In many plants the wedge-form plates, intermediate between 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, that 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 septa 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 beech-wood, 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 the sap 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 the cambiwm. 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 subsetanee or wood; the bark also receives a minute addition yearly, and the remainder of the fluid returns to the earth as an excretion.*
* In the Cissampelos Pareira, belonging to the natural order Menispermaceae, 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.
The other order of plants grows in an entirely different man-ner, namely by a deposition from within, whence they are said to be endogenous; these include all the grasses, bamboos, palms, etc. 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 towards the center, until they finally disappear, and leave a central cavity, or a loose cellular structure. Fig. 5 represents the horizontal, and fig. C 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, etc, denote each yearly increase by one of 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 in part inclined, as in the figure*.
* The reader is referred to the following articles in the three editions of Dr. Lindley's Introduction to Botany, namely - " Exogenous structure," and " Of the stem and origin of wood;" and also " Exogens," and " Endogens," by the same author in the Penny Cyclopaedia: all are replete with physiological interest.
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 and beams.