Every tree has a complete circulatory system in which water and dissolved mineral salts, collected by the roots and commonly known as 'sap', are drawn upwards in the growing season through innumerable pipes or hollow vessels by the rapid evaporation from the surfaces of the leaves. In turn, the action of sunlight on the chlorophyll or green colouring matter in the leaves converts the rising sap. through a complex process of photosynthesis, into the essential food sugars and starches which then travel down the bast or inner skin underlying the bark, and are conveyed into the heart of the tree at every point by an elaborate system of radial medullary rays, clearly visible in the so-called 'flash' or flower of quartered oak. There is also a respiratory system whereby the leaves inspire oxygen and carbon dioxide during the day, which is again diffused throughout the living structure of the tree, while the waste products of combustion, chiefly carbon dioxide. are transpired at night. All this constant passage of essential water, food and oxygen throughout every part of the tree necessitates a complex honeycomb structure of pipes or vessels, soft pithy cells and structural wood fibre, which, even when fully dried, retains sufficient elasticity to behave as a typical sponge, capable of absorbing or giving up moisture in a ceaseless effort to maintain itself in a constant state of equilibrium with its surroundings.
The actual structure of the tree is illustrated in Figure 2 and the sectional drawings show the details. The function of the bark is to protect the tree from injury and shield the precious bast layer, which is the sole conveyor of food. Immediately under the bast is the cambium or growth layer, one cell thick and therefore only visible as a coat of slime. This layer creates new bast on one side and sapwood on the other, while the sapwood in turn ages and creates fresh heartwood. The function of the sapwood is to convey the water to the leaves, while the heartwood acts as a stiffening rib to the tree. At the exact centre of the annual rings, but not necessarily at the centre of the tree, lies the pith which is merely the decayed remnants of the original twig.
The growth in the cambium layer is very rapid during the spring when the constant evaporation from the leaf surfaces and the continual unfolding of fresh leaves exercise the strongest pull on the rising sap, therefore ensuring an abundant supply of food. It is not so rapid in summer, when the first impetus has been lost, and ceases altogether in winter when the tree remains dormant. Both rapid spring and slower summer growth combine together to create one annual ring whose number determines the exact age of the tree, while the heart-wood itself is sapwood which, deprived of essential food and oxygen, has been literally choked to death by the ever-encircling rings of sapwood. Once the tree has started to make heartwood the amount of sapwood remains constant throughout the life of the tree, and for each new growth ring added, one matures and dies. Before it dies, however, all food is withdrawn from its cells, the fibres harden forming stiffer, denser heartwood, while the typical darker coloration evident in most—but not all—woods is the result of subtle chemical changes in the nature of the wood. The point to be stressed is that these chemical changes must take place within the living tree; no amount of after-treatment will darken sapwood, harden its fibres or drive out the sugars and starches locked in its cells which form the essential foodstuffs of all destructive wood-boring pests. This is the only reason why sapwood might not be recommended for use in furniture, although apart from that there is little, if any, difference in the structural strength. Sapwood will.
however, always be wetter, for the pores of heartwood are blocked with gum-like bladders (tyiosis) which prevent the free movement of water, and therefore they contain more air and froth.
Figure 1 shows a slice cut from a laburnum tree (laburnum oyster) illustrating very clearly the sharp division between heartwood and sapwood which need not necessarily follow the annual rings. The spacing of the rings, the alternation between rapid and slow growth in succeeding years, the greater freedom of growth on one side due possibly to climatic conditions, prevalent winds, etc., or overshadowing or poor soil on the narrow side, is clearly visible, as are the medullary rays radiating inward from every point on the bast to the centre pith. These rays are always the weakest link as the split plainly shows, for they deflect the longitudinal fibres and thus form the natural cleavage-lines along which most timbers will readily split.
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