so that they are perforated by many minute
openings.
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Transverse section of a sheathed bundle-pair from Corn (Zea mays), p, parencliyraa cells ; within these the sclerenchyma sheath ; i'ti pitted vessel with its fellow opposite, and two annular vessels between, with the adjacent tissues mark the sylem bundle ; the area above and between the pitted vessels is the phloem bundle.
The xylem- and phloem-strands have a definite relation to one another in position. In the pteridophytes, the phloem-region often envelops file xylem region completely, though in a few cases it is enveloped by the xylem. In most of the spermatophytes, however, these strands lie side by side, the xylem as a rule nearest the cen- tre and the phloem nearest the periphery of the
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Cross-section of a single bundle pair, pa, in- closing parenchyma ; p, phloem bundle ; ^, xylem bundle; c, cambium; a, accompanying scleren- chyma, sometimes called bast fibres.
axis. They extend into the leaves, in which they occupy the so-called ribs or veins, the xylem nearest the upper side, the phloem nearest the tnider side. Together they form the smaller vein- lets, becoming slenderer and having fewer ele- mentswith successive branching, until the phloem- strand disappears, leaving the xylem-strand to form the finest veinlets, invisible to the naked eye, which end 'blindly amongst the green cells, or form a network with other small strands. Near its ending in the leaf the xylem-strand is composed exclusively of tracheids. The xylem- and phloem-strands originate near the growing points by the ditl'erentiation of the tissues which arise from the plerome. Taken all together, they constitute a central cylinder in the root and stem, known as the stele. In some stems, espe- cially among the pteridophytes, there are sev- eral independent steles, but more often this ap- pearance is produced by the branching of the
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Diagram of a cross section of dicotyledonous stem, showing a single circle of bundle pairs (the cross-hatched region — phloem bundle, the rest — xylem bundle) dividing the outer region (cortex) from the central (pith).
stele, as in the stalks of leaves. In the stems of many plants, especially dicotyledons, a for- mative region, the stelar cambium, arises in the stele between the .xylem- and phloem-strands. This may give rise to additional xylem- and phloem-tisstie, and so increase the size of these strands. More often, however, it extends from one pair of strands to another, and so eonstittites a complete zone, by means of which not only is the thickness of the original strands increased, but that of the intervening tissues as well. Usu- ally new xylem- and phloem-strands are produced by the stelar cambium between the older ones. Thus it may come about that the stele shows a massive development of secondary xylem in the centre and secondary phloem around it, the two separated by a thin sheet of cambium. This is the condition in all deciduous and coniferous trees. The centre of the trunk is composed of old and dead xylem-tissue, its outside of bark, most of which is likewise dead, the only living parts being the cambium and adjacent tissues.
(4) The Nutritive System. The ntitritive system consists of cells, usually thin-walled, among whose organs are found one or more chloroplasts (q.v.;. The massing of these cells gives the green color to the nutritive regions. If the plant body be more than a few cells in thickness the nutritive tissues are limited to the surface, because the green coloring matter, chlo- rophyll (q.v.). can be produced and maintained only'under adequate illumination. The interior tissues, therefore, are colorless, because of the absorption of light by the outer ones. The nu- tritive tissues may occupy the surface of the stem only, but their most effective disposition is in the leaves. In some liverworts and in the mosses, .the so-called leaf consists of a single layer of cells: they are not lik? the leaves of the higher plants "either in mode of origin or in struc- ture, although they serve the same function. (For structure of the leaf of the higher plants,
