noun
definition
The outer layer of an internal organ or body structure, such as the kidney or the brain.
definition
The tissue of a stem or root that lies inward from the epidermis, but exterior to the vascular tissue.
Examples of cortex in a Sentence
The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function.
The whole cylinder is enclosed by the peculiarly differentiated innermost cell-layer of the cortex, known as the endodermis.
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 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.
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.
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.
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.
In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.
G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed).
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.
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.
In both stem and root early walli separate the cortex from the stele.
The primary cortex between stele and periderm has perished.
This passes gradually into the thinner-walled parenchyma of the inner cortex.
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.
In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex.
When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex.
Outermost layer of root-cap. c. Wall marking the inner limit of the outer cortex.
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.
Multitudes of such hairs on the branches of the roots cause the entry of great quantities of water, which by a subsequent similar osmotic action accumulates in the cortex of the roots.
It is this relatively huge development of cortex cerebri which is the pre-eminent structural character of man.
In the lower animal forms there is no such nervous structure at all as the cortex cerebri.
Larger and thicker in the rabbit, when excited it gives rise in that animal to movements of the eyes and of the fore-limbs and neck; but it is only in much higher types, such as the dog, that the cortex yields, under experimental excitation, definitely localized foci, whence can be evoked movements of the fore-limb, hind-limb, neck, eyes, ears and face.
Experiment shows that in the manlike (anthropoid) apes the differentiation of the foci or "centres" of movement in the motor field of the cortex is even more minute.
The movement evoked from a point of cortex is not always the same; its character is determined by movements evoked from neighbouring points of cortex immediately antecedently.
The movements obtained by point-to-point excitation of the cortex are often evidently imperfect as compared with natural movements - that is, are only portions of complete normal movements.
Thus among the tongue movements evoked by stigmatic stimulation of the cortex undeviated protrusion or retraction of the organ is not found.
Again, from different points of the cortex the assunIption of the requisite positions of the tongue, lips, cheeks, palate and epiglottis, as components in the act of sucking, can be provoked singly.
Again, no single point in the cortex evokes the act of ocular convergence and fixation.
The motor field of the cortex is, taken altogether, relatively to the size of the lower parts of the brain, larger in the anthropoid than in the inferior monkey brains.
The excitable foci of the cerebral cortex are well myelinated long before the unexcitable are so.
The areas of intervening cortex, arriving at structural completion later than the above sense-spheres, are called by some association-spheres, to indicate the view that they contain the neural mechanisms of reactions (some have said "ideas") associated with the sense perceptions elaborated in the several sensese spheres.
The distinction, therefore, between the movement of the eyeballs, elicited from the occipital (visual) cortex, and that of the hand, elicited from the cortex in the region of the central sulcus (somaesthetic), is not a difference between motor and sensory, for both are sensori-motor in the nature of their reactions; the difference is only a difference between the kind of sense and sense-organ in the two cases, the muscular apparatus in each case being an appanage of the sensual.
In the dog it has been proved that after removal from the animal of every vestige of its cortex cerebri, it still executes habitual acts of great motor complexity requiring extraordinarily delicate adjustment of muscular contraction.
In man, actually gross sensory defects follow even limited lesions of the cortex.
The motor field, therefore, though absolutely larger, forms a smaller fraction of the whole cortex of the brain than in the lower forms. The statement that in the anthropoid (orang-outan) brain the groups of foci in the motor fields of the cortex are themselves separated one from another by surrounding inexcitable cortex, has been made and was one of great interest, but has not been confirmed by subsequent observat'on.
When this differentiation of cortex, with its highest expression in man, is collated with the development of the cortex as studied in the successive phases of its growth and ripening in the human infant, a suggestive analogy is obvious.
In the larger of the two great groups into which the Angiosperms are divided, the Dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex.
The fertilized egg charged with food reserves rests for a considerable period, surrounded by its cortex, the whole having assumed a reddish-brown colour.
Much damage is often caused by species of Peridermium, which often invade the cortex and cambium to such an extent as to " ring " the stem or branch, or to cause an abnormal formation of turpentine which soaks into the wood and stops the upward passage of water; this causes the parts above the diseased area to perish.
In man, both size and complexity contribute to the increased area of the cortex or outer layer of the brain, which has been fully ascertained to be the seat of the mysterious processes by which sensation furnishes the groundwork of thought.
A leaf-trace, as it passes through the cortex, has a collateral structure, the protoxylem being situated at the inner edge of the xylem; when it reaches the leaf-base the position of the spiral tracheids is gradually altered, and the endarch arrangement (protoxylem internal) gives place to a mesarch structure (protoxylem more or less central and not on the edge of the xylem strand).
Secretory sacs occur abundantly in the leaflamina, where they appear as short lines between the veins; they are abundant also in the cortex and pith of the shoot, in the fleshy integument of the ovule, and elsewhere.
The secondary phloem contains numerous thick-walled fibres, parenchymatous cells, and large sieve-tubes with plates on the radial walls; swollen parenchymatous cells containing crystals are commonly met with in the cortex, pith and medullary-ray tissues.