40

PITH-MEDULLARY SHEATH.

exogens, while in warm and tropical regions such plants are far more abundant than endogens and acrogens. The embryo exogenous stem consists entirely of parenchyma, but, at the end of the first year's growth, we find: 1, A central mass of parenchyma, m, which is called the Medulla or Pith; 2, An interrupted ring of spiral vessels, t, called the Medullary sheath; 3, An interrupted zone or ring of wood-cells and vessels, forming the Wood; 4, A layer of very delicate thin-walled cells, the Cambium layer; 5, Radiating lines, r, connecting the pith with the cambium layer and bark, the Medullary rays; and 6, The Bark, b, a mass of parenchyma surrounding the whole stem, and containing in its interior liber-cells, etc., and invested on its external surface by the Epidermis.

Each succeeding year's growth repeats that of the first year, except as regards the pith and spiral vessels; the former of which does not increase in size after the first year, and the latter are never repeated. Old stems have no more distinct regions than those of the first year. Exogenous stems then present the following parts: namely, pith, medullary sheath, wood, medullary rays, cambium layer, and bark.

(1) The Pith is a central column of parenchyma which is always connected with the terminal bud but is not as a rule continued into the root. Out of this parenchymatous or fundamental tissue the spiral and other vessels and woodcells are formed; and, as they increase in number, they encroach upon it till it has the appearance shown in Fig. 90. In after years the pith is more or less broken up by the rapid growth of the outer parts, and in many fast-growing herbaceous plants it mostly disappears early and leaves. large air-cavities in its place. The diameter of the pith varies in different plants. It is usually small in hardwooded plants, as ebony, and large in soft-wooded ones, as elder.

(2) The Medullary Sheath, as seen in Figs. 90, 91, is

FORMATION OF WOOD.

4I

not continuous around the pith, but spaces are left through which the medullary rays pass outward.

(3) The Wood or Xylem.Figs. 85, 90, begins its development by the formation of the spiral vessels forming the medullary sheath (Fig. 90, t) (Fig. 91, B, d), and which are never produced after the first year. Outside of this sheath the wood consists of woody tissue, among which pitted vessels are distributed in perennial plants, and annular and others in herbaceous ones.

When the stem lasts more than one year a second ring of wood is

FIG. 90.-Horizontal section of the first year's stem or branch of an Exogenous stem. m. Pith. r. Medullary rays. t. Spiral vessels forming the medullary sheath, on the outside of which are the other elements of the fibrovascular bundle. b. Bark.

formed from the cells of the cambium layer, which are placed on the outside of the first ring. This second

FIG. 91.-Diagram showing the structure of an Exogenous stem three years old. A. Horizontal section. B. Vertical section. The figures 1, 2, 3, refer to the years of growth of the wood, and the letters mark similar parts in both sections. a, a. Pith. d. Spiral vessels. b, b, b. Pitted vessels. c, c, c. Wood-cells. e. Cambium layer. f. Inner layer of bark or liber (phloëm). g. Middle layer of bark. h. Outer layer of bark. i. Medullary rays. (After Carpenter.)

42

ANNUAL RINGS.

ring (Fig. 91, B, 2) resembles in every respect that of the first year, except that no medullary sheath is formed; it consists, therefore, entirely of woody tissue and pitted vessels, c, b. In the third year of growth another ring of wood is produced precisely resembling the second (Fig. 91, 3), and the same is the case with each succeeding annual ring as long as the plant continues to live. These stems are hence called exogenous. In the stems of Gymnospermous plants, as those of the Fir, the well-marked annual rings of wood consist essentially of wood-cells with bordered pits. Hence, upon making a transverse section of the wood of such trees, the openings will be found of nearly the same size; while in the transverse section of the wood of other exogenous trees the pitted vessels may be at once distinguished from the wood-cells by the larger size of their openings (Fig. 91, A, b, b, b).

The pitted vessels in ordinary trees are also commonly more abundant on the inner part of each annual ring, the wood-cells forming a compact layer on the outside (Fig. 91, A, c, c, c). In such cases the limits of each ring are accurately defined. In those trees which have the pitted vessels more or less diffused throughout the woody tissue, as in the Lime and Maple, the rings are not so evident, and can only be distinguished by the smaller size of the wood-cells on the outside of each layer, owing to their diminished growth toward the end of the season.

When the annual layers are first formed, the walls of their component wood-cells and vessels are pervious to fluids, and very thin, and their cavities gorged with sap. But, as they increase in age, their walls become so thickened that their cavities are ultimately almost or entirely obliterated, and they are then impervious to fluids.

This internal portion, in which the wood-cells and vessels have thickened walls, are impervious to fluids, hard in texture, of a dry nature, and commonly more or less colored, is called the Duramen, or Heart-wood; an

AGE OF DICOTYLEDONOUS TREES.

43

outer portion, in which the wood-cells and vessels have thin sides, are pervious to and full of sap, soft in texture, and pale or colorless, has the name of Alburnum or Sapwood.

Age of Dicotyledonous Trees.-As each ring of wood in an Exogenous stem is produced annually, it should follow that, by counting the number of rings in a transverse section of the stem of such a tree, we ought to be able to ascertain its age. This is true with a few exceptions, when such trees are natives of cold climates, because in these the annual rings are usually distinctly marked; but in Dicotyledonous trees of warm climates it is generally difficult, and frequently impossible, to ascertain their age in this manner, in consequence of several disturbing causes : thus, in the first place, the rings are by no means so well defined; secondly, more than one ring may be formed in a year; thirdly, some trees, such as Zamias, only produce. one ring as the growth of several years; fourthly, some plants, as certain species of Cacti, never form annual rings, but the wood, whatever its age, only appears as a uniform mass; while, lastly, in some trees, such as Guaiacum, the rings are not only indistinct, but very irregular in their growth.

It is probable that, from not sufficiently taking into account the variations in the growth of the annual rings at different periods of their age, and their varying thickness on the two sides of the tree, the ages of such trees have been sometimes overestimated. But still there can be no doubt that Dicotyledonous trees do live to a great age; when we consider that the new rings of wood are developed from the cambium cells which are placed on the outside of the previous rings, and that it is in these new annual rings that all the active functions of the plant are carried on, there can be, under ordinary circumstances, no direct limit to their age.

Size of Dicotyledonous Trees. As there is no as

44

SIZE-CAMBIUM LAYER.

signable limit to the age of Dicotyledonous trees in consequence of their mode of growth, the same circumstance leads, in many cases, to their attaining a great size. Thus the Sequoia gigantea has been measured one hundred and sixteen feet in circumference at the base; and even Oaks in this country have been known to measure more than fifty feet in circumference; many remarkable examples might be given of trees attaining an enormous size, which is also an evidence of their great age.

(4) The cells composing the cambium-layer are of a very delicate nature, and consist of a thin wall of cellulose, containing a nucleus, protoplasm, and watery cell-sap; in fact, all the substances which are present in young growing cells. The cambium-layer is called a generating tissue or meristem, because its cells are capable of dividing and forming permanent tissue, or that in which the cells have ceased to divide, but have assumed their definite form.

(5) The cells forming the medullary rays, like those of the pith, are part of the fundamental tissue of the stem (page 40); but they differ from them in form, and become much flattened (Fig. 26) in a radial direction, owing to the pressure which the neighboring wedges of the fibrovascular bundles have exerted upon them. As new rings of wood are formed in successive years, fresh additions are made to the ends of the medullary rays from the cambium, so that, however large the space between the pith and the cellular portion of the bark ultimately becomes, the two are always kept in connection by their means. Besides the medullary rays, which thus extend throughout the entire thickness of the wood, others are also commonly developed between them in each succeeding year, which extend from the rings of those years respectively to the bark; these are called secondary medullary rays.

The medullary rays are composed of flattened sixsided cells, which are placed one above the other in one or more rows, like the bricks in a wall, hence the tissue