cortex(redirected from cortex of thymus)
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cortex,in botany, term generally applied to the outer soft tissues of the leaves, stems, and roots of plants. Cortical cells of the leaves and outer layers of nonwoody stems contain chloroplasts, and are modified for food storage (usually in the form of starch) in roots and the inner layers of stems and seeds. Because of the combination of its soft texture (especially after cooking) and its role as a food storage tissue, the cortex is the predominant plant tissue eaten by humans and other animals.
The mass of primary tissue in roots and stems extending inward from the epidermis to the phloem. The cortex may consist of one or a combination of three major tissues: parenchyma, collenchyma, and sclerenchyma. In roots the cortex almost always consists of parenchyma, and is bounded, more or less distinctly, by the hypodermis (exodermis) on the periphery and by the endodermis on the inside.
Cortical parenchyma is composed of loosely arranged thin-walled living cells. Prominent intercellular spaces usually occur in this tissue. In stems the cells of the outer parenchyma may appear green due to the presence of chloroplasts in the cells (see illustration). This green tissue is sometimes called chlorenchyma, and it is probable that photosynthesis takes place in it.
In some species the cells of the outer cortex are modified in aerial stems by deposition of hemicellulose as an additional wall substance, especially in the corners or angles of the cells. This tissue is called collenchyma, and the thickening of the cell walls gives mechanical support to the shoot.
The cortex makes up a considerable proportion of the volume of the root, particularly in young roots, where it functions in the transport of water and ions from the epidermis to the vascular (xylem and phloem) tissues. In older roots it functions primarily as a storage tissue.
In addition to being supportive and protective, the cortex functions in the synthesis and localization of many chemical substances; it is one of the most fundamental storage tissues in the plant. The kinds of cortical cells specialized with regard to storage and synthesis are numerous.
Because the living protoplasts of the cortex are so highly specialized, patterns and gradients of many substances occur within the cortex, including starch, tannins, glucosides, organic acids, crystals of many kinds, and alkaloids. Oil cavities, resin ducts, and laticifers (latex ducts) are also common in the midcortex of many plants.
in plants, the peripheral part of stems and roots external to the cambium; it is formed of tissues that vary according to structure and origin. A distinction is made between primary and secondary cortex. Primary cortex is formed at the growing point from outer layers of the meristems between the primary protective tissues (the epidermis in stems and the epiblema in roots) and the central cylinder. It is usually composed of parenchyma and is well developed in the roots of all plants; in dicotyledons and gymnosperms it is well developed in the rhizomes and stems. In the stems of dicotyledonous plants, the outer layer of primary cortex consists primarily of collenchyma; in the roots of monocotyledons, primarily of exodermis. The layer of primary cortex that is attached to the central cylinder (endodermis) is not always clearly differentiated. Secondary cortex (secondary phloem) is formed by the cambium and is composed of sieve-tube members, companion cells, phloem parenchyma, and phloem fibers; it serves for transporting the products of phostosynthesis down through the plant, functioning as a transmittal tissue for one to two years. Old cortex does not perform this function but separates into layers of cork, which thus becomes part of the cortex of many varieties of trees. The cortex serves as mechanical and chemical protection.
The chemical composition of thoroughly dried cortex (of cork phloem) is approximately 16–23 percent cellulose, 7–15 percent pentosans, 6–16 percent hexosans, 8–10 percent polyuronides, 27–33 percent lignin, and 14–30 percent extractive substances.
In practice, the cortex is usually called bark. It can be easily separated from the wood of the stem and the roots of arboreal plants. The bark of young trees is smooth; cracks appear as the trees age. The surface of the bark is furrowed, scaly, fibrous, or warty.
The cortex has great practical significance and is used in a variety of ways. The corky tissue, which develops best in the cork oak and the cork tree, is used for manufacturing corks and heat-insulating sheets. The cork of birch bark is used to make tar oil. Linden phloem yields bast when soaked. The cortex of the roots of the spindle tree contains approximately 7 percent gutta, and the bark of oak, spruce, and willow yields tannins used for tanning hides. The barks of the cinchona and the buckthorn are used in medicinal preparations. The amount of cortex obtained in peeling a round tree trunk amounts to 10–15 percent of the mass of the processed wood.
O. N. CHISTIAKOVA and V. V. FEFILOV