly practised for want of the knowledge of a few physiological facts, which are essentially necessary to be stated before any rules for pruning can be properly given. If these facts be understood few rules will be required, but otherwise a volume of directions are liable to be misapplied. It ought therefore to be under stood, that the sap ascends from the stem chiefly through the albur num, or outer layers of young wood. It then enters the leaves, when it is exposed to the influence of light. Having in consequence under gone an elaboration, it descends on the outside of the alburnum, that is to say, through the inner bark, the internal surface of the latter possessing a great degree of lubricity, whereby the passage of a portion of the juices at least is facilitated to the roots, whilst part is detained and becomes organised-by the action of the medullary rays, forming a new layer of woody matter. The thickness of the layer so formed is very different in different species; but in the same species, all other circumstances being the same, it is in proportion to the surface of leaves. The aggregate horizontal growth, or in other words the dia metrical extension of the stem, branch, and even roots, depends upon the quantity of healthy foliage. The diametrical increase of any par ticular part corresponds with the greater or less proportion of foliage above that part. Again, it may be stated that if any one tree has a clear stem of 20 feet in length, and another has one of 40 feet, the roots and foliage of both being equal, the layer of new wood will be much thinner in the latter case than in the former; for the deposition of woody matter will have to extend over double the surface in the tall tree. It may be also remarked, that, in general, if a tree be left entirely to nature, it will ultimately produce a greater bulk of wood, taking stems and branches together, than would be the case if sub jected to pruning. Except however in the case of growing for fuel, pruning is more or leas necessary. The value of a timber-tree depends chiefly on the stem, the branch timber being of less value. The object to be kept in view in pruning for timber is the production of a clean stem with as small a proportion of branches as possible. But it has been explained that the growth of the stem depends on the quantity of foliage, and without branches a sufficient quantity of foliage cannot be maintained. Whilst, on the one hand, an undue preponderance of branches is prevented by pruning, yet, on the other, as much foliage should be preserved as circumstances will permit.
An excessive privation of foliage should never be occasioned at any one time. It is therefore advisable that pruning should be commenced early, in order, as it were, to repress irregularities in the bud. At all events, whenever an irregularity can be clearly distinguished as such, it should be checked. Thus, when a shoot, occupying the position of a branch, ascends in competition with the top, or leading shoot, and is allowed to go on for years, the stem will have a forked character; or if the competing portion be amputated after it has acquired a large size, the timber will be rendered unsound. But if the shoot from which this aspiring limb took its rise had been stopped in the first or second season of its growth in summer, the tree might have been grown with a clean regularly tapering stem. It is only great irregularities however
that should be meddled with during the early growth of the plant ; otherwise the increase of roots is prevented, such increase being reciprocal with that of the branches.
Young shoots of the above description may be checked by being shortened In summer; but no large branches should be cut off at that season, nor after the sap begins to flow briskly in spring. Towards autumn, however, when the leaves have nearly ceased to carry on their functions, pruning may be very properly performed ; and some species, for example the birch and maple, require to be pruned at that time, otherwise they are apt to bleed. The period at which bleeding, that is, an extravasation of sap, takes place with greatest force, is immediately before the bursting of the buds in spring. I f a branch be cut off when in foliage, the remaining portion draws the sap and prevents effusion at the wound. Some derangement will of course be produced in the flow of sap ; and a diminution of the process of lignification and the formation of roots will be occasioned, owing to the abstraction of a certain extent of foliage. But when the sap is in motion, and the leaves at the same time not expanded, the amputation of a branch is followed by a flow of sap, which appears to drain from every part of the tree. The sap becomes stale on exposure to the air ; it then affects the cambium and inner bark, often to a considerable extent below the wound, and if these are not in all cases so far completely killed, yet they are generally more or less injured, and canker is apt to ensue.
Supposing it were required to rear a tree so as to have a stem of clean-grown timber, as tall and as thick as could possibly be obtained in any given time. The first consideration is the root, the plant being supposed to be young, with a top or upright leading shoot, and several side branches. All these should be left undisturbed in the first instance, unless such of the latter as may require a check if growing too strong for the others or for the leader. The laterals should be left, for the sake of the roots, till their presence on the stem becomes incon sistent with the object of producing it as free as possible from knots, taking also into consideration that the difficulty in healing the wound increases as the branches get older. A few of the largest shoots should be those first removed. They may be shortened in the first instance after midsummer, and afterwards cut off smoothly by the lowest circular wrinkles which form round their base, close to the stem, but without slicing off a portion of the bark ; a slovenly practice, of which only bad pruners are guilty, and which can only have the effect of diminishing the power of the bark to close over the wound neces sarily made by the pruning-knife. Such unnecessary wounds cause an extravasation of sap, as may be seen especially in coniferous trees, where they are generally covered with resinous exudations. In the following season the next largest branches should be in like manner displaced, care being still taken, on account of the importance of the foliage, as already explained, that the quantity removed shall be no more than is absolutely necessary.