The whole cylinder is enclosed by the peculiarly differentiated innermost cell-layer of the cortex, known as the endodermis.
The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function.
The inner layer of the cortex (phloeoterma) may form a well-marked endodermis, or differ in other ways from the rest of the cortex.
In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.
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 largel) in providing the stereom of the plant.
Sclerenchyma may bi formed later in various positions in the cortex, according to loca needs.
Scattered single stereids or bundles of fibres are no imnrornmnn in the rnrtev of the root The innermost layer of the cortex, abutting on the central cylinder of the stem or on the bundles of the leaves, is called the jthloeoterma, and is often differentiated.
Thi vascular system is connected in various ways with that of th(parent axis by the differentiation of bundle-connections across thi cortex of the latter.
G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed).
In a few cases the hydrom strand is continued into the cortex of the stem as a leaf-trace bundle (the anatomically demonstrable trace of the leaf in the stem).
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 other cases it does not differ histologically from the parenchyma of the rest of the cortex, though it is often distinguished by containing particularly abundant starch, in which case it is known as a starch sheath.
The centre of the S~hooo- stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith ~ together being classed as ground tissue.
They can be traced upwards from any given point till they are found to pass out of the cylinder, travel through the cortex of the stem and enter a leaf.
In some cases this individualization is carried ftirther, the cortex and pith becoming continuous between the bundles which appear as isolated strands em- Aberrant bedded in a general \, L.~/ ~ Typesof ground-tissue.
In both stem and root early walli separate the cortex from the stele.
This may have a radial stem-like organization, a central cell-thread giving off from every side a number of short sometimes unicellular branches, which together form a cortex round the central thread, the whole structure having a cylindrical form which only branches when one of the short cell-branches from the central thread grows out beyond the general surface and forms in its turn a new central thread, from whose cells arise new short branches.
This bundle is continued down into the cortex of the stem as a leaf-trace, and passing very slowly through the sclernchymatous external cortex and the parenchymatous, starchy internal cortex to join the central cylinder.
Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma.
The stereom of the moss is found mainly in the outer cortex of the stem and in the midrib of the leaf.
Special tissues (stereom) may be developed for this purpose in the cortex, or in immediate connection.
In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodernlis, appears in the centre.
Sieve-tubes with accompanying corn- diameter of the stem, the cortex sd.p~cierizedperid~sm.
2o.Laticiferous vessels from the cortex of the root Scoyzonera hispanica, tangential secf ion.
The initials of the cortex and central cylinder Whether these art always in layers which remain separate is not known, but it is certair that in many cases such layers cannot be distinguished.
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, a difference produced by the stopping of transverse and the increased frequency of longitudinal divisions.
The periblem, 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.
This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through whicF air can pass to the deeper lying tissues.
The ne~ root thus laid down burrows through the cortex of the mother-root and finally emerges into the soil.
The connections of its stele witl that of the parent axis are made across the pericycle of the latter Its cortex is never in connection with the cortex of the parent, but with its pericycle.
Its most usual seat of origin in the stem is the external layer of the cortex immediately below the epidermis; in the root, the pericycle.
The whole of the cortex, stereom and parenchyma alike, is commonly living, and its cells often contain starch.
The cortex, as has been said, is in its origin the remains of th~ primitive assimilating tissue of the plant, after differentiatioi of the surface layer and the conducting system.
The branches of the stem arise by multiplication of the cells 01 the epidermis and cortex at a given spot, giving rise to a protuber ance, at the end of which an apical meristem is established.
It conducts plastic substances inwards from the cortex, and its cells are frequently full of starch, which they store in winter.
Periderm; c, cortex; ph, phlocni with alternating strands of fibres, sieve-tubes and parenchyma; ~r.r, principal ray; Sr., subordinate rays; ca, cambium.
The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem.
An ordinary cambium is scarcely ever found in the Monocotyledons, but in certain woody forms a secondary meristem is formed outside the primary bundles, and gives rise externally to a little secondary cortex, and internally to a secondary parenchyma in which are developed numerous zones of additional bundles, usually of concentric structure, with phloem surrounded by xylem.
In the epidermis itself (rarely), in any layer of the cortex, or in the pericycle.
In the former case the formation of phelloderm is trivial in amount; in the latter, considerable, since this tissue has to replace the cast-off cortex, as a metabolic and particularly a storage tissue.
The formation and gradually increasing thickness of its bark are explained by the continually increasing need of adequate protection to the living cortex, under the strain of the increasing framework which the enormous multiplication of its living protoplasts demands, and the development of which leads to continual rupture of the exterior.
This pressure of the turgid cortex on the central stele is known as root pressure, and is of very considerable amount.
These bodies, known technically as chioroplaIts, are found embedded in the protoplasm of the cells of the mesophyll of foliage leaves, of certain of the cells of some of the leaves of the flower, and of the cortex of the young twigs and petioles.
Some make their way through the cells of the outer part of the cortex towards the root-tip, and form a mycelium or feltwork of hyphae, which generally occupies two or three layers of cells.
From this branches pass into the middle region of the cortex and ramify through the interior half of its cells.
One of these hairs can be seen to be penetrated at a particular spot, and the entering body is then found to grow along the length of the hair till it reaches the cortex of the root.
If a piece of bark and cortex are torn off, the occlusion takes longer, because the tissues have to creep over the exposed area of wood; and the same is true of a transverse cut severing the branch, as may be seen in any properly pruned tree.
Cecidia or galls arise by the hypertrophy of the subepidermal cells of a leaf, cortex, &c., which has been pierced by theovipositor of an insect, and in which the egg is deposited.
Frost blisters are pustular swellings due to the up-growth of callus-tissue into cavities caused by the uprising of the superficial cortex under the action of intense cold.
The usual necrosis of the injured cortex occursdrying up, shrivelling, and consequent stretching and cracking of the dead cortex on the wood beneath.
There is very little grey matter in the cortex of the hemispheres, the surface of which is devoid of convolutions, mostly quite smooth; in others, for instance pigeons, fowls and birds of prey, a very slight furrow might be compared with the Sylvian fissure.
The midbrain is represented chiefly by the optic lobes, the cortex of which alone is homologous with the corpora quadragemina of the mammals.
The localization of function in the cerebral and in the cerebellar cortex has doubtless been the main cause of this progress, and has proceeded poi passu with an extended insight into the structure and connexions of the parts concerned.
The cerebral cortex, and, more definitely, the cortical elements (nerve cells), formed the seat of the activity of the soul, and were ordered into departments according to various functions.
The spores germinate on a damp surface and enter the cortex through small cracks or wounds in the protecting layer.
The cylindrical branches of the fruticose forms are usually radially symmetrical, but the flattened branches of these forms and also the thalli of the foliaceous form show a difference in the cortex of the upper and lower side.
All the longest routes thus formed traverse late in their course the cortex of the cerebral hemisphere.