T .stru•lure.
The internal structure of the plant does not give rise to such definite parts or organs as appear in the ext,. rnal conformation. The plant-body is made up of cells. Some of these cells perform one work and some perform another work. The fundamental tissue is parenchyma. In this tissue the cells are very simi lar one to another, more or less cubical or equal sided, or at least not greatly elongated. The vital processes take place in the parenchyma. Out of the parenchyma the other and special kinds of tissue develop.
The special cellular structures in the stem are chiefly mechanical tis sues of two general kinds of elongated cells,—those that support the plant or contribute to maintain its form and stature, those that transport the fluids. The supporting tissues, giving rigidity to the plant, are of two kinds in respect to the structure of the cell-wall : those in which the cells are thick ened or strengthened in the angles (collenchyma, Fig. 24), and those in which the cell-walls are thickened throughout (sclerencyhma). The conducting tissues are also of two kinds : those with trachea-like walls, marked with rings or pits, and those with punctured or sieve-like walls. The supporting tissues may be in the epidermis of young or of small stems, in the bark, or placed inside the woody cylinder.
The conducting tissues are usn a 11y definitely placed, and these we may consider further.
The development of these mechanical tis sues (for transportation and support) results in the formation of vessels, or systems of spe cialized tissue in particular parts of the stem. Vessel-bearing plants are said to be vascular, in distinction from certain very low orders of plants in which no special tissues of this kind have been developed.
It is well known that trees of temperate climates and very many other plants have a distinct and separable bark and that they increase in diameter by "rings" added on the woody cylin der. On the other hand, palms, grasses, bananas and many other mostly herbaceous plants increase in diameter by means of tissues scattered through the stem; these plants do not make an annual ring, and they rarely branch extensively. The former kinds of plants were formerly called exogens, or outside growers, and the latter endogens or inside-growers. These terms are now given up, however, as not expressing good anatomical distinctions. These classes of plants are now named from the cotyledon or seed-leaf characteristics,—the former having two leaves on the embryo plant, and called dicotyledons ; the latter having one leaf in the seed or embryo, and called monocotyle dons.
In most dicotyledonous plants we all recognize three fairly distinct parts of the stem, at least at some epoch in the life of the plant : the bark, the woody part, the pith. These
parts are usually not clearly set off in the minute anatomical structure, however ; but we may pause a mo ment to discuss them. Long tissues, extending lengthwise the stem or leaf, formed of elongated cells placed end to end or closely interlapping, are usually associated in more or less definite strands or bundles (Fig. 21). It is these strands, or parts of them, that produce the commercial fibers.
The bundle, running lengthwise the stem, is composed of two parts or regions : the xylem or wood part, lying on the inward side of the bundle as it stands in the stem ; the phloem or bark part, lying on the outward side. These bundles stand side by side around the outside of the woody cylinder, with the pith or undifferen tiated parenchyma at the center of the cylinder. These bundles therefore make a continuous ring. However, the bundles are themselves supplied, when growing, with living parenchyma, called cambium, from which new cells are formed for both the xylem and phloem regions of the vascular bundles. Inasmuch as the bundles form a ring about the stem, so the cambium that accompanies them also forms a ring. The parenchyma tissue extends outward from the pith between the bundles (or the bundles are imbedded in the parenchyma), causing the rayed appear ance of the stem in cross-section. The xylem part of the bundle contains the trachea like spiraled or pitted vessels. These are the routes through which the water ascends from the root. The phloem part containing the sieve-tubes transports the organized food, or "elaborated sap," after it has been formed in the leaves; this food is transported to all parts of the plant to build new cells, or sometimes to be stored until needed. The supporting tissue may be associated with the vascular, or fibre-vascular, bundles. Bast is schleren chyma tissue growing with the phloem. The xylem and phloem regions separate as they grow, the former becoming part of the wood and the latter part of the inner bark. The outer separable part commonly called bark is a very complex structure, being formed of the cortex or skin of the stem and the cork and strengthening tissues formed therein, the old and dead or dying phloem, and the new phloem that is just forming from the cambium in the vascular bundle. The xylem grows old and dies ; the dead tissue becomes filled and hardened in firm wood ; new xylem tissues are added on the outward side. The phloem grows old and dies ; the dead parts are added to the bark ; new phloem tissues are added on the inward side. The fibers of hemp and flax are derived from the phloem.