The ornamental figure or grain of many of the woods, appears to depend as much or more upon the particular directions and mixings of the fibres, as upon their differences of colour. I will first consider the effect of the fibre, assisted only by the slight variation of tint, observable between the inner and outer surfaces of the annual layers, and the lighter or more silky character of the medullary rays.
If the tree consisted of a series of truly cylindrical rings, like the tubes of a telescope, the horizontal section would exhibit circles; the vertical, parallel straight lines; and the oblique section would present parts of ovals; but nature rarely works with such formality, and but few trees are either exactly circular or straight, and therefore although the three natural sections have a general disposition to the figures described, every little bend and twist in the tree disturbs the regularity of the fibres, and adds to the variety and ornament of the wood.
The horizontal section, or that parallel with the earth, only displays the annual rings and medullary rays, as in fig. 1, p. 14, and this division of the wood is principally employed by the turner, as it is particularly appropriate to his works, the strength and shrinking being alike at all parts of the circumference, in the blocks and slices cut out of the entire tree, and tolerably so in those works turned out of the quarterings or parts of the transverse pieces.
But as the cut is made intermediate between the horizontal line, and the one parallel with the axis, the figure gradually slides into that of the ordinary plank, magnified portions of which are shown in figs. 2 and 3: and these are almost invariably selected for carpentry, etc.
The oblique slices of the woods possess neither the uniformity of grain of the one section, nor the strength of the other, and it old be likewise a most wasteful method of cutting up the timber; it is therefore only retorted to for thin veneers, when some particular figure or arrangement of the fibres has to be obtained for the purrposes of Ornamental eabinet-work.
The perendicular cut through the heart of the tree is not only the hardest but the most diversfied, because therein occurs the greatest mixture and variety of the fibres, the first and the last of which, in point of age, are then presented in the same plank; but of course the density and diversity lessen as the board is cut further away from the axis. In general the radial cut is also more ornamental than the tangental, as in the former the medullary rays produce the principal effect, because they are then displayed in broader masses, and are considered to contain the greater proportion of the colouring matter of the wood.
The section through the heart displays likewise the origin of most of the branches, which arise first as knots, in or near the central pith, and then work outwards in directions corresponding with the arms of the trees, some of which, as in the cypress and oak grow out nearly horizontally, and others, as in the poplar, shoot up almost perpendicularly.
Those parts of wood described as curls, arc the result of the confused filling in of the space between the forks, or the springings of the brnnehes. Fig. 9 represents the section of a piece of yew-tree, which shows remarkably well the direction of the main stem A B, the origin of the branch C, and likewise the formation of the curl between B and C; fig. 10 is the end view of the stem at A. In many woods, mahogany especially, the curls are particularly large, handsome and variegated, and are generally produced as explained.
It would appear as if the germs of the primary branches were set at a very early period of the growth of the central stem, and gave rise to the knots, many of which however fail to penetrate to the exterior so as to produce branches, but are covered over by the more vigorous deposition of the annual rings. All these knots and branches, act as so many disturbances and interruptions to the uniformity of the principal zones of fibres, which appear to divide to make way for the passage of the off-shoots, each of which possesses in its axis a filament of the pith, so that the branch resembles the general trunk in all respects, except in bulk, and again from the principal branches smaller ones continually arise, ending at last in the most minute twigs, each of which is distinctly continuous with the central pith of the main stem, and fulfils its individual share in causing the diversity of figure in the wood.
The knots are commonly harder than the general substance, and that more particularly in the softer woods; the knots of the deals, for example, begin near the axis of the tree, and at first show the mingling of the general fibres with those of the knot, much the same as in the origin of the branch of the yew in fig. 9, but after a little while it appears as if the branch, from elongating so much more rapidly than the deposition of the annual rings upon the main stem, soon shot through and became entirely detached, and the future rings of the trunk were bent and turned slightly aside when they encountered the knot, but without uniting with it in any respect.
This may explain why the smooth cylindrical knots of the outer boards of white deal, pine, etc, so frequently drop out when exposed on both sides in thin boards; whereas the turpentine in the red and yellow deals may serve the part of a cement, and retain these kinds the more firmly.
The elliptical form of the knots in the plank, is mostly due to the oblique direction in which they are cut, and their hardness, (equal to that of many of the tropical hard woods,) to the close grouping of the annual rings and fibres of which they are themselves composed. These are compressed by the surrounding wood he parent stem, at the time of deposition; whereas the principal layers of the stem of the tree are opposed alone by the loosened and yielding bark, and only obtain the ordinary density.