416
ANATOMY
In some cases where there is apparently a well-marked plerome at the apex, this is really the young pith, the distinction between the stelar and cortical initials, if it exists, being, as is so often the case, impossible to make out. The young tissue of the stelar cylinder, in the case of the modified siphonostele characteristic of the dicotyledonous stem, differs from the adjoining pith and cortex in its narrow elongated cells, produced by the stopping of transverse and the increased frequency of longitudinal divisions. This is especially the case in the young vascular bundles themselves {desmogen strands). The protoxylem and protophloem are developed a few cells from the inner and outer margins of the desmogen strand, desmogenic tissue left over giving rise to the segments of endocycle and pericycle capping the bundle. Differentiation of the xylem progresses outwards, of the phloem inwards, but the two tissues never meet in the centre. Sometimes development stops altogether, and a layer of undifferentiated parenchyma, the mesodesm, is left between them ; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side. In this case the formation of the primary bundle passes straight over into the formation of secondary tissue by a cambium, and no line can be drawn between the two processes. The differentiation of the stelar stereom, which usually takes the forms of a sclerized pericycle, and may extend to the endocycle and parts of the rays, takes place in most cases later than the formation of the primary vascular strand. In the very frequent cases where the bundles have considerable individuality, the fibrous pericyclic cap very clearly has a common origin from the same strand of tissue as the vascular elements themselves. The separation of layers in the apical meristem of the root is usually very much more obvious than in that of the stem. The outermost is the calyptrogen, which gives rise to the root cap, and in Dicotyledons to the piliferous layer as well. The pcriblem, one cell thick at the apex, produces the cortex, to which the piliferous layer belongs in Monocotyledons ; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular cylinder. In a few cases the boundaries of the different layers are not traceable. The protoxylems and the phloem strands are developed alternately, just within the outer imit of the young cylinder. The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed. Differentiation is very much more rapid—i.e., the tissues are completely formed much nearer to the apex than is the case in the stem. This is owing to the elongating region (in which protoxylem and protophloem alone are differentiated) being very much shorter than in the stem. The root hairs grow out from the cells of the piliferous layer immediately behind the elongating region. The branches of the stem arise by multiplication of the cells of the epidermis and cortex at a given spot, giving rise to a protuberance, at the end of which an apical meristem is established. The vascular system is connected in various ways with that of the parent axis by the differentiation of bundle-connexions across the cortex of the latter. This is known as exogenous branch-formation. In the root, on the other hand, the origin of branches is endogenous. The cells of the pericycle, usually opposite a protoxylem strand, divide tangentially and give rise to a new growing-point. The new root thus laid down burrow's through the cortex of the mother-root and finally emerges into the soil. The connexions of its stele with that of the parent axis are made across the pericycle of the latter. Its cortex is never in connexion with the cortex of the parent, but with its pericycle. Adventitious roots, arising from the stems, usually take origin in the pericycle, but sometimes from other parts of the conjunctive. In most of the existing Pteridophytes, in the Monocotyledons, and in annual plants among the Dicotyledons, Seconda ^ere ^ n0 furt^er growth of much structural tissues? importance in the tissues after differentiation from the primary meristems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs, and in fossil Pteridophytes belonging to all the great groups, certain layers of cells remain meristematic among the permanent tissues, or after passing through a resting stage reacquire meristematic properties, and give rise to secondary tissues. Such meristematic layers are called secondary meristems. There are two chief secondary meristems, the cambium and thephellogen. The formation of secondary tissues is characteristic of most woody plants, to whatever class they belong. Every great group or phylum of vascular plants, when it has become dominant in the vegetation of the world, has produced members with the tree habit arising by the forma-
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tion of a thick woody trunk, in most cases by the activity of a cambium. The cambium in the typical case, which is by far the most frequent, continues the primary differentiation of xylem and phloem in the desmogen strand (see above), or arises in the resting mesodesm or mesocycle and adds new (secondary) xylem and phloem to the primary tissues. New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and the phloem. A tissue-mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to—(1) a tracheid ; (2) a segment of a vessel; (3) a xylem-fibre ; or (4) a vertical file of xylem-parenchyma cells. In the last case the mother cell divides by a number of horizontal walls. A tissue-mother-cell of the phloem may give rise to—(1) a segment of a sieve-tube with its companion cell or cells ; (2) a phloem fibre ; (3) a single phloem-parenchyma (cambiform) cell, or a vertical file of short parenchyma cells. At certain points the cambium does not give rise to xylem and phloem elements, but cuts off cells on both sides which elongate radially and divide by horizontal walls. When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one straight line through the xylem and phloem. Such a cellplate is called a medullary ray. It is essentially a living tissue, and serves to place all the living cells of the secondary vascular tissues in communication. It conducts plastic substances inwards from the cortex, and its cells are frequently full of starch, which they store in winter. They are accompanied by intercellular channels serving for the conduction of oxygen to, and carbon dioxide from, the living cells in the interior of the w'ood, which would otherwise be cut off from the means of respiration. The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether. The parenchyma is often arranged in tangential bands between the layers of sievetubes and tracheal elements. The xylem parenchyma is often found in strands associated with the tracheal elements. These strands are not isolated, but form a connected network through the wood. The xylem parenchyma cells are connected, as are the medullary ray cells, with the tracheal elements by one-sided bordered pits—i.e., pits with a border on the tracheal element side, and simple on the parenchyma cell side. The fibres are frequently found in tangential bands between similar bands of tracheee or sieve-tubes. "The fibrous bands are generally formed towards the end of the year’s growth in thickness. The fibres belong to the same morphological category as the parenchyma, various transitions being found between them ; thus there may be thin-walled cells of the shape of fibres, or ordinary fibres may be divided into a number of superposed cells. These intermediate cells, like the ordinary parenchyma, frequently store starch, and the fibres themselves, though usually dead, sometimes retain their protoplasm, and may also be used for starch accumulations. The vessels and tracheids are very various in size, shape, and structure in different plants. They are nearly always aggregated in strands, which, like those of the parenchyma, are not isolated, but connected with one another. In a few cases some of the tracheids have very thick walls and reduced cavities, functioning as mechanical rather than water-conducting elements. All transitions are found between such forms and typical tracheids. These fibre-tracheids are easily confused on superficial view with the true wood-fibres belonging to the parenchymatous system ; but their pits are always bordered, though in the extreme type they are reduced to mere slits in the wall. The sieve-tubes of the secondary phloem usually have very oblique end-walls bearing a row of sieve-plates ; plates also occur on the radial side-walls. The tissue-elements just described are found only in the more complicated secondary vascular tissues of certain Dicotyledons. A considerable evolution in complexity can be traced in passing from the simplest forms of xylem and phloem found in the primary vascular tissues both among Pteridophytes and Phanerogams to these highly differentiated types. In the simplest condition we have merely tracheae and sieve-tubes, respectively associated with parenchyma, which in the former case is usually amylom, and in the latter consists of pfoteid cells. This type is found in nearly all Pteridophytes and, so far as is known, in Cycadofilices, both in primary and secondary tissue. The stereom is furnished either by cortical cells or by the tracheal elements, in a few cases by fibres which are probably homologous with sievetubes. Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube. In other cases the proteid cells of the secondary
