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ANATOMY

OF PLANTS assimilating cells to other parts of the plant. Rarely in the leaf, root-cap consisting of a number of layers of tissue whose cells break down into mucilage towards the outer surface, thus fairly frequently in the stem (particularly in Pteridophytes), and facilitating the passage of the apex as it is pushed between the universally in the root, the phloeoterma is developed as an endodermis (see below). In other cases, it does not differ histologiparticles of soil. The cortex, as has been said, is in its origin the remains of the cally from the parenchyma of the rest of the cortex, though it is primitive assimilating tissue of the plant, after differentiation often distinguished by containing particularly abundant starch, ofnsists the surface layer and conducting system.living It in which case it is known as a starch sheath. Cortex. CO primitively and the mainly of typical One of the most striking characters common to the two highest parenchyma ; but its differentiation may be extremely varied, groups of plants, the Pteridophytes and Phanerogams, is the since in the complex body of the higher plants its functions are possession of a double (hydrom-leptom) conducting very various. In all green plants, where there is a special protec- system, such as we saw among the highest mosses, Vascular tive epidermis, the cortex of the shoot has to perform the but with sharply characterized and peculiar features, system. primitive fundamental function of carbon assimilation. In the almost certainly indicating common descent throughout leafy shoot this is mainly localized in the cortical tissue of the both these groups. It is confined to the sporophyte, which forms leaves, known as mesophyll, which is essentially a parenchy- the leafy-plant in these groups and is known as the vascular matous tissue containing chloroplasts, and penetrated by a system. Associated with it are other tissues, consisting primilacunar system so that the surfaces of the assimilating cells are tively of parenchyma, mainly starchy, and in the Phanerogams brought into contact with air to as large an extent as possible particularly, of special stereom. The whole tissue system is in order to facilitate gaseous interchange between the assimilating known as the Stelar system. It has no direct phylogenetic concells and the atmosphere. At the same time the cells of the nexion with that of the mosses. The origin of the Pteridophyta mesophyll are transpiring cells—i.e., the evaporation of water {q.v.) is very obscure, but it may be regarded as certain that from the leaf goes on from them into the intercellular spaces. it is not to be sought among the mosses, which are an extremely The only pathways for the gases which thus pass between the specialized and peculiarly differentiated group. Furthermore, cells of the mesophyll and the outside air are the stomata. A both the hydrom and leptom of Pteridophytes have marked typical land plant has always to protect itself against over-tran- peculiarities to which no parallel is to be found among the spiration, and for this reason the stomata are placed mainly or Bryophytes. Hence we must conclude that the conducting exclusively on the lower side of the leaf, where the water-vapour system of the Pteridophytes has had an entirely separate evolution. that escapes from them, being lighter than air, cannot rise away All the surviving forms, however, have a completely established from the surface of the leaf, but remains in contact with it and double system with the specific characters alluded to, and since thus tends to check further transpiration. The stomata are in there is every reason to believe that the conditions of evolution direct communication with the richest lacunar system, which is of the primitive Pteridophyte must have been essentially similar found in the loosely arranged mesophyll {spongy tissue) on that to those of the Bryophytes, the various stages in the side. This is the main transpiring tissue, and is protected from evolution of the conducting system of the latter are direct ilhrmination and consequent too great evaporation. The main very useful to compare with the arrangements met assimilating tissue, on the other hand, is under the upper epidermis, with in the former. The hydroid of a Pteridophyte or of a where it is well illuminated, and consists of oblong cells densely Phanerogam is characteristically a dead, usually elongated, cell conpacked with chloroplasts and with their long axes perpendicular taining air and water, and either thin-walled with lignified spiral to the surface {palisade tissue). The intercellular spaces are here or annular thickenings, or with thick lignified walls, incompletely fewer and narrower. The whole lacunar system thus forms a perforated by pits (usually bordered pits) of various shapes. kind of funnel, with its narrow end in the palisade and its wide When a number of such cells, called tracheids, placed end to end under the stomata, so that the double necessity for the limi- end, have open communication with one another, the resulting celltation of transpiration and the illumination of the palisade cells fusion is called a vessel. Vessels are very rare among Pterilessens the amount of carbon dioxide which can reach the latter. dophytes, though common among Phanerogams. The tracheids Leaves whose blades are placed vertically possess palisade tissue or vessels, indifferently called tracheal elements, together with and stomata on both sides {isobilateral leaves), since there is no the immediately associated cells (usually amylom in Pteridifference in the illumination, while those which are cylindrical dophytes) constitute the xylern of the plant. This is a morphoor of similar shape {centric leaves) have it all round. The leaves logical term given to the particular type of hydrom found in both of shade plants have little or no differentiation of palisade. In Pteridophytes and Phanerogams, together with the parenchyma fleshy leaves which have little or no chlorophyll, the central or stereom, or both, included within the boundaries of the hydrom mesophyll is abundant and acts as water-storage tissue. The tissue strand. The leptoid of a Pteridophyte is also an elongated cortex of a young stem is usually green, and plays a more or less cell, with a thin lining of protoplasm, but destitute of a nucleus, important part in the assimilative function. It also always and always in communication with the next cell of the leptom possesses a well-developed lacunar system communicating with strand by relatively large perforations (in Pteridophytes often not the external air through stomata (in the young stem) or lenlicels easily demonstrable), through which pass strings of protoplasm. (see below). This lacunar system not only enables the cells of These are often converted into a peculiar substance called callose, the cortex itself to respire, but also forms channels through which is also frequently formed over the surface of the perforated, which air can pass to the deeper-lying tissues. The cortex of the often extremely oblique, end-walls. The structure formed by a older stem and of the root frequently acts as a reserve store-house number of such cells placed end to end is called a sieve-tube for food, which generally takes the form of starch, and it also (obviously comparable with a xylem-vessel), and the end-wall or assists largely in providing the stereom of the plant. In the area of end-wall occupied by a group of perforations, a sieve-plate leaf-blade this often appears as a layer of thickened subepidermal (see Cytology). The sieve-tubes, with their accompanying parencells, the hypoderm, often also as subepidermal bundles of chyma or stereom, constitute the tissue called phloem. This is sclerenchymatous fibres or of similar bundles extending right likewise the term for a morphologically defined tissue system, i. e. across the leaf from one epidermis to the other, and thus acting the leptom found in Pteridophytes and Phanerogams with its assoas struts. Isolated cells {idioblasts), thickened in various ways, ciated cells, and is entirely parallel with the xylem. The sieveare not uncommonly found supporting the tissues of the leaf. In tubes differ, however, from the tracheids in being immediately assothe larger veins of the leaf, especially the midrib, in the petiole ciated, apparently constantly, not with starchy parenchyma, but and in the young stem, an extremely frequent type of mechanical with parenchymatous cells containing particularly abundant tissue is collenchyma. This consists of elongated cells with cellu- proteid contents which seem to have a function intimately conlose walls, which are locally thickened along the original corners of nected with the conducting function of the sieve-tubes, and which the cells, reducing the lumen to a cylinder, so that a number of we may call proteid-cells. vertical pillars of cellulose connected by comparatively thin walls The xylem and phloem are nearly always found in close assoform the framework of the tissue. This tissue remains living and ciation in strands of various shapes in all the three main organs is usually formed quite early just below the epidermis, where it of the sporophyte—root, stem, and leaf—and form a. provides the first peripheral support for a still growing stem or connected tissue-system running through the whole ^" petiole. Sclerenchyma may be formed later in various positions body. In the primary axis of the plant among Pterijs . in the cortex, according to local needs. Scattered single stereids dophytes and many Phanerogams, at any rate in its straa( central or bundles of fibres are not uncommon in the cortex of the root. first formed part, the xylem and phloem are associated .. . The innermost layer of the cortex, abutting on the central in the form of a cylinder, with xylem occupying the cy e. ? cylinder of the stem or on the bundles of the leaves, is called the centre, and the phloem (in the upward growing part monos e e* phloeoterma, and is often differentiated. In the leaf- or primary stem) forming a mantle at the periphery (Fig. 3). In Phloeo- blade it takes the form of special parenchymatous the downward growing part of the axis (primary root), however, terma. sheaths to the bundles. The cells of these sheaths are the peripheral mantle of phloem is interrupted, the xylem coming often distinguished from the rest of the mesophyll by to the surface of the cylinder along (usually) two or (sometimes) containing little or no chlorophyll. Occasionally, however, they more vertical lines. Such an arrangement of vascular tissue is are particularly rich in chloroplasts. These bundle sheaths are called radial, and is characteristic of all roots (Figs. 2 and 9). important in the conduction of carbohydrates away from the The cylinder is surrounded by a mantle of one or more layers of