Medullary rays

When the plant is injured, the cell walls of the pith and of the medullary rays are gradually transformed into gum. The gum absorbs water and produces internal pressure within the stems, thus forcing the gum to the surface through the incision caused by the injury. It gradually hardens owing to the evaporation of water. 

Features Stem two feet high, thick, smooth, branched leaves, pinnate or lobed. Flowers (May) large, red, single, terminal. Transverse section of root is starchy, medullary rays tinged purple. Taste sweet, becoming bitter. 

Features Rootbark is a bright, rusty brown, soft and brittle. Short, corky, layered fracture, with many oil cells. Chips of the woody root are commonly used—they are brownish-white in colour, showing concentric rings and slender medullary rays. 

The anatomy of the tuber is shown diagrammatically in Figure 11.2. Parts that can be distinguished are the skin (periderm) with the lenticels, the eyes, the bud and stem ends, the cortex, the ring of vascular bundles, the perimedullary zone, and the pith with medullary rays which are homologous with the medulla of the stolon.7... 

The bark of Phellodendron amurense (Rutaceae) is known as an important Japanese medicine for stomach and intestinal diseases. The bark is yellowish in color and contains two major alkaloids, berberine and palmatine. Yellow alkaloids do not usually have distinguishing characteristics in tissues when observed microscopically. The bark was soaked in 5% nitric acid solution for 3 days and sectioned for microscopical studies (1). The alkaloids were crystallized as nitrate salts in the phloem, medullary rays, and cortex. Crystals were removed by... 

Cell death induced by AmB in the medullary thick ascending limb is prevented by ouabain [108]. A reasonable explanation for this observation is that ouabain, by inhibiting transport, decreases the oxygen demand of an area of the nephron that already has a hmited oxygen supply. This is consistent with the observation that AmB exhibits preferential damage to the medullary ray, an area that is vulnerable to hypoxic injury [48]. It is also conceivable that AmB-induced renal vasoconstriction and ischemia to this section of the nephron enhances cell death produced by a direct toxic action. Thus, any maneuver that improves renal perfusion, or decreases oxygen demand, would be expected to be protective. This may explain the salutary effect of salt loading, theophylline, calcium channel... 

Andoh et al developed an experimental model of TAC-induced chronic nephrotoxicity using salt-depletion in rats [687, 704]. A particular characteristic of this model is that renal functional changes and structural injury occur with TAC blood levels equivalent to those found in treated patients, in a striking contrast with the CSA chronic nephrotoxicity model, where extremely high CSA blood levels are necessary to produce injury. In this TAC model, there is an early and dose-dependent decrease in GFR and RBF with a parallel RVR increase followed by a late development of renal interstitial fibrosis involving the itmer strip and medullary rays, arteriolar hyahnosis, tubular atrophy and hypertrophy and medullary thick ascending limb size variance. Structural injury showed a significant positive correlation with decreased renal function [687, 704,705]. 

Rosen S, Greenfeld Z, Brezis M. Chronic cyclosporine-induced nephropathy in the rat. A medullary ray and inner stripe injury. Transplantation 1990 49 445-452. 

Pig. 19.—Transverse section ot white pine stem ot four years growth, showing cork (o), cortex (6), phloem (c), cambium (d), xylem (e), secretion reservoir (J), pith (g) and medullary ray (A). (Photomicrograph.) X340. 

These are bands of parenchyma cells which extend radially from the cortex to the pith (primary medullary rays) or from a part of the xylem to a part of the phloem (secondary medullary rays). In tangential-longitudinal sections they usually appear spindle shaped while in radial-longitudinal sections they are seen crossing the other elements. Their primary function is to supply the cambium and wood with elaborated sap formed in the leaves and conveyed away by the sieve tubes, and phloem parenchyma and to supply the cam-... 

Patches of cells of the inner layer of pericamhium divide rapidly and are called, interfascicular cambium. These join the intrafasci-cular iambium to form a continuous cambium ring which then cuts off additional secondary xylem on its inner face and secondary phloem on its outer face pushing inward the first-formed or proto-xylem and outward the first-formed or protophloem. The medullary rays become deepened. 

Strands of cells extending radially from the cortex to the center of the section separating each open fibro-vascular bundle from its neighbors. These are called medullary rays. 

Pig. 64.—Photomicrograph of a transverse section of an old portion of California Privet root, showing completed secondary development. Note the prominent medullary rays (mr) cork (cfe) phellogen (,ph) secondary cortex (between ph and p ) protophloem (p )i secondary phloem ( ) cambium (c) secondary xylem tracheae (0 wood fibers (w/) and piotoxylem ( ). 

Fig. 68.—Photomicrograph of cross-section of stem of Aristolochia sipko, where cambial activity is just beginning, a, Epidermis b, coUenchyma c, thin-walled parenchyma of the cortex, the innermost cell layer of which is the starch sheath or endodermis d, sclerenchyma ring of the pericycle e, thin-walled parenchyma of the pericycle /, primary medullary ray g, phloem h, xylem interfascicular cambium medulla or pith. X 20. (From Stevens.)...

Fibro-vascular bundles of open collateral type arranged m a circle with primary medullary rays between the bun es. 

Between the bundles certain cells of the primary medullary rays become very active and form interfascicular cambium which joins the cambium of the first-formed bundles (intrafascicular cambium) to form a complete cambium ring. By the rapid multiplication of these cambial cells new (secondary) xylem is cut off internally and new (secondary) phloem externally, pushing inward the first-formed, or protoxylem, and outward the first-formed, or prolophloem, thus increasing the diameter of the stem. The primary medullary rays are deepened. Cambium may also give rise to secondary medullary rays. 

Fibro-vascula r bundles of open collateral type which are now arranged into a compact circle, and between which are found primary and often secondary medullary rays. 

Fig. 71.—Portion of cross-section of four-year-old stem of Aristolochia sipho, as shown by the rings of growth in the wood. The letters are the same as in Pig. 68 but new tissues have been added by the activity of the cambium and a cork cambium has arisen from the outermost collenchyma cells and given rise to cork. The new tissues are I, cork cambium k, cork g, secondary phloem from the cambium, and just outside this is older crushed phloem , secondary xylem produced by the cambium m, secondary medullary ray made by the cambium (notice that this does not extend to the pith). Half of the pith is shown. Notice how it has been crushed almost out of existence. Compare Figs. 68 and 71, tissue for tissue, to find out what changes the primary tissues undergo with age, and to what extent new tissues are added. Photomicrograph x 20. (From Stevens.)...

In Phytolacca, etc., there first arises a ring of primary bundles with broad loose medullary rays. Then the stem cambium ceases its... 

Fig. 74.—Part o a transverse section of a twig of the linden, four years old. m. Pith ms, medullary sheath x, secondary wood Ph, phloem 2, 3, 4, annual rings c, cambium pa, dilated outer ends of medullary rays b, bast pr, primary cortex k, cork. (From Sayre after Vines.)...

Fig. 75.—Part of a cross-section through branch of Cytisus laburnum. (The branch was cut from the tree at the end of October.) Prom A to E the last annual ring of wood from A to B the spring growth with large tracheal tubes T, T, T) between B and C and D and D are wood-fibers between C and D and D and E, wood parenchyma from E to F, cambium F to G, phloem portion C to H, cortical parenchyma M, medullary ray. Below A the last woodrfibers and wood parenchyma formed the previous year. From Sayre after Haberlandt.)...

In a tangential longitudinal section which has been cut through the phloem, the exact range in width of the medullary rays may be ascertained. In this bark the medullary rays are spindle-shaped in tangential view and one to four cells in width. 

Fig. 76.—Photomicrograph of transverse section of Cascara Sagrada bark k, cork g, cork cambium c, cortex st, group of stone cells bf, group of bast fibers mr, medullary ray.

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