Page:EB1911 - Volume 21.djvu/765

Rh supply. The stomata serve for all gaseous interchange between the plant and the surrounding air. The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage. Sometimes the epidermis is considerably more developed by tangential division of its cells, forming a many-layered water-tissue. This is found especially in plants which during certain hours of the day are unable to cover the water lost through transpiration by the supply coming from the roots. The water stored in such a time supplies the immediate need of the transpiring cells and prevents the injury which would result from their excessive depletion.

The epidermis of a very large number of species bears hairs of various kinds. The simplest type consists simply of a single elongated cell projecting above the general level of the epidermis. Other hairs consist of a chain of cells; others, again, are branched in various ways, while yet others have

the form of a flat plate of cells placed parallel to the leaf surface and inserted on a stalk. The cells of hairs may have living contents or they may simply contain air. A very common function of hairs is to diminish transpiration, by creating a still atmosphere between them, as in the case of the sunk stomata already mentioned. But hairs have a variety of other functions. They may, for instance, be glandular or stinging, as in the common stinging nettle, where the top of the hair is very brittle, easily breaking off when touched. The sharp, broken end penetrates the skin, and into the slight wound thus formed the formic acid contained by the hair is injected.

Mention may be made here of a class of epidermal organ, the hydathodes, the wide distribution and variety of which have been revealed by recent research. These are special organs, usually situated on foliage leaves, for the excretion of water in liquid form when transpiration is diminished so that the

pressure in the water-channels of the plant has come to exceed a certain limit. They are widely distributed, but are particularly abundant in certain tropical climates where active root absorption goes on while the air is nearly saturated with water vapour. In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connexion with the vascular system. The cells concerned, like all secreting organs, have abundant protoplasm with large nuclei, and sometimes, in addition, part of the cell-wall is modified as a filter. In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem. The epithem is penetrated by a network of fine intercellular spaces, which are normally filled with water and debouch on one or more intercellular cavities below the epidermis. Above each cavity is situated a so-called water-stoma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guard-cells which have nearly or entirely lost the power of movement. The pores of the water-stomata are the outlets of the hydathode. The epithem is frequently surrounded by a sheath of cuticularized cells. In other cases the epithem may be absent altogether, the tracheal strand debouch in directly on the lacunae of the mesophyll. This last type of hydathode is usually situated on the edge of the leaf. Some hydathodes are active glands, secreting the water they expel from the leaf. [Many other types of glands also exist, either in connexion with the epidermis or not, such as nectaries, digestive glands, oil, resin and mucilage glands, &c. They serve the most various purposes in the life of the plant, but they are not of significance in relation to the primary vital activities, and cannot be dealt with in the limits of the present article.] The typical epidermis of the shoot of a land plant does not absorb water, but some plants living in situations where they cannot depend on a regular supply from the roots (e.g. epiphytic plants and desert plants) have absorptive hairs or scales on the leaf epidermis through which rain and dew can be absorbed. Some hydathodes also are capable of absorbing as well as excreting water.

The surface layer of the root, sometimes included under the term epidermis, is fundamentally different from the epidermis of the stem. In correspondence with its water-absorbing function it is not cuticularized, but remains usually thin-walled; the absorbing surface is increased by its cells

being produced into delicate tubes which curl round and adhere firmly to particles of soil, thus at once fixing the root firmly in the soil, and enabling the hair to absorb readily the thin films of water ordinarily surrounding the particles (fig. 1, U). The root-hair ends blindly and is simply an outgrowth from a surface cell, having no cross-walls. It corresponds in function with the rhizoid of a Bryophyte. At the apex of a root, covering and protecting the delicate tissue of the growing point, is a special root-cap consisting of a number of layers of tissue whose cells break down into mucilage towards the outer surface, thus facilitating the passage of the apex as it is pushed between the particles of soil. 2.—Transverse Sections of Leaves.

A, Dorsiventral leaf&emsp;&emsp;B, Isobilateral leaf.

ep, epidermis; st, stoma; mes, mesophyll; pal, palisade; spo, spongy tissue; i.sp, intercellular space; w.t., water tissue; x, xylem; ph, phloem; phlt, phloeoterma; scl, sclerenchyma. The cortex, as has been said, is in its origin the remains of the primitive assimilating tissue of the plant, after differentiation of the surface layer and the conducting system. It consists primitive of typical living parenchyma; but its differentiation may be extremely varied, since in the complex

bodies of the higher plants its functions are numerous. In all green plants which have a special protective epidermis, the cortex of the shoot has to perform the primitive fundamental function of carbon assimilation. In the leaf shoot this function is mainly localized in the cortical tissue of the leaves, known as mesophyll, which is essentially a parenchymatous tissue containing chloroplasts, and is penetrated by a system of intercellular spaces so that the surfaces of the assimilating cells are brought into contact with air to as large an extent as possible, in order to facilitate gaseous interchange between the assimilating cells and the atmosphere. At the same time the cells of the mesophyll are transpiring cells—i. e. the evaporation of water from the leaf goes on from them into the intercellular spaces. The only pathways for the gases which thus pass between the cells of the mesophyll and the outside air are the stomata. A land plant has nearly always to protect itself against over-transpiration, and for this reason the stomata of the typical dorsiventral leaf (fig. 2, A), which has distinct upper and lower faces, are placed mainly or exclusively on the lower side of the leaf, where the water vapour that escapes from them, being lighter than air, cannot pass away from the surface of the leaf, but remains in contact with it and thus tends to check further transpiration. The stomata are in direct communication with the ample system of intercellular spaces which is found in the loosely arranged mesophyll (spongy tissue) on that side. This is the main transpiring tissue, and is protected from direct illumination and consequent too great evaporation. The main assimilating tissue, on the other hand, is under the upper epidermis, where it is well illuminated, and consists of oblong cells densely packed with chloroplasts and with their long axes perpendicular to the surface (palisade tissue). The intercellular spaces are here very narrow channels between the palisade cells. Leaves whose blades are normally held in a vertical position possess palisade tissue and stomata on both sides (isobilateral leaves) (fig. 2, B), since there is no difference in the illumination and other external conditions, while those which are cylindrical or of similar shape (centric leaves) have it all round. The leaves of shade plants have little or no differentiation of palisade tissue. In fleshy leaves which contain a great bulk of tissue in relation to their chlorophyll content, the central mesophyll contains little or no chlorophyll and acts as water-storage tissue. The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function. It also always possesses a well-developed lacunar system communicating with the external air through stomata (in the young stem) or lenticels (see below). This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through which air can pass to the deeper lying tissues. The cortex of the older stem of the root frequently acts as a reserve store-house for food, which generally takes the form of starch, and it also assists largely in providing the stereom of the plant. In the leaf-blade this sometimes appears as a layer of thickened subepidermal cells, the hypoderm, often also as subepidermal bundles of sclerenchymatous fibres, or as similar bundles extending right across the leaf from one epidermis to the other and thus acting as struts. Isolated cells (idioblasts), thickened in various ways, are not uncommonly found supporting the tissues of the leaf. In the larger veins of the leaf, especially in the midrib, in the petiole, and in the young stem, an extremely frequent type of mechanical tissue is collenchyma. This consists of elongated cells with cellulose walls, which are locally thickened along the original corners of the cells, reducing the lumen to a cylinder, so that a number of vertical pillars of cellulose connected by comparatively thin walls form the framework of the tissue. his tissue remains living and is usually formed quite early, just below the epidermis, where it provides the first peripheral support for a still growing stem or petiole. Sclerenchyma may be formed later in various positions in the cortex, according to local needs. Scattered single stereids or bundles of fibres are not uncommon in the cortex of the root.