longitudinal incision made with a knife during the summer in the outer bark will immediately contract upon each side of the incision, leaving the cut much wider than the instrument could make in the case of a healthy tree; but, if the disease be of long standing, this contraction will not take place, as the bark will have lost its natural elasticity. Another, but only an indirect, consequence of this disease in a tree is that the proper lateral and top branches make small and weak annual shoots; and each year, as the disease becomes more confirmed, the annual shoots become the more weakly, owing to the

loss of natural vigour in the organism. This abnormal condition of the stem is due to a want of elasticity with regard to the turgidity or state of tension in the various tissues of the bark and the wood. But, under normal circumstances of growth, when this turgidity, or state of tension between the hydrostatic pressure on the one hand and the elasticity of the cell-wall on the other, becomes interfered with by loss of expansive power on the part of the cells forming the cambium and the rest of the bark, then a mechanical pressure arises, which

1 For further particulars see Vines' Lectures on the Physiology of Plants, 1886, p. 40.

creates a disturbance within the organism by preventing the free upward flow of the sap from the roots to the leaves, and also by hindering the descent of the assimilated nourishment between the wood and the bark.

When trees become bark-bound in this manner, the only true sylvicultural treatment is to remove them and cultivate other species for which the soil and situation are more favourable. This may easily be done by cutting out the diseased individual stems during the ordinary operations of thinning.

Sun-burn or Scorching of the Bark is a diseased condition occasioned by exposure of the stem to the burning influence of the direct rays of the sun or to those refracted from the soil. It causes the bark to lose its turgidity, and consequently to become dry and dead. It only occurs on the south, south-west, and west sides of trees, and is, in close-canopied crops, only found when trees are suddenly exposed to the sun after having long enjoyed the protection of adjoining trees; but it can often be the result of injudicious thinning or pruning in ornamental woods or in park trees, when this is carried so far as to remove stems or branches that had previously protected the lower portions of the bole of the tree affected.

Concerning this disease the following remarks are made in the editor's translation of Kauschinger-Fürst's Protection of Woodlands, 1893, p. 20:

"The trees which suffer from sun-burn are invariably such as, even in their later stages of growth, have a smooth bark without any rugged corky protection; that is to say, the Beech in particular, and in a less degree Hornbeam, Ash, Maple, Sycamore, and young Spruce and Silver Fir. Species with rough corticeous bark, like Oak and Elm, do not suffer at all, or are at any rate only exposed to danger during the earliest period of their growth, before the thicker fissured bark begins to form. The danger begins about the pole-forest stage of growth; but older trees are more sensitive in this respect, and exposed to greater danger. In the case of Beech, which is very sensitive to insolation, the appearance of sun-burn can often be noted even on large trimmed transplants; it usually results in their dying off.

"Preventive measures consist in the avoidance, so far as possible, of sudden exposure of the edges of woodlands, and of the retention of all branch development there; timely formation of a protective belt by planting up with Spruce can also prove efficacious. Beech-trees that have hitherto been growing in the close canopy of high forests, but are now retained as standards for the production of large timber, almost invariably after a short. time show the occurrence of sun-burn; hence the retention of such individual standards is not advisable. Sun-burnt stems along the edge of woods should, however, be retained as long as possible, as they remain for a long time in active vegetation; whilst, if they be removed, the stems immediately behind them would then be exposed to precisely the same danger."

3. Moss or Lichen upon the Bark.—In a normal state of growth, the bark of healthy trees is clean and free from any undue development of moss and lichens. They may, however, be found to exist to a slight extent upon trees in a variety of circumstances, soils, and situations, without always being a symptom of unhealthiness. In any district of country where much rain falls, almost all trees will, to a certain extent, be found to support moss and lichens, without the

stems being in what might reasonably be termed an unhealthy condition of growth. But the tendency to become a support for organisms of lowlier form, and to be much overgrown with lichen, exists to the greatest extent in a damp humid atmosphere, where the trees have not a free circulation of air about them. In this case a diseased state of the trees cannot be said to be the cause of the lichen growing upon them; yet such trees, existing in a state of air favourable to the development of the lichen-spores upon their stems, may be ultimately much injured by the plants choking the lenticels of the bark. And in this case, draining of the soil so as to carry off all superfluous moisture, thinning the trees judiciously so as to admit a free current of air, and removing the lichen from the bark, will generally completely restore them. But, if those requisites of health have been too long neglected, it may often be impossible to effect a cure. Trees existing in a high and exposed situation, more especially if the subsoil upon which they are growing is cold and damp, are almost always apt to be injured both by moss and lichen.

In the Centralblatt für das gesammte Forstwesen for 1889, p. 275, a case is detailed in which the woodlands in a damp mountain valley, with a crop of Spruce, Larch, and Scots Pine, suffered to such an extent from being overgrown with eight species of lichens, that about 50 per cent. of the trees were dying prematurely.

4. Diseases occasioned by Frost include not only the damage done by early and late frosts, often resulting in the death of the seedlings and young plants, but also frost-shakes made on the outer portions of poles or trees, and heart-shakes, occasioned by the action of hard winter frost.

The cause of the damage done by early frosts in autumn, and more particularly by late frosts in spring, is the withdrawal of water from the cells into the intercellular spaces, so as to cause loss of turgidity in the tissue, in consequence of which the parts affected die off when a thaw sets in, as they are no longer able to perform their normal functions. Species of plants that break early into leaf in spring are naturally less sensitive to the effects of sudden changes in the atmospheric temperature than Oak, Chestnut, and Robinia, which only flush their young foliage between the middle and the end of May.

The use of strong, sturdy transplants is to be recommended for localities liable to be visited by frosts in spring and autumn; whilst the sylvicultural means at our command for obviating risk and minimising danger are thus summarised by Kauschinger-Fürst (op. cit., p. 10):

"The chief preventive measures which can be applied on a large scale for the protection of young plantations and thickets consist in the formation and reproduction of crops of trees liable to suffer from frost, especially of Beech and Silver Fir, under protective or parent standards, maintenance of fairly dense canopy or shelter overhead, and gradual removal or clearance of the same, avoidance of sudden, usually very injurious, exposure of the young growth thus formed or reproduced, and maintenance of a protective belt on the east and north-east sides till the chief period of danger for the young crop is at an end. Hardy softwoods, like Aspen and Birch, which often assert themselves spontaneously in large numbers among the young growth,

may form a very desirable protection for the more sensitive species, in which case they should only be gradually cleared or weeded out.

“Where natural protective standards are wanting in localities exposed to danger from frost, they can be artificially produced by means of hardy woods of speedy growth, like Pine, Birch, or Alder, which should be planted out in rows somewhat wide apart. When these have attained a height suitable to afford protection, the planting out of the species requiring shelter can take place between the rows; and when the main crop has developed so far as to be above the frost-height, the protective species can then be gradually removed. Patches that are essentially frost-holes should always, when possible, be planted up with hardy species.'

Frost-shakes are caused by excessive and rapid shrinkage of the bark and sapwood during intense frost. Whenever the extent and the rate of shrinkage exceed a certain limit, varying according to the species of tree, a sudden rupture of the woody-fibrous tissue takes place along the line of least resistance, and is usually accompanied by a loud noise. The clefts or frost-shakes thus formed sometimes extend all along the bole. When the thaw occurs, the woody-fibrous tissue again expands so as to nearly close up the wound, which cicatrises by the formation of a ridge of callus tissue. This operation may be repeated almost every winter, or the cleft may remain closed during a succession of mild winters; but the frost-shake remains visible as a long excrescence standing out somewhat from the ordinary contour of the stem. With regard to this abnormal or diseased condition of the trunks of trees Kauschinger-Fürst remark (op. cit., p. 13) that—

"Hardwoods with strongly developed medullary rays, like Oak, Elm, and Sweet Chestnut, are chiefly exposed to the danger of forming frostshakes; they are also, but less frequently, formed in Beech, a few softwoods (Lime, Poplar, Willow), and Conifers. They are especially to be found on old isolated stems, or on standards in copse or in high-forest, and usually occur on the east and north-east sides of the stems, as the harder frosts only set in when the wind comes from these directions, and consequently work most powerfully on these sides. Low-lying tracts and localities with fresh or moist soil are more exposed than others to this particular danger.

"Preventive measures can hardly be applied to mitigate this evil; but the early utilisation of trees with bad frost-shakes is recommendable in view of the liability to which they become exposed to fungoid disease, and consequent depreciation in value."

5. Stag-Headedness.-This symptom is easily distinguished from all others by the top and upper branches of the tree affected becoming dead and quite bare of all leaves and young twigs, and having the appearance of stag's horns. Willows and Poplars, which luxuriate in a soil rather damp than otherwise, generally become stag-horn-topped when grown in a soil too dry for their healthy development. This occurs as soon as they have arrived at such maturity upon the soil upon which they may be growing as its nature is capable of bringing them to. It at once indicates their premature state; and the wisest thing is to cut them down, seeing that the soil is not adapted for their growth. Elm, Oak, Ash, Sycamore, &c., generally become stag-headed, when they are exposed freely to light and air after having stood in close canopy, through the sap being diverted, in its upward flow, towards the forma

tion of shoots from adventitious or dormant buds upon the stem, which are now for the first time called into active life by the sunlight and warmth; hence the upper portion of the crown does not receive its proper share of food, and consequently gradually dies of starvation.

Regarding this local condition of disease, Professor R. Hartig (Lehrbuch der Baumkrankheiten, 1889, p. 14), remarks :

"Oaks which have grown up in close canopy along with Beech, and have only a slightly-developed crown of foliage, acquire a predisposition towards dying off at the top of the crown (stag-headedness) when they become fully exposed to light and air; whilst, under similar conditions, trees with well-developed crowns do not suffer from this disease."

This atrophy of the upper portion of the crown may be obviated by gradually accustoming standard trees to increased growing-space and larger supplies of light and air.

And, as the editor has elsewhere explained with reference to partial clearances in valuable crops approaching maturity (Studies in Forestry, 1894, chap. x.)::

"The nature of the development of the crown is, however, of greater importance than the age of the crop; for if the previous thinnings have been neglected, and the canopy has been allowed to remain so dense that the woods are crowded, it often happens that, on receiving increased growingspace, the trees with their weakly crowns are unable to avail themselves of the advantages thereby offered, and exhibit, as is often the case with Oaks, a tendency to development of dormant buds, which leads to stag-headedness, or death of the crown and malformation of the bole."

6. Premature Seeding is not in any way a disease, but is merely the symptom of an abnormal condition, which finds its natural expression in an abnormal tendency towards reproduction of the species. Trees in a healthy rapid-growing state are seldom found to produce seed till they have arrived at a considerable age and size. Generally speaking, any forest-tree bearing much seed under forty years of age is not likely, ultimately, to arrive at a valuable size. Any tree, however healthy a state it may be in, may be made to bear seed by creating an unnatural disturbance within the organism, as, for example, by mutilating its roots or its stem; but where a young tree produces seed largely, at any abnormally premature age, without any visible outward injuries having been inflicted, it can hardly be in a healthy condition. The premature bearing of much seed in the Larch, for example, is almost a sure index of heart-rot in the trees; and even in the cases of Pines of any kind, the bearing of much seed in their young state at once points to weakness of constitution, and indicates that the trees are not likely to become valuable as timber, but that they will culminate in increment prematurely at a comparatively early stage. Many of the newer coniferæ are found to bear seed at a very early stage, and still have the appearance of continuing to make fair progress; but this early bearing of seed is probably a sign that they have not yet thoroughly acclimatised themselves.