TIMBER. The terms "wood" and "timber" are not synonyms, for wood occurs in the veins of leaves and as strings in thick palm stems, also in thicker cylindrical form in twigs of shrubs and trees. The term timber, synonymous with the North American term lumber, applies solely to wood of considerable dimensions pro duced by trees. (See LUMBERING.) Two great classes of trees provide timber. (I) Conifers, includ ing pines, spruces and larches, usually have more or less needle like leaves, and their naked seeds are most frequently borne on cones : they yield the so-called softwoods. (2) Dicotyledons, in cluding oak, ash, beech and teak, commonly have broader flat leaves and their seeds are always produced in closed seed-cases: they provide the hardwoods.
The trunk and branches grow in length solely at their tips, so that the oldest part of a trunk or branch is its base and the youngest is its slender tip. But after a portion of a trunk or branch has ceased to grow in length it can grow in thickness year after year and especially by producing new wood. This takes its origin from a microscopically thin sheet, the cambium, lying between the bark and the wood : the cambium deposits new wood outside the pre-existing wood. In the cool-temperate climate the cambium produces new wood only during spring and part of the summer and is inactive during autumn and winter : the wood produced first in each year is frequently (e.g., ash, Scots pine) different from that produced later in the year, so that a distinction is drawn between the early spring-wood and the later summer wood (frequently termed autumn-wood). In such cases a cross section of the tree-trunk will show a number of concentric annual rings, whose number is equal to the age of the region of trunk cut. In the case of the beech and a number of other trees, the annual rings show no marked differentiation into spring-wood and summer-wood, but are recognisable because a very thin layer produced latest in the growing season differs in appearance from the rest of the wood. Finally many kinds of evergreen tropical
trees (e.g., mahogany) show no distinct annual rings.
In certain kinds of trees, for instance species of pines and leaf shedding oaks, after the wood has attained a certain age it darkens in colour, so that when a cross-cut of a mo-year old part of the trunk is taken the darker older central wood contrasts as heart wood with the surrounding pale sap-wood.
In addition to the fibres, certain softwoods, including those of true pines, have very thin resin-containing tubes, the resin ducts, which run parallel with the fibres. In structure each duct in microscopic miniature recalls a tall factory chimney, as its central tubular hollow is surrounded by microscopic, short, more or less brick-shaped cells, which are hollow and have thin walls. All commercial softwoods (excluding that of the yew) contain resin but some of them (e.g., Californian redwood) have in place of resin-ducts merely resin-containing short cells.
Traversing the wood at right angles to the fibres are thin string-like or ribbon-like structures that run from the outside of the wood radially inwards towards or actually to the pith. These are the medullary rays, which are usually nearly or quite invisible to the naked eye in cross-sections of softwoods. They consist of more or less brick-shaped cells, which in the sap-wood contain albuminous substances and at times such other nutritive sub stances as sugar, starch or fatty oil: in the sap-wood they conse quently invite attack by animals and fungi. Softwoods that, like true pines, have resin-ducts in the wood also possess these in the thicker medullary rays.