STRUCTURE OF THE WOODS. 15 cian for that instrument; the principal lines alone are represented, and these are magnified to about twice their linear distances, for greater perspicuity. Fig. 1, which represents the horizontal or transverse section of a young tree or a branch, shows the arrangement of the annual rings around the center or pith; these rings are surrounded by an exterior covering, consisting also of several thinner layers, which it will suffice to consider collectively, in their common acceptation, or as the bark. The fibres which are seen as rays proceeding from the pith to the bark, are the medullary rays or plates. Figs. 2 and 3 are vertical sections of an older piece of beechwood. Fig 2 is cut through a plane, such as from a to a, in which the edges of the annual rings appear as tolerably parallel fibres running in one direction, or lengthways through the stem; the few thicker stripes are the edges of some of the medullary rays. Fig. 3 is cut radially, or through the heart, as from b to b. In this the fibres are observed to be arranged in two sets, or to run crossways; there are first the edges of the annual rings, as in fig. 2, and secondly, the broad medullary rays or plates. The whole of these figures, but especially the last, show the character of all the proper woods, namely, those possessing two sets of fibres, and in which the growth of the plant is accomplished, by the yearly addition of the external ring of the wood, and the internal ring of the bark, whence these rings are called annual rings, and the plants are said to be exogenous, from the growth of the wood being external. In fig. 1 the medullary rays are the more distinctly drawn, in accordance with the appearance of the section, as they seem to constitute more determinate lines; whereas the annual rings consist rather of series of tubes arranged side by side, and in contact with each other, and which could not be represented on so small a scale. At the outer part of each annual ring these tubes or pores appear to be smaller and closer; the substance is consequently more dense, from the greater proportion of the matter forming the walls of the tubes; and the inner or the softer parts of the annual rings have in general larger vessels, and therefore less density. In many plants the wedge-form plates, intermediate between 16 STRUCTURE OF THE WOODS. the medullary rays, only appear as an irregular cellular tissue full of small tubes or pores, without any very definite arrangement*. The medullary rays constitute however, the most characteristic part of the structure, and greatly assist in determining the difference between the varieties of the exogenous plants, as well as the wide distinction between the entire group and those shortly to be described. The medullary rays also appear, by their distinct continuity, to constitute the principal source of combination and strength in the substance of the woods; most of the medullary rays, in proceeding from the center to the circumference, divide into parts to fill out the increased space. In the general way, the vertical fibres of the annual rings, and the horizontal fibres of the medullary rays, are closely and uniformly intermingled; they form collectively the substance of the wood, and they also constitute two series of minute interstices, which are viewed to be either separate cells or vessels, the majority of which proceed vertically, the others radially. In many, as the oak, sycamore, maple, and sweet chesnut, the medullary rays, when dissected, exhibit a more expanded or foliated character, and pervade the structure, not as simple radial tubes, but as broad septæ or divisions, which resemble flattened cells or clefts amongst the general groups of pores, giving rise to the term silver-grain, derived from their light and glossy appearance they vary considerably in size and number. The beechwood, fig. 3, has been selected as a medium example between this peculiarity and the ordinary crossings of the fibres, which in the firs and several others seem as straight as if they were lines mechanically ruled; and even in the most dense woods are in general easily made out under the microscope. The vessels or cells running amidst the fibres are to the plant what the blood-vessels and air-cells are to the animal; a part of them convey the crude sap from the roots, or the mouths of the plant, through the external layers of the wood to the leaves, in which it is evaporated and prepared; the fluid afterwards returns through the bark as the elaborated sap, and combines with that in the external layers of the wood, the two constituting * In the Cissampelos Pareira, belonging to the natural order Menispermacea, this structure is singularly evident; the medullary rays are very thick, and almost detached from the intermediate wedge-form plates, which are nearly solid, except the few pores by which they are pierced, much like the substance of the common cane. ex STRUCTURE OF THE PALMS, ETC. 17 the cambium. The latter ultimately becomes consolidated for the production of the new annual ring that is deposited beneath the loosened bark, and which is eventually to constitute a part of the general substance or wood; the bark, also receives a minute addition yearly, and the remainder of the fluid returns to the earth as an excretion *. The other order of plants grows in an entirely different manner, namely by a deposition from within, whence they are said 6. Fig. 5. to be endogenous; these include all the grasses, bamboos, palms, &c.Endogens are mostly hollow, and have only one set of fibres, the vertical, which appear in the transverse section, (fig. 4,) as irregular dots closely congregated around the margin, and gradually more distant 4. towards the center, until they finally disappear, and leave a central cavity, or a loose cellular structure. Fig. 5 represents the horizontal, and fig. 6 the vertical section of portions of the same, or the cocoa-nut palm, (Cocos nucifera,) of half their full size. All the endogens are considered to commence from a circular pithy stem which is entirely solid; some, as the canes, maintain this solidity, with the exception of the tubes or pores extending throughout their length. The bamboos extend greatly in diameter, so as to become hollow, except the diaphragms at the knots; these are often used as cases for rolls of papers. The palms generally enlarge still more considerably to their extreme size, which in some cases is fifty times the diameter of the original stem, the center being soft and pithy. Some of the palms, &c. denote each yearly increase by one of *The reader will find the articles in the three Editions of Dr. Lindley's Introduction to Botany, " Exogenous structure," and " Of the stem and origin of wood;" and also "Exogens," and "Endogens," by the same author in the Penny Cyclopedia, replete with physiological interest. 18 STRUCTURE OF THE PALMS, ETC. the rings or markings upon their stems, which are always soft in the upper part, like a green vegetable, and terminate in a cluster of broad pendent leaves, generally annual, and when they drop off they leave circular marks upon the stem, which are sometimes permanent, and indicate by their number the age of the plant. The vertical fibres above referred to, proceed from the leaves, and are considered to be analogous to their roots, and likewise to assimilate in function to the downward flow of the sap from the leaves of the exogens: whereas in the palms they constitute separate and detached fibres, that first proceed inwards, and then again outwards, with a very long and gradual sweep, thereby causing the fibres to be arranged in part vertically, and partly inclined, as in the figure*. The substance of the stems of the palms, is not allowed by physiological botanists to be proper wood, (which in all cases grows exteriorly, and possesses the two sets of fibres shown in fig. 3;) whereas the endogenous plants have only the one set, or the vertical fibres; and although many of this tribe yield an abundance of valuable gifts to the natives of the tropical climates in which they flourish, only a portion of the lower part of the shell of the tree is available as wood; amongst other purposes, the smaller kinds are used by the natives as tubes for the conveyance of water, and the larger pieces as joists, &c. &c. 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 leaves 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, &c. The ordinary canes and bamboos are too well known to require more than to be named.-See article 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; but 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 each of the annual rings 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 circumstances of its situation, and the general rapidity of its growth. "But in many hot countries the difference between the growingseason and that of rest, if any occur, is so small, that the zones are, as it were, counfounded, and the observer finds himself incapable of distinguishing 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, considered by Mr. Fincham† to be exceedingly similar, in specific gravity, * Dr. Lindley's Introduction to Botany, second edition, p. 74, |