from a technical point of view lies in the fact that, along with the water contained, they form the main cause of decomposition in wood, because it is from them that saprophytic fungi obtain some of their essentially necessary food-supplies.

2. Carbohydrates and Glycosides. To this class starch, glucose, sugar, and gums belong. The glycosides include all the substances capable of being transformed by chemical agencies into sugar or any other combination belonging to an aromatic or fatty class. The glycosides are chiefly contained in the bark, although some are also to be found in the wood-sap, as, for instance, the coniferin (C16H2O) present in the cambial-sap of all Conifers. Starch and sugar are chiefly found in the younger parts of the tree, where they are stored as reserve nutrients. As starch and sugar form the food of insects, wood containing any large proportion of these is specially liable to be attacked.

3. Resins, Oils, Oleo-resins, and Aromatic substances.-Every kind of wood has its own peculiar aroma, both when green and after seasoning. The most important of this class of substances in European trees is the resin or turpentine contained in Conifers. It is found in the resin-ducts occurring among the tracheids and passing along the medullary rays, the largest number of longitudinal resin-ducts being formed in or near the summer zone of each annual ring. Resin may impregnate the whole of the wood, and it is of importance as regards the technical qualities of coniferous timber (see also Chap. V. p. 608). 4. Tannic, Oxalic, and other Plant Acids. Tannin or tannic acid occurs in almost every kind of wood, but is chiefly to be found in the bark (e.g., Oak). Among European trees Sweet-Chestnut contains most tannin in its wood. Tannic acid corrodes iron imbedded in or brought into contact with it. Thus Oak is not used for the construction of steamships built of iron and steel, but has given place to the Teakwood (Tectona grandis) of India, Burma, and Siam, the æthereal oil contained in which is preservative of metal bolts, &c.

5. Dye-stuffs.—These give to each kind of wood, and often to the different parts of the wood (sapwood, heartwood), its own peculiar colour. Wood darkens with age by a gradual process of slight oxidisation, while its colour may be considerably deepened by steaming it.

6. Ash or Mineral substances are contained partly in the sap and partly in the wood, especially in the young wood, and usually average from 0-2 to 0.6 per cent of the weight of barked, seasoned stem-timber. The specific composition of the ash varies according to the soil on which the tree has been grown; but the chief constituents are always potash, lime, magnesia, and phosphoric acid, while the proportion of mineral acids is small. The metallic oxides are mostly found in the form of organic acid salts, which furnish carbonates on the wood being burned. Hence carbonic acid is always very conspicuous in wood-ashes, although it really forms no component part of the wood itself. As wood-ashes consist mostly of carbonates, potashes can easily be prepared from wood, and till about forty years ago this was the only mode of producing them.

The sapwood and the youngest parts of trees are paler in colour, more watery, and contain more substances held in solution, and especially more nitrogenous substances, than the older portions, and they are therefore more easily decomposed. As the sapwood grows older it usually undergoes a process of variation differing with each kind of tree, although in many softwoods and alsoŝin some other trees there is no very definite division or line of demarcation between sapwood and heartwood. Thus a classification may be made into (1) Sapwood-trees, in which the alburnum or sapwood undergoes but little or no real or visible change, such as Aspen, Birch, Lime, Alder, Maple, Sycamore, Plane, and Hornbeam; (2) Heartwood-trees, in which duramen or heartwood is produced-containing much less water and becoming of a distinctly different colour from the younger portions of the stem, as in Oak, Elm, Chestnut, Robinia, Cherry, Walnut, Apple, Larch, Pine, Thuja, Juniper, and Yew; and (3) trees with imperfect (duramen, in which a definite transformation from sapwood to heartwood really takes place as regards the amount of water contained, though without this being accompanied by any very marked and distinctive deepening in the colour of the older wood, such as in Ash, Beech, Willow, Poplar, Horse-Chestnut, Pear, Spruce, and Silver Fir.

Perfectly formed duramen or heartwood is wood that has ceased to perform

vital functions; there is no ascent of sap throughout the heartwood. It contains much less water, protein, and starch than the sapwood, and the walls and lumina of the woody substance are impregnated and filled with colouring matters, resins, oleo-resins, gums, or mineral matters, usually, but not always, rendering the timber heavier, harder, and more durable than the sapwood. Consequently, the greater the proportion of heartwood to sapwood in any tree, the more durable and valuable is usually the wood it contains.

If an Oak, Larch, or Pine be "girdled" (as is done to season Teak in Burma, and to render it floatable) by cutting a ring all round the stem, right through the sapwood into the darker heartwood, the ascent of the sap ceases, the leaves will soon wither, and the tree will die. But if a Birch or Aspen be ringed by cutting a girdle all round the stem, the trees will not necessarily be killed, because the sap can still ascend to the crown through the older, non-indurated sapwood, and the girdled zone will in course of time cicatrise and become overgrown. The broader, however, and the deeper such a girdle be made, the greater is the physiological disturbance in the sapwood-trees, and the more likely they are to die from its effects, especially if the operation be repeated one or two years later.

The general proportion which exists between the quantity of timber, branchwood, and brushwood in woodland crops varies according to (1) the species of tree; (2) the density of the crop; (3) the age of the crop ; and (4) the nature of the soil and situation.

Among the broad-leaved trees the most regular stems are formed by Alder, Oak (especially Sessile Oak), and Ash, then by Poplar, Birch, and Aspen ; but the Conifers generally, and more particularly the shade-bearing kinds (Douglas Firs, Spruces, Silver Firs), surpass even the light-demanding broad-leaved trees in their formation of a good bole.

As a general rule, the greater the density of crop, the larger will be the proportion of timber, and the smaller the relative proportion of branchwood and brushwood. Hence highwood crops produce a larger proportion of long logs than is obtainable from standards over coppice.

In young woods the proportion of timber is of course small; but as the crops approach maturity the percentage of branchwood and brushwood decreases in wellmanaged woods, so that, in general, in most tree-crops the percentage of timber contained in the stem amounts to between 80 and 90 per cent of the total quantity of wood. And as regards the nature of the soil and situation, the percentage of timber contained in the stems of the trees forming the crop is more or less directly proportional to the quality of the land.

With regard to the straightness of stem, the various kinds of timber-crops differ greatly. The straightest boles are formed by Larch, Douglas Fir, Spruce, and Silver Fir, then by Scots and Weymouth Pines, Poplar, Alder, and Sessile Oak. But the density of the crop exerts a decided influence in this respect with most kinds of trees. Oak, Ash, Scots Pine, Beech, Maple and Sycamore, and Hornbeam all gain in straightness by being grown as close together as does not interfere with their normal activity in assimilation and in producing woody-fibrous tissue; while at the same time the freedom of the bole from branches and knots directly depends on the density of canopy maintained in any given crop. The importance of these two factors can hardly be over-estimated with regard to the technical value of timber for general purposes, which usually determines its market-value. The relative proportion of the top-end of the bole as compared with the butt-end is another factor determining the selling-price of timber; hence any sylvicultural measures are advantageous which will raise the form-factor of the stems forming the crop. It is also often profitable to stimulate the increment in timber-crops approaching maturity by partial clearance when once they have completed their main growth in height, after being previously grown in close canopy.

The percentage of the different classes of wood yielded at an advanced age by the different kinds of highwood crops of normal density, and growing on favourable soil and situation, has been tabulated by Gayer as follows:

The percentage in copse standards is, according to Lauprecht :

As already stated, the technical properties dependent on the anatomical structure and chemical composition of different kinds of wood may be classified with regard to (1) Ornamental Qualities, (2) Physical Properties, and (3) Mechanical Properties. These three groups include the following different properties:

I. Ornamental Qualities or Outward Appearance.-1. Colour; 2. Lustre; 3. Grain; 4. Texture and Marking; 5. Odour.

II. Physical Properties or Material Condition.-1. Density and Weight; 2. Percentage of Water contained: Moistness and Seasoning; 3. Relation towards Water: Shrinkage, Cracking, Warping, and Expansion; 4. Defects and Unsoundness.

III. Mechanical Properties or Relation towards External Influences.1. Strength; 2. Elasticity, Flexibility and Toughness; 3. Fissibility 4. Hardness; 5. Heating-Power; 6. Durability. Nos. 1 to 4 concern changes in form which do not destroy the cohesiveness of the woody substance, whereas Nos. 5 and 6 concern changes in form which destroy the cohesiveness of the wood.

I. Ornamental Qualities or Outward Appearance include all those that can be tested by sight, touch, or smell. They are natural properties which may vary with the soil and the rate of growth, but which are only partially subject to the control of the forester; and they chiefly determine the marketvalue of the wood for decorative purposes, such as making art-furniture, panelling, parquetry, &c.

1. Colour varies in wood not only in its primary composition, but also in tone or depth. When much colouring matter is contained it may, besides giving a rich look to the wood, be extracted as a dye-stuff (as in the case of the Sumach, Rhus

cotinus, and the root-wood of barberry among European trees and shrubs, and as in many sub-tropical and tropical trees). Most of the richly coloured decorative woods (Mahogany, Ebony, &c.) are imported from hot climates. There is a distinct fashion in woods as regards colour-e.g., in the preference given to Alder for clogs, and to Willows and Maples for bread-platters and kitchen utensils. Apart from variations in normal colour, due to soil and rate of growth, in some cases the heartwood of old trees assumes a specially dark colour, giving additional value to the wood for furniture and other decorative purposes. Such is the case with the "brown Oak," obtained chiefly from the English midland counties, the deep rich colour of which is probably only a sign of abnormally perfected heartwood, although it may perhaps be due either to the deposition of soluble matter decomposed in younger parts of the tree, or to incipient decomposition in the heartwood itself. Similarly, some of the Larch grown on the Barnard Castle estate (King's County, Ireland) is almost liver-coloured when green, and exceedingly hard.

Green freshly-felled wood is generally lighter in colour than seasoned wood, but the difference in colour between the sapwood and the heartwood is usually increased by seasoning. Bright-coloured woods often turn greyish after long contact with the air. Spruce retains its colour better than Silver Fir, and is therefore preferred for interior work. Alder quickly changes by oxidisation from fleshy-red to yellow-red, and especially in the younger parts containing most water. The wood of trees grown on suitable soil and with a large individual growing-space is, both when green and seasoned, more richly coloured than that of trees grown in close canopy or on wet land. Uniformity of colour is a good sign, and the richer the colour of the heartwood, the better the timber usually is for general purposes. The usual colour of healthy wood is about as follows:

Note. The natural colour of wood is always darkened by steaming, and can of course easily be changed by staining with acids, washing with lime, &c. Beech turns reddish to red-brown on being steamed. Unpolished, unvarnished Oak furniture, wainscotting, stair-steps, balustrading, &c., can be deepened in colour and made to assume an ancient look by being occasionally rubbed with linseed oil. Nearly every conservative process darkens the colour of wood.

2. Lustre. Like other smooth surfaces, wood, when planed, is more or less lustrous. This power of reflecting light varies in different kinds of trees, and can be greatly increased by polishing and varnishing; but the differences in lustre are noticeable even on ordinary split surfaces without any artificial treatment. When wood is split radially, so as to expose the medullary rays lengthways and to their full height, many kinds of wood show a considerable degree of lustre, as in the "silver grain" of Oak, and in Maple and Sycamore, although in Aspen and some other Poplars the medullary rays somewhat diminish the lustre. Like 2 E

VOL. II.

colour and texture, lustre is one of the ornamental qualities determining the value of wood for decorative use.

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3. Grain. The fineness or coarseness of the grain in wood is of importance as regards the method of working up the wood and the future value of the finished product. Woods which show few or no visible signs of their structure are termed 'fine-grained," while others are more or less "coarse-grained." In fine-grained woods it is difficult to distinguish the annual rings on the transverse section, or the spring and summer zones of each ring on the radial section, because there is not much variation in the size of the different woody fibres and cells in different parts, in different annual rings, and in the old and the young wood. Even when the fibres and cells are rather large, the wood may sometimes be even-grained (e.g. Lime), though most fine-grained woods have small fibres and cells. In finegrained wood the woody tissue is so close together that the medullary rays are not visible on the transverse section; the cells of these rays are about the same size as those of the wood-parenchym, and the smaller rays are closely interwoven with the woody tissue. In coarse-grained wood the opposite is the case; the woodvessels can be easily recognised, the wood is marked by a clear grouping of the elementary tissues, by the differences and marked distinctions between the annual rings, and between the spring and summer zones in each, and by the breadth and height of the medullary rays. Oak, Elm, and Corsican and Austrian Pines are typical coarse-grained woods, difficult to plane or turn, while Box, Apple, and Pear have a much finer grain than any of our true woodland trees. Most coarse-grained woods can, however, take a high polish, as the material used settles easily in the pores.

4. Texture and Marking.—Texture is really synonymous with the anatomical structure characterising each kind of wood, although the term is commonly applied to the "marking" or outward appearance of the converted surface of wood. The coarser the grain of the wood, the more easily distinguishable is its texture, and the more striking its marking. The marking of the wood varies according as it is seen on a transverse, a radial, or a tangential section; the first only showing the annual rings, while the other two exhibit the more or less parallel layers of wood. The greatest variety of marking is of course produced in the radial section, the "flowering" of which, along with its colour, determines the value of wood for furniture-making and interior decoration. Wherever a stem forks, or a branch is thrown out, the annual rings are no longer formed in more or less parallel-straight lines, and the "flowering" thus obtainable in the wood gives the timber a special value for furniture-making, &c. Anything, in fact (e.g., branches, wounds), which causes deviation from the ordinary parallel position of the annual rings, such as is desired in clean straight timber for other technical purposes, or which leads to abnormal distribution of colouring matters, resins, &c., tends to improve the marking of wood for ornamental purposes; and this is especially the case with the "burrs" produced by masses of adventitious, dormant buds, usually near the base, on Poplar, Elm, Oak, Ash, Alder, Birch, Walnut, Maple, and Lime, although these excrescences can only be considered defects as regards the use of such timber for constructive purposes. The larger the burr, and the more fantastic the markings to which it gives rise, the more valuable are such pieces for veneering and furniture-making.

5. Odour. Every kind of wood emits its own characteristic odour when green, and many of them retain it strongly after seasoning. This is particularly the case with Conifers, and they may sometimes be so resinous as to be unsuitable for interior work. Most of the strongly-scented woods are produced only in hot climates (e.g., Deodar, Cinnamon, Sandalwood, Camphorwood, &c.), and even in the south of Europe the odour of those of our woodland trees which also thrive there is stronger than in this climate. Oakwood smelling strongly like tanningbark is said to be always of superior quality.