Anatomy of Plants

The Cortex.

The cortex consists primarily of living paren chyma, but its differentiation may be extremely varied, and in the complex bodies of vascular plants its functions are varied and may be modified. The cortex of a young stem is usually green as its cells possess chloroplasts, and a fine system of inter cellular spaces communicates with the external air through stomata. With age the inner cortical layers may lose their chlorophyll and become storage tissue, and the outer layers may be extensively modified (see p. 1o, secondary tissues). In many pteridophytes (e.g., ferns) the mature outer cortical layers of the stem and root are hard sclerenchyma with thick brown walls, but in flowering plants, while the stems are still slender, the cells of the outer cortical layers elongate and become cylindrical so that their intercellular spaces are widened. Vertical bands of cellulose thicken those regions of the walls which are in contact with the intercellular spaces, but the remaining regions of the walls usually remain thin. Such a tissue is known as collenchyma. It provides peripheral support for the growing stem, and as its cells are alive, is open to later modifications with changing needs in stems which undergo annual increase in girth. The wonderful recovery, after bending and stretching, which is often observable in herbaceous stems, is in large measure due to the effective dis tribution of this elastic collenchyma. On the other hand scleren chyma is commonly dead and provides a more rigid but usually unmodifiable supporting tissue. Its adequacy will he evident for stems such as those of modern ferns which do not increase annually in girth. Less commonly, scattered cells or cell-groups which increase little in length may become stone cells, with irregularly but inordinately thickened hard walls, while in some cases longitudinal rows of cells disintegrate thus forming canals, such as resin canals, the cortical wall-cells of which are excretory. Many such minor modifications of the original cortex of the stem occur in association with special localized functions.

Phloeotherma.—The innermost layer of the living cortex in vesting the central cylinder of the stem is, however, often early and peculiarly modified as a continuous cylinder called the phloeotherma. Throughout its entire course its cell-walls remain in close contact so that the system of intercellular spaces in the cortex is not continued into the central cylinder of conductive tissue. Towards the upper limit of the phloeothermal cylinder, while the stern is still young, the component cells may be rich in plastids in which starch grains are formed. In some localities the phloeotherma is known as a starch sheath, which is variously thought to play some role in the controlled conduction of carbo hydrates from the assimilatory cortical cells to the vascular cylinder, or in some connection with the directional growth of the young stem. In the more mature regions of the stem starch grains are absent from the phloeothermal cells, the walls of which are somewhat thickened. In particular, a band of the lateral longi

tudinal and of the end walls is modified, being impregnated with a fatty substance which is thought to prevent radial transfer of foods in solution athwart the phloeotherma save under proto plasmic control of its cells. The phloeotherma is here known as an endodermis, and is thought to serve as a physiological barrier between the cortex and the central conductive cylinder.

Mesophyll.—The cortex of the leaf is primarily concerned with the primitive fundamental function of carbon assimilation, and to the active performance of this function the maximum exposure of living parenchymatous tissue containing chloroplasts is neces sary. Thus the cortex of the leaf is essentially a green expanse of thin-walled parenchyma, known as the mesophyll, and is pene trated by a large and elaborate system of intercellular spaces which serve at one and the same time for the promotion of tran spiration and respiration. The pathways for the gases which thus pass between the mesophyll and the outer air are the stomata, which in land plants with dorsiventral leaves are mainly or exclusively placed on the lower leaf-surface. By this means over transpiration tends to be avoided, since the liberated water vapour, being lighter than air, tends to remain in contact with the lower leaf-surface and checks evaporation. The stomata are in direct communication with the ample system of intercellular spaces in the mesophyll, the lower layers of which are commonly loosely arranged as spongy tissue with extensive intercellular spaces. This is the main transpiring tissue of the leaf, and in it chloroplasts may be comparatively few. It is protected from direct illumination and is at the same time liable to over-evapora tion. The main assimilating tissue, on the other hand, is under the upper epidermis, is well illuminated, its cells are densely packed with chloroplasts, and are commonly oblong with their long axes perpendicular to the leaf-surface. The intercellular spaces are here narrow as between sticks in a bundle, and are in open communication with the spaces in the spongy tissue below. By reason of its resemblance to the boards in a fence, when viewed in a transverse section of the leaf, this portion of the mesophyll is called the palisade tissue. Leaves with blades held in a vertical position commonly possess palisade tissue on both sides or have little or no distinction in the form and arrangement of the cells of the mesophyll, since there is no differ ence in the illumination or other external conditions, while cyl indrical or approximately cylindrical leaves may have palisade tissue all round. The leaves of shade plants have little or no differentiation of the palisade tissue. In fleshy leaves, which con tain a great bulk of tissue in relation to their chlorophyll content, the central mesophyll contains little or no chlorophyll and may consist of water-storage tissue, while locally a layer or mass of sub-epidermal cells may be thick-walled and fibrous.