The Living Tree

The growth of a tree is always continuous although it becomes slower in the course of time. Giant sequoias (Sequoiadendron giganteum) in Califor- 

Longitudinal growth (primary growth), which takes place in the early season, proceeds at the end of the stem, branches and roots. The growth points are located inside the buds, which have been formed during the preceding autumn. 

Radial growth begins in the cambium which is composed of a single layer of thin-walled living cells (initials) filled with protoplasm (cf. Fig. 1-2). The cambial zone consists of several rows of cells, which all possess the ability to divide. On division the initial cell produces a new initial and a xylem mother cell, which in its turn gives rise to two daughter cells each of the latter is capable of further division. More cells are produced toward the xylem on the inside than toward the phloem on the outside phloem cells divide less frequently than xylem cells. For these reasons, trees always contain much more wood than bark. 

At the beginning of the growth the tree requires an effective water transportation system. In softwoods thin-walled cells with large cavities are formed in hardwoods special vessels take care of the liquid transportation. Comparatively light-colored and porous earlywood is thus formed. Later, the rate of growth decreases and latewood is produced. It consists of thick- 

INNER LAYER MIDDLE LAYER OUTER LAYER PRIMARY WALL 

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Drying. The living tree holds much water in its cells. A southern pine log, 5 m long and 0.5 m in diameter, for example, may weigh as much as 1000 kg and contain 47% or 0.46 m (16 fE) water. 

Many terpenes are derived from renewable plant oil resources like essential oils. a-Pinene and -pinene from turpentine may be the best known examples, because they represent a very large volume. Turpentine was originally obtained from pine trees by tapping gum oleoresin from the stem of the living trees followed by steam distillation of the crude oleoresin and subsequently separation into rosin and turpentine by distillation. The ratio of a-pinene to -pinene in this turpentine varies considerably and depends a lot on the pine species from... 

The burning of charcoal by the method still much used of submitting wood to partial combustion in earth-covered mounds is mentioned. The recovery of pitch from resinous trees was either by making incisions in the living tree and collecting the pitch which accumulated, or by a process somewhat similar to the charcoal burning, a process interestingly described by Theophrastus as follows ... 

A piece of wood from the doorway of a Mayan temple is subjected to radioactive carbon dating techniques and found to have 12 7 cpm per gram of carbon The living trees from which the wood came had 15 3 cpm per gram of carbon Flow old must this temple be, knowing that the half-life of t4C is yr9... 

After the living tree is harvested to provide wood for a painting surface, or perhaps for a painting support or frame, CO2 uptake ceases and the ratio of J C to begins to decrease. A piece of wood from an art object can be analyzed for carbon-14 content. If it is... 

Roots should be collected only after the herb itself has fallen back for the season, no matter whether the plant is annual, biennial or perennial. Barks should never be taken from the living tree, but only in the spring time, from a tree that has been cut down the previous autumn. If naturally and carefully dried, the therapeutic virtues will remain for long periods. 

Other Polysaccharides In addition to xylan and glucomannan minor amounts of miscellaneous polysaccharides are present in hardwoods, partly of the same type as those occurring in softwoods. They might be important components for the living tree, although of little interest when considering the technical applications. 

Wood differs from most materials used for construction and other purposes in that it is continually exchanging moisture with its surroundings. This is true in both the living tree as well as under conditions of final use. 

Water in the Living Tree. Wood in the living tree is formed and functions in an essentially water-saturated environment. The functioning sapwood cells are a part of the vascular system that conducts water and solutes from the roots to the leaves through a continuous water-saturated network of wood cells (2). When the tree is felled the water in the wood is cut off from the soil water and the wood commences to lose most of its moisture. 

Even wood that has no discernible defects has extremely variable properties as a result of its heterogeneous composition and natural growth patterns. Wood is an anisotropic material in that the mechanical properties vary with respect to the three mutually perpendicular axes of the material (radial, tangential, and longitudinal). These natural characteristics are compounded further by the environmental influences encountered during the growth of the living tree. Yet wood is a viable construction material because workable estimates of the mechanical properties have been developed. 

The role of the acetate groups in the cell wall of the living tree is not clear. It has been suggested that they serve the function of decreasing the hydration capacity of the xylan, which could, otherwise, conceivably lower the tensile strength of the woody tissue. This appears doubtful, in view of... 

The actual state of the xylans and glucomannans in the living tree is still largely unknown. Since they do not crystallize readily without some chemical modification, such as partial depolymerization or removal of side chains, or both, it would seem likely that they occur in the amorphous state in the wood. The possibility that they could form microfibrils, as does mannan B in vegetable ivory, cannot be entirely excluded.More probably, however, they occur as a powder between and around the cellulose microfibrils. Using a polarized infrared technique, Marchessault and coworkers have obtained indications that, not only the cellulose, but also the xylans and glucomannans, may be oriented in the direction of the fiber axis in wood. 

All these lipophilic components perform a function for the living tree. The exact function is not understood for all substances. The composition of the lipophilic wood extractives varies from species to species, and is far from homogeneously distributed in the tree. All woods contain sterols, waxes, glycerides and other aliphatic extractives. However, softwood contains considerable quantities of rosin acids (e.g. abietic acid), but these acids do not exist in hardwood (e.g. birch and aspen). Certain components are predominantly located in the bark of the tree. The sterol fraction from birch and aspen bark comprises almost exclusively beta-sitosterol by way of example. 

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