Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/110

100 is formed on the outside ; and in this way the stem increases in thickness. This growth in thickness, however, ceases periodically, and is renewed with each new period of vege tation, and thus the wood is formed in concentric layers, sharply marked off from succeeding layers, each being an annual ring of wood, and the same is seen in the cortex (fig. 74). The rings of wood are thus formed successively outside those pre-existing, while in the cortex the new layers are produced inside those already formed. The inner vessels of the primary fibro-vascular bundles immediately surround the pith and often project into it, and form what is termed the medullary sheath, which consists of spiral vessels, and through this sheath the primary medullary rays pass. These medullary rays extend from the pith to the cortex, but as new zones of wood and cortex are produced, new rays are formed in them, which increase by additions from the cambium layer. The secondary cortex, formed from the cambium ring, constitutes what is commonly known as the inner bark or endopMceum ; the primary cortex, which forms the outer bark, consists of two layers of cells, which have been respectively termed the mesophlceum and the epiphloeum. Outside all is the epidermis. This, however, does not remain as thin-walled cells, but is usually converted into periderm, and this may in turn be completely supplanted by the bark. Thus, if a transverse and a longitudinal section of a Dicotyledonous stem be made, the following structures will be seen as repre sented in fig. 74. In the centre is the cellular pith a, a, immediately surrounding it comes the medullary sheath d, then the secondary layers of wood b, c, of successive years growth, outside this the cambium ring e, separating the wood- layers from the cortical layers. Of these last, the inner,/, is the bast layer or endophlceum, g is the middle cellular layer or mesophlceum, and d is the outer epiphloeum. The latter two constitute the outer bark. Connecting the meso phlceum with the pith is seen a medullary ray i. Outside all are the epidermal tissues. Fig. 74. Diagram of the structure of an Exogenous or Dicotyledonous stem of three years growth, A being a transverse section, and B a vertical section. The figures 1, 2, 3, mark the years of growth, and the letters refer to the same parts in both figures, a, a, medulla or pith, consisting of hexagonal parenchyma ; 6, 6, 6, pitted or dotted vessels, and c, c, c, wood-cells, of successive annual layers ; d, spiral vessels of the medullary sheath ; e, layer of cambium cells between wood and bark ;/, inner bast layer of bark (Liber, Endophlmum) ; g, cellular envelope, forming middle layer of bark (Mesophloeum) ; h, outer corky layer of bark (Epiphlceum) ; t, i, medullary ray which, in the transverse section, is seen running continuously from the pith to the bark. (After Carpenter.) Let us now examine the different parts of an Exogenous stem proceeding from the centre to the circumference:—

The Pith, or the central part of a Dicotyledonous stem, is composed of cellular tissue. In the young stem it is succulent, the cells being full of fluid and frequently of a greenish hue ; but in process of time it becomes pale- coloured, dry, and full of air. These changes take place first in the central cells. Sometimes the pith is broken up into cavities, which have a regular arrangement, as in the Walnut, Jessamine, and Cecropia peltata ; it is then called discoid or disciforrn. At other times, by the rapid growth of the outer part of the stem, the pith is ruptured irregu larly, and forms large cavities, as in the fistular stem of Umbelliferous plants. In some cases fibro-vascular bundles are found in the pith, as in Elder, Pitcher-plant, and Ferula, and occasionally its cells are marked by pores indicating a thickening of the cell-wall. The extent of pith varies in different plants and in different parts of the same plant. In Ebony it is small, while in the Elder it is large. In yEschynomene aspera (Shola plant, the Rice- paper plant of India), the interior of the stem is almost entirely composed of cellular tissue or pith ; from this a kind of paper is made, and light hats. The same kind of tissue occurs in the Papyrus of the Nile. Large pith is also seen in Fatsia (Aralia) papyrifera (Tung-tsaou or Chinese rice-paper plant), and in Scasvola Taccada of the Malay archipelago. When the woody circle of the first year is completed, the pith usually remains stationary as regards its size, retaining more or less its dimensions, even in old trunks, and never becoming obliterated.

The Medullary Sheath is the fibro-vascular layer immediately surrounding the pith. It is the inner layer of the fibro-vascular bundle of the first jear (fig. 74, d), and consists chiefly of true spiral vessels, with annular and reticulated vessels, intermixed with long woody fibres, which continue to exercise their functions during the life of the plant, and which extend into the leaves. Between the ves sels of the sheath the medullary rays from the pith pass.

The Wood.—The layers of wood (fig. 74, b, c) are formed outside the medullary sheath in concentric rings in the manner already described. On account of this mode of formation of wood-layers successively outside pre-existing layers the stem increases indefinitely. There are no annular or spiral vessels present ; these have been replaced by pitted and punctated vessels along with wood-cells. The steins have been called exogenous and also indefinite, and Dicotyledonous plants have sometimes received the name of Cyclogens, in consequence of exhibiting concentric circles in their stems. On a transverse section each zone or circle is usually seen to be separated from that next to it by a well-marked line of demarcation. This line, as in the Oak and in the Ash, is indicated by holes which are the openings of large pitted vessels, the remainder of the tissue in the circle being formed by pleurenchymatous tubes with thickened walls and of smaller calibre. In some trees, as the Lime, Hornbeam, and Maple, the line is by no means so well marked, as the openings are smaller and more generally diffused ; but there is iisually a deficiency of pitted vessels towards the outer part of the circle. In cone-bear ing plants, as the Fir, in which the woody layers consist entirely of piinctated tissue, withoiit any large pitted vessels, the line of separation is marked by the tissue becoming dense and often coloured. In some kinds of wood, as Sumach, the zones are separated by a marked development of cellular tissue. The separation between the zones is owing to the interruption in the growth of the tree during autumn and winter, and hence it is well defined in trees of temperate and cold climates. But even in tropical trees, the lines, although often inconspicuous, are still visible, the dry season, during which many of them lose their leaves, being their season of repose. The woody layers vary in their texture at different periods. At first all the tissues are pervious and full of fluid; but by degrees they become thickened, and the channels of the vessels get filled up and obliterated. The first-formed layers are those which soonest become thus 