Experience has proved the accuracy of his theory, which he has carried out in practice for some ten years past in America, where vulcanised wood has come extensively into use. The process consists simply in placing the timber, by small waggon-loads, in an air-tight retort and subjecting it to the high pressure of superheated air, the temperature of which varies from 300° to 700°. The time of exposure to the heat varies with the character of the timber, ordinary wood requiring about eight hours, and the hardwoods used for decorative and similar purposes requiring from ten to twenty hours to vulcanise. The heat is found to permeate the whole section of all the timber in the retort, thereby accomplishing the same result as the charring process, only in a much more thorough and efficient manner, without losing any of the sap or chemical properties of the wood and without waste. The process, however, goes further than this, inasmuch as the high temperature kills all the germs that may be found in the albuminoid portions of the timber which tend to breed and nourish fungi. In short, the effect of the process is to solidify the albuminoid matter throughout the tree, and at the same time to destroy bacteria or fungi, thus preserving the wood against decay. Another important result is attained through the wood becoming harder and stronger. Professor Alfred P. Trantwein, of Steven's Institute of Technology, Hoboken, New Jersey, who has extensively tested vulcanised wood in this connection, reports that his experiments establish the fact that vulcanising strengthens the wood, the modulus of rupture by transverse stress being 21 per cent. higher, and the crushing strength being increased 23 per cent.

An examination of samples of wood treated by Haskin's process seems to bear out all that is claimed for it, and the advantages are confirmed by the reports of Professor Chandler, Professor Theurston, and other wellknown American experts and practical men. For example, a portion of a Pitch Pine sleeper, which has been under traffic on the Erie Railway for nine years, is apparently quite hard and sound. According to Mr Hain, the manager of the New York elevated railways, vulcanised Yellow Pine timbers have been in use on that line for the past six years, and do not show any signs of rot, whereas untreated timbers have decayed. It is stated that the cost of treating wood by the new process is far less than that of the creosoting and other processes, and that, so far as present experience goes, vulcanised wood is proof against the white ant and the teredo.

So far, however, as the true province of Sylviculture is concerned, the best means that the forester has at his command, in order to improve the technical quality of the raw produce he brings to the market, consist in1. The selection of suitable species of trees to form the crops under the given conditions of soil and situation;

2. The formation of woods of proper density for the species of trees selected, and the maintenance of normal density throughout all stages of the development of the crop;

3. The proper conduct of the operations of tending (weeding, cleaning, thinning);

4. And in rational treatment of the crop when it approaches maturity, either by thinning, or, preferably, by the method of partial clearance with simultaneous under-planting in the case of the light-demanding species.

CHAPTER XII.

THE PATHOLOGY OF FOREST-TREES.

I. The Healthy and Normal Growth of Trees.-A tree is an organised, living body, constructed mainly of cells and vessels, the vitality and growth of which are primarily maintained by means of the absorption of mineral food through its root-system and of the assimilation of carbonic acid in the foliage when exposed to light and warmth. The body of a tree is formed out of a few earthy substances, together with water and air. It is fixed to the earth by its roots, and rises into the air in the form of a stem, bearing leaves, flowers, and fruit; but it has no power of locomotion.

As has elsewhere been remarked by the editor (Soil and Situation in Relation to Forest Growth, 1893, published by the Government of India for official circulation, p. 2):-

"In agriculture the productive capacity of land is to a great extent commensurate with the inorganic richness of the soil; but, as tree-growth utilises comparatively very much smaller quantities of mineral constituents (especially of those less common) than are demanded by agricultural produce, the sylvicultural capability of any soil is not determinable by the quantity of inorganic matter available for food-supplies, except perhaps in the case of the very poorest classes of forest-land.

"The actual quantities of mineral substance annually utilised per acre, as found in the ash of agricultural and forest produce, have been shown 1 to be as follows:

1 Ebermayer's Physiologische Chemie der Pflanzen, 1882, vol. i. p. 761.

"Weber 1 gives the average analysis of the dry timber of different species of European forest-trees as :

"As the total demand on mineral food-supplies made by woodland growth barely exceeds half of that made by agricultural produce, and as through the fall and decomposition of the leaves by far the greater portion of it is returned to the soil, it will be at once apparent that forests of normal density do not exhaust the soil to anything like the same extent as agricultural

crops.

"According to Professor R. Hartig of Munich: 2

"The quantity of mineral food taken up from the soil depends chiefly on the species, for some plants extract greater supplies than others. Thus the Spruce, for example, when growing along with the Beech in forests forming close canopy, extracts more raw mineral substance for ash than the latter. At fifty years of age the relative proportion of production between the Spruce and the Beech is—

These figures prove that Spruce not only withdraws more nourishment from the soil than Beech, but that it also produces more timber than the Beech from any given quantity of mineral nutriment. For the production of any given cubic quantity of Beech timber more mineral matter is requisite than for the production of the same volume of Spruce.'"

A young tree is produced by seed derived from its parent tree. The seed of a tree, when acted upon by certain stimulating agencies, begins to exhibit life by germinating; these agencies are water, heat, light, air, and nutriment. The seed of a tree, if placed in earth at a temperature not exceeding 32° Fahrenheit, will remain inactive till it decays; but if it be placed in earth moistened with water, and having a temperature of some degrees above 32°, it gradually absorbs part of the moisture that surrounds it and swells. Its tissue thus becomes softened and stretches; the water is decomposed, and a part of its oxygen, combining with the carbon of the seed, forms carbonic acid, which is thrown off. Food for the young plant is prepared through the reserves of starch that are stored up within it being converted into dextrin or grape-sugar, by means of which the vital action of the embryo begins on the requisite temperature being attained.

The embryo lengthens downwards into the earth by the radicle or young root, and upwards into the air by the cotyledons. The latter acquire a green colour through the presence of a slight quantity of 1 Aufgaben der Forstwirthschaft (Lorey's Handbuch der Forstwissenschaft), 1886, p. 72. 2 Anatomie und Physiologie der Pflanzen, 1891, p. 227.

iron in their chlorophyll, during the process of decomposition of the carbonic acid which they absorb from the earth and the air.

A certain degree of moisture in the soil is a necessary condition for maintaining the vitality and the healthy development of trees at all stages of their existence, as their various organs are made up of cells, tubes, and vessels, requiring a free circulation of the sap throughout their structure.

The germination of the seed having taken place by the agencies of water, heat, light, and air, the young root lengthens at its point, and absorbs nourishment from the earth. This nutrient being conveyed to the summit of the plant by means of its cellular substance, it is partly forced into the seed-leaves, now expanding where it is exposed to the action of light and air. Here moisture is transpired; whilst at the same time nourishment is prepared for the maintenance and for the development of the organism by the processes of assimilation of atmospheric carbon, and the formation of carbo-hydrates to be utilised. for structural and physiological purposes.

The current of sap rises continuously, by means of capillary attraction chiefly, upwards from the roots to the growing-point, which lengthens and forms a small twig; and at the same time the first portion of woody matter is formed from the descending sap in the centre of the radicle. The growing point, as it receives the sap, aërates it and sends it downwards. The root is thus enabled to form ramifications, while at the same time the gradual formation of woody matter takes place within the vessels of the young plant by the deposition of assimilated nourishment.

Before the growing-point begins to extend, it acquires the rudimentary state of a leaf; and this continues to develop as the growing point lengthens, until, when the first space between two leaves on the little twig ceases to increase, the first leaf has really attained its complete formation.

When the first leaf is fully developed, it carries on in a much more perfect mannner the functions previously performed by the seed-leaves; for assimilation becomes more vigorous, and the elaborated nutriment is passed in larger quantities downwards through the cambial layer of the bark to the root. Here it forms the first stratum of woody matter or cellulose, one portion of which is incorporated with the bark, whilst the remainder begins the formation of sapwood.

While this initial process of development is taking place, the cellular tissue of the stem is forming and expanding horizontally, in order to make room for the matter passing into it; hence development is at the same time progressing both in a horizontal and in a perpendicular direction.

Simultaneously with the primary development of woody-fibrous

tissue there also goes on the processes of formation of young roots, and their increase and ramification by constant, gradual increment of the cellular substance of their points. They thus assume something like a definite proportion to the leaves they are to support, and upon whose assimilative capacity their increment directly depends.

After the first leaf is produced by the growing point, others successively appear around it; all are formed alike, are connected with the stem in the same manner as the first, and perform in all respects similar functions. At last the growing-point or axis ceases to lengthen for the season, and the old leaves gradually fall off. Meanwhile a new set of leaves, instead of expanding after their formation, remain in their rudimentary state, harden, and fold over one another, so as to be a protection to the delicate point of growth,— or, in other words, become the bracts or scales of a leaf-bud. The ascending axis now consists of a shoot with a woody axis, and a distinct pith and bark, the whole assuming a more or less conical shape.

With the spring of the second year, and the return of warm weather, active vegetation recommences. The uppermost buds, and any others which were formed during the previous year, gradually unfold when the flow of sap is induced by the rise of temperature in spring; the place of the water transpired and the sap utilised is instantly filled by continuous supplies from below; circulation of sap is effected by an impulse being given to the fluids from the summit to the roots; fresh extension is given to the roots by new fibrils being thrown out; fresh food-supplies are absorbed from the earth, and sent upwards through the soft sapwood formed during the last year; and the phenomenon called the flow of the sap is fully accomplished, which continues with greater or less velocity till the return of winter. The growing-point lengthens upwards, forming leaves and buds in the same way as the parent shoot; a horizontal increase of the whole of the cellular system of the stem takes place; and each bud sends assimilated nourishment down through the cambium between the bark and the wood of the shoot from which it sprung, thus forming, on the one hand, a new layer of sapwood or alburnum, and on the other, a fresh deposit of bark or liber.

The food-supply, which is drawn from the earth and brought into circulation as sap by the unfolding leaves, are exposed, as in the previous year, to the chemical action of air and light, in connection with the processes of assimilation of atmospheric carbon and the preparation of carbo-hydrates; it is then returned through the leaf-stalk to the stem, and is sent downwards through the cambial layer to be from it either conveyed to the root, or distributed horizontally by diffusion throughout the cell-walls, or through the medullary rays connecting the periphery with the centre of the stem. At the end of the