The larger palms generally reach us in slabs measuring about the sixth or eighth part of the circle, as in fig. 4, the smaller sizes are sent entire; fig. 5 represents a small piece near the outside, with the fibres half size; but the different palms vary considerably in the shapes, magnitudes and distances of the fibres, and the colours and densities of the two parts.
In the vertical section, fig. 6, which is also drawn half size, the fibres look like streaks or wires embedded in a substance similar to cement or pith, which is devoid of fibrous structure; the inhabitants of the Isthmus of Darien pick out the fibres from some of the palms and use them as nails, they are generally pointed, and in the specimens from which the drawing was made, they are as hard as rosewood, whereas the pithy substance is quite friable. Some of the smallest palms are imported into this country for walking-sticks, under the names of partridge and
* The loaves of the exogens are by some thought to send down similar roots or fibres, between the bark and wood for the formation of the annual ring.
Penang canes, etc. The ordinary canes and bamboos are too well known to require more than to be named. - Sec artical palms, in the catalogue.
To return to the more particular examination of the woods that most concern us, it will be observed that the central pith in fig. 1, happens to be of an irregular triangular shape. This, the primary portion of the plant, is in the first instance always cylindrical; it is supposed to assume its accidental form, (which is very frequently hexagonal,) from the compression to which it is subjected. The pith governs, in a considerable degree, the general figure or section, as all the series of rings will be observed, in fig. 1, page 14, to have a disposition to project at three points; hut with the successive additions, the angular form is gradually lost, as it would be if we wound a ribbon upon a small triangular wire; for after a time, no material departure from the circular form would be observable.
A greater variation amongst the rings is due to the more or less favourable growth of the successive years, and to the different exposure of the tree to the sun and air, which develop that side of the plant in an additional degree; whereas the tree growing against a wall or any other obstruction, becomes remarkably stunted on that side of its axis, from being so shielded.
The growth of a tree is seldom so exactly uniform that its section is circular, or its heart central, often far from it; and as every annual ring is more consolidated, and of a deeper colour on its outer surface, they frequently serve to denote very accurately, in the woods growing in cold and temperate climates, the age of the plant, the differences of the seasons, the circum-stances of its situation, and the general rapidity of its growth.
"But in many hot countries the difference between the growing-season and that of rest, if any occur, is so small, that the zones are as it were confounded, and the observer finds himself incapable of distinguishg: with exactness the formation of one year from that of another."*
It is, however,difficult to arrive at any satisfactory conclusion respecting the qualities of woods from the appearance of their annual rings; for instance, in two specimens of larch, cousiderd by Mr. Fincham† to be exceedingly similar, in specific gr&\ strength and durability; in the one, Scotch larch, there were only three annual rings in five-eighths of an inch, whereas in Italian larch there were twenty-four layers in the same space. In some of the tropical woods the appearance of the rings can scarcely be defined, and in a specimen of the lower or butt end of teak, now before me, three annual rings alone, cover the great space of one inch and three-eighths.
* Dr. Lindley's Introduction to Botany, second edition, p. †principal builder of Her Majesty's Dock-yard, Chatham.
The horizontal section of a tree, occasionally looks as if it were the result of two, three, or more separate shoots or stems consolidated into one; in some of the foreign woods in particular, this irregularity often gives rise to deep indentations, and most strange shapes, which become eventually surrounded by one single covering of sap; so that a stem of considerable girth may yield only an insignificant piece of wood, scarcely available for the smallest purposes of turnery, much less for cabinet work.*
The circulation of the sap is considered to be limited to a few of the external layers, or those of the sap-wood, or alburnum, which are in a less matured state than the perfect wood, or duramen, beneath. The last act of the circulation, as regards the heart-wood, is supposed to be the deposition of the colouring matter, resin or gum, through the agency of the medullary rays that proceed from the bark towards the center, and leave their contents in the layer outside the true wood perfected the year previous. We may fairly suppose by analogy, that as one ring is added each year, so one is perfected annually, and thrown out of the circulatory system.
That the circulation has ceased in the heart-wood, and that the connexion between it and the bark has become broken, is further proved by the fact, that numbers of trees may be found in tolerably vigorous growth within the bark, whereas at the heart they are decayed and rotten. In fact some of the hardest foreign woods, as king-wood, tulip-wood, and others, are rarely sound in the center, and thus indicate very clearly that their decay commenced whilst they were in their parent soil; and as in these, the apparance of annual rings is scarcely to be distinguished, this also appears to indicate a great term of age, enough to account for this rclatively premature decay.
* This is not peculiar to the tropical woods; for example, some of the yew-trees in Hampton Court gardens, appeal to have grown in this manner from three or four separate stems, that have joined into one at a short distance above the ground. As an instance of the singular manner in which the separate branches of trees thus combine, I may mention that stones, pieces of metal and other substances, are occasionally met with in the central parts of timber, from having been accidentally deposited in a cleft, or the fork of a branch, and entirely inclosed or overgrown by the subsequent increase of the plant.