leaves. The fluid contents of these cells are, therefore, as manifold as that of the parenchyma cells. The bast-cells of one plant contain quite a different fluid matter to that of another plant. In one, the fluid is poisonous; in another, nutritive here it is a white, yellow, or orange-colored milk sap: there, caoutchouc, chlorophyl, or resinous matters are present.

Although the bast-cells form the innermost layer of bark, in no case do they rest immediately on the wood. They are always separated by a bed of cells more or less thick, called the cambium layer.

THE WOOD.

The wood includes nearly the whole of that part of the stem situated beneath the bark. It consists of a number of ligneous circles, visible on the cross-section, which are traversed by lines radiating from the centre to the circumference of the stem, that is to say, from the medullary canal to the bark, called the medullary rays.

This disposition of the wood in circular beds or layers, takes place in all the trees of countries where the season of growth has only a limited duration, and is followed by a period of cold and vegetable inactivity. Each year there is formed in spring a new bed of wood, and at the same time a fresh layer of bark. The age of a tree is, therefore, in most cases, the same as the number of ligneous circles which can be counted on the cross-section of its stem. In the same manner the age of the branches may be computed.

This fact must be mentioned with some restrictions; for, in certain circumstances, as for instance, when a warm spring is succeeded by a wet, cold summer, vegetation receives a check, and a ring is formed prematurely; with the return of more favorable weather, the tree again makes a rapid growth, so that at the close of the vegetative season two rings have been formed during the same year. The rings become less distinct in Exogenous trees as we travel South. On approaching warmer climates, where vegetation continues almost without interruption throughout the whole

year, the rings become confounded one with the other; they are much thinner, and so multiplied that they cease to be reliable, and do not indicate in any manner the age of the tree.

The trees of temperate climates usually thicken themselves in their whole circumference, and the unequal development of only one side must always be regarded as an exceptional case. In tropical countries there are, however, trees whose stems take the most wonderful forms, in consequence of the unequal development of their sides. Some of them belong to the genus Bauhinia, Natural Order, Leguminosa. The most striking example among the species of this genus is furnished by the stem of Heretiera Fomes. The first year the stem of this tree is normally formed, a small wood-ring surrounding the pith; but afterward it annually thickens by crescent-like deposits of wood on two opposite sides; the stem thus presents a flattened, compressed appearance. "I examined such a stem," says Dr. Herman Schacht,* "which was eighteen inches in one direction, in the other, on the contrary, it was only two inches broad," presenting "in some measure, the appearance of a natural plank, surrounded by a weak bark."

Anomalous forms of Exogenous stems also exist amongst tropical trees belonging to the Natural Orders Bignoniaceæ, Malpighiaceae, Menispermaceæ, and Aristolochiaceæ.†

In the wood of beech-trees, two distinct species of cells can be recognized, the fibre-cells and the vasiform or ductcells.

The fibre-cells form the principal part of the wood of each ligneous deposit. They are elongated and extremely attenuated cells, tapering to either extremity, and lying together in bundles more or less compact, which are developed vertically. It is through the fibre-cells of the wood that the main current of the sap flows in the spring. Their vital

* "Lehrbuch der Anatomie und Physiologie der Gewachse," page 345, Berlin, 1856.

† See, "Precis de Botanique et de Physiologie Vegetale," par A. Richard; page 75-80, Paris, 1852.

activity, however, only continues for a short time. Their walls are soon thickened by earthy matter, which goes on accumulating, until their cavities are finally closed, and the sap ceases to circulate through them. The color then changes, and they no longer take any further part in the vital operations of the tree. Their function is now purely

a mechanical one; for the very same matter which terminates life endows them with force and persistence. The * fibre-cells thus lignified, form, as it were, the skeleton or framework of the tree, and withstand outer influences as well as inner decomposition much longer than the other tissues. They are the very last to yield to dissolution.

The vasiform or duct-cells are spread through the mass of fibrous tissue. They originate out of a row of cells, the cross walls of which are absorbed, so that, when fully developed, they form one continuous tube. These ducts may be readily distinguished from the wood-cells among which they are interspersed, as their interior diameter or bore is much more considerable, and they remain permanently open. The open mouths of the ducts are very conspicuous on the transverse section of common pine-wood, where they resemble pores. There are several varieties of this species of cell termed by botanists, dotted, annular, spiral, and scalariform ducts. The whole of these vessels at first contain sap, which is afterward displaced by air. They may be regarded, in fact, as the air-vessels of plants, by means of which the sap in their interior is brought into communication with the atmosphere. This is the reason why these ducts or air-tubes are placed amongst the fibrecells or sap-tubes, as inspection plainly shows.

The ducts and fibre-cells are, however, arranged in the stem according to a definite law, as inspection plainly shows; for the former preponderate in number toward the interior portion of the ring, whilst the fibre-cells are most abundant towards its exterior. The bounding line of each year's growth is easily distinguished by this internal arrangement of the ducts, and its cause is to be sought for in the vital economy of the tree itself. The inner portion

of the wood-ring, with its loose and porous structure, is constructed in spring, when the greatest amount of sap is needed for the nutrition of the growing leaves, shoots, and flowers. Hence the wood-cells formed at this time have wide cavities, and their walls are but slightly thickened. They have been called by botanists ducts, and through them the current of sap flows in early spring, when it is most in demand. But in summer and autumn, when the new leaves and shoots are fully developed, less sap is needed. The outer and more compact portion of the wood is formed at this time, and the wood-cells then developed are adapted in their tubular capacity, to the diminished necessities of the tree; their cavities are much smaller and their walls thicker. It is the long, attenuated forms of these cells which has led to their being called fibre-cells.

The fibre and duct-cells die early. There is a gradual cessation of their vitality, inseparably connected with their formation. So soon as a cell ceases to form new cells, or to develope or carry nourishing matter in it, so soon as its fluid contents disappear and it becomes filled with air, it may be considered as dead. This is the condition of the fibre and duct-cells when fully developed; then the sap disappears from their cavities, and we find in them neither protoplasm nor cell-nuclei. Their nitrogenous contents have been expended either in the lignification of their walls, or they have been absorbed by the neighboring cells. With the disappearance of this formative material, the life of these cells necessarily terminates.

THE PITH AND THE MEDULLARY RAYS.

These are parenchyma cells; the most widely diffused, important, and variable tissue of plants. The parenchyma cells of the pith are spherical or ovoid when they are but slightly united together, but more often they become more or less polyhedral by reciprocal pressure.

In the young stem, the pith is of considerable importance; it abounds in nutritive matter, which serves to nourish the young buds on its surface, and is often of a green color,

more or less intense. But when the buds develope into branches, supporting leaves, flowers, and other appendages, the nutritive liquids accumulated in the pith are absorbed, the particles of green matter disappear; and when the vegetation commenced in spring is arrested in autumn, the cells of the pith are dry, colorless, and empty : they are then, in fact, dead cells.

The parenchyma cells of the medullary rays are of a quadrilateral form, and develope in horizontal radiating lines from the pith to the bark. These lines are easily distinguished on the cross-section of the stem, when the wood is compact and not too deeply colored: the oak, for example. Their looser structure and lighter color renders them in such circumstances more visible. The medullary rays develope vertically as well as horizontally, and partition off the wood into a number of wedges, in the form of elongated triangles, of which the point that is a little obtuse corresponds to the medullary canal.

The medullary rays are of great service to the old wood. They maintain an exchange of sap between the cells of the pith, wood, and bark; and when the pith-cells are quite dead, they unite the older annual layers of wood with the younger and with the bark, and thus continue the communication. They therefore survive the death of the pith-cells, and even of the wood-cells, in the midst of which they radiate. The medullary rays of a five or six years' old woodring, are still vitally active cells, filled with sap.

We have now given the anatomical and physiological peculiarities of the different species of cells which, united, form the tissue or substance of the stem of a beech tree. We proceed to investigate the physiological phenomena of these cells, as combined together into a continuous tissue, and thus give the reader as clear and philosophical an idea as possible of the growth of the tree.

The remarkable disposition of the substance of the bark and wood in circular strata or layers, results from the annual formation of a new stratum or bed of wood on the exterior of that already existing there, and of one or more