686. Structure. The growing rootlet of the germinating plant exhibits under a microscope a nearly uniform mass of cellular tissue. The cells composing it are soft and delicate, with thin, porous walls adapted to absorb moisture, which it has already begun to do. It grows by the accession of cell to cell through their division and enlargement at its point, or rather just behind the advance layer which constitutes its cap (pileorhiza § 725.
687. The earliest tissue. The same structure also appears in the expanding cotyledons and the opening bud of the plumule. At this early stage, therefore, all plants alike in all their parts are composed of simple parenchyma. Subsequent changes in structure occur, giving to each tribe its several peculiarities. Still the growing points of the axis, both ascending and descending, advance by the formation of the same tissue, and the vessels, if formed at all, follow a little later.
688. The changes. The rootlet soon becomes a root, assumes a corky layer instead of the tender, spongiform epidermis, and ceases to absorb. But new rootlets spring from the radicle, or branch from the axis, which in their turn absorb, harden, divide and subdivide; and so on indefinitely.
689. The increasing demand for moisture is thus met by the multiplication of these root ends, which have been called the spongelets. The absorbing surface is also greatly increased by the hair-like processes of the epidermis; - the fibrillae (§ 724) which multiply generally in proportion to the dryness of the soil.
G90. There are four general modes of growth and structure, whereby the vegetable kingdom is distinguished into as many great classes, viz.:
The outside-growers (Exogens), The inside-growers (Endogens), The point-growers (Acrogens), The mass-growers (Thallogens).
691. The exogenous structure. A cross section of the stem or branch of any dicotyledonous plant (mustard, maple), exhibits zones of different structures, which are distinguished as pith, medullary sheath, wood, and bark.
692. The pith occupies the central part of the stem. It consists of parenchyma, is chiefly abundant in herbaceous plants and all young stems. When new, it is filled with fluids for the nourishment of the buds until they can make food for themselves. As the plant advances in age, the pith loses its vitality, is filled with air only, is often torn into irregular cavities, or disappears.
693. The medullary sheath immediately surrounds the pith. It is a thin, delicate tissue consisting of spiral vessels. It communicates with every bud, and sends off detachments of its vessels to the petioles and veins of every leaf. Its tubes secrete oxygen from carbonic acid or water and convey it to the leaves.
694. The wood consists of pleurenchyma and ducts (§ 666) arranged more or less distinctly in concentric zones or layers. The first or inner layer, together with the medullary sheath and pith, is the product of the first year. One new layer is formed each successive year, during the life of the plant.
695. Annual circles. The ducts are usually first formed and lie in the inner part of the strata next the center, while the wood-fibers are produced toward the end of the season, and deposited in the outer part. The former are distinguished by the large size of their open ends, while the fibers are minute and compact. This circumstance renders the limits of each layer distinctly perceptible in a cross section, and their number, if counted at the base, will correctly indicate the age of the tree.
696. Exceptions. There are doubtless some exceptions to this rule. In tropical countries, where there is no distinction of seasons, there may bo several zones deposited annually, or on the other hand, several or all the annual layers may bo so blended by the uniform mixture of the ducts with the wood-tissue as to be un-distinguishable. The layers of the beet-root are certainly not annual. They seem to correspond with the number of leaf cycles (§ 228).
697. The alburnum and duramen - the sap-wood and heart-wood, are well-known distinctions in the wood. The former, named from al-bus, white, is usually of a light color and softer structure. It is the living part of the wood through whose vessels mainly the sap ascends.
698. How formed. The interior layers of the alburnum gradually harden by the deposition of solid matter in their vessels, and the thickening of the cell-walls, until fluids can no longer pass through them.
Thus the duramen (durus, hard) is formed of a firm and durable texture, the only part valued as timber. Its varying colors in cherry, walnut, rose-wood, are well-known.
595, Cross-sections of an exogenous stem (Elm), of 2 years' growth; 1, pith, 2, 3, annual layers of wood, next the cambium, 4, bark; 596, and endogenous stem (Sorghum or Millet), where there is no distinction of layers.
C99. The duramen is of no account in vegetation, and is in thi3 respect dead. Hence it often decays, leaving the trunk hollow, and the tree at the same time as flourishing as ever.
700. The bark succeeds and replaces the epidermis, covering and protecting the wood. It is readily distinguished into three parts, viz.:
The inner, white bark (liber), The middle, green bark (cellular), The outer, brown bark (cortical). The substance of all these is parenchyma and arranged, like the wood, in layers.
701. The liber or white bark contains scattered bundles of pleuren-chyma and cienchyma with its cellular tissue. Its wood-cells are very long (§ 666), called bast-cells, and are strengthened with secondary deposits until quite filled up. Hence the strength and toughness of flax and hemp. The strong material of "Russian matting" is from the liber of the linden-tree, and the "lace" of the South Seas from the lace-bark tree. The liber of other trees is not remarkable for strength.