Xylem parenchyma

In addition, the parenchymatous cells in the xylem appear to yield at least two lignitic materials. One is a dark red-brown material that may be derived from inclusions originally present in the cell. This material is common and may be observed in Figures 2, 3, 5, 6, 7, and 8. The other coalified product associated with this cell type is yellowish in color and derived from the wall of the cell (Figures 2 and 6). Ray parenchyma and longitudinal xylem parenchyma typically form only a primary wall, and usually the wall is devoid of lignin. It is of interest that this wall layer is preserved and that it is frequently possible to observe the simple pits that are characteristic of this cell type (Figure 2). 

At the stem apex, alkaloids are present in all the young undifferentiated cells. According to Molle (6), the most recently formed cells have comparatively little, the precipitations increasing to a maximum density at a short distance behind the actual apex. The zone of tissue differentiation is also abundantly supplied, but as differentiation proceeds, alkaloids disappear from the vascular strands, and then from the central tissues of the pith. When differentiation is complete, the alkaloids are located principally in three concentric layers, in the epidermis and outer cortical layers just below it, in parenchyma within and adjacent to the phloem, and in the periphery of the pith just inside the xylem strands. The xylem parenchyma and medullary rays also possess alkaloids after they have disappeared from the conducting elements. 

Street P F S, Roble J, Ellis B E 1986 Secretion of vascular coating components by xylem parenchyma cells of tomatoes infected with Verticillium alboatrum. Protoplasma 132 1-11... 

Although this report demonstrated that HA could move along vascular bundles slowly and stain the cell walls of xylem parenchyma, it did not show that the humic material could be taken up by roots. 

Added to this is the adhesion of water molecules to the molecules of the wall lining of the xylem which prevents the water filaments from being detached from the vessel walls, thus making their rupture more difficult. The sheath of living xylem parenchyma around the conducting, dead xylem elements serves an additional purpose. It prevents the infiltration of air into the conducting elements, which are subject to considerable suction at the time of intensive transpiration. 

Wang, G., Lin, Q., Xu, Y., 2007. Tetraenea mongolica Maxim can accumulate large amounts of tria-cylglycerol in phloem cells and xylem parenchyma of stems. Phytochemistry 68, 2112-2117. 

Five- to six-month-old tobacco plants (Nicotiana tabacum var. Samsun) grown in a glasshouse at 20°C were used for this study. Commercial synthetic substrates employed both for histochemical and biochemical assays were guaiacol, p-phenylenediamine-pyrocatechol (PPD-PC), 3-3 di-aminobenzidine (DAB), tetramethylbenzidine (TMB) and syringaldazine. Isopropylamine and monosodium salts of ferulic acid were also used as substrates as well as their / -fluorinated analogues substituted with a fluorine atom on the / -carbon (Fig. 1). Histochemical observations were done on hand-made transverse sections of fresh tobacco stems. Biochemical assays were performed separately on bark (inner cortical parenchyma, phloem and fibres) and xylem fractions. Technical data of incubation, enzyme extraction, spectrophotometric and electrophoretic assays were given elsewhere (5-7). Synthesis of fluorinated compounds was performed as previously described (4). 

Substrate Parenchyma Bark Fibres Phloem Young Xylem... 

Typical mature roots have different shapes (conical, conical-cylindrical, cylindrical, fusiform) and different sizes (3 to 15 centimeters in diameter), depending on variety, age and growth conditions. The color of the outer peel varies from white to dark brown. The cross-section of cassava roots shows the two major components which are the peel and the central pith (Figure 12.1). The peel is composed of the outer layer (called the periderm) and the inner layer (called the cortical region or cortex), which contains sclerenchyma, cortical parenchyma and phloem tissue. The large central pith of the roots is the starch-reserve flesh, comprised of cambium and parenchyma tissue and xylem vessels. 

Parenchyma cells have the function of storing nutrients and are located between the sieve elements in the inner bark. Both vertical parenchyma cells and horizontal phloem rays are present. The latter are direct continuations of the xylem rays, but much shorter. 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

Besides vessel members and tracheids, parenchyma cells and fibers also occur in the xylem (see Fig. 1-3). Xylem fibers, which contribute to the structural support of a plant, are long thin cells with lignified cell walls they are generally devoid of protoplasts at maturity but are nonconducting. The living parenchyma cells in the xylem are important for the storage of carbohydrates and for the lateral movement of water and solutes into and out of the conducting cells. 

Nicotiana alkaloids, which serve as chemical defence compounds, are synthesized in the roots and are transported to other plant organs, such as aerial parts, via the xylem. These alkaloids accumulate in vacuoles. PMT and A622 oxidoreductase are strongly expressed in the endodermis and outer cortex cells of tobacco root tips and to a lesser degree in other parts of the cortex and parenchyma cells surrounding the xylem (Shoji et ah, 2002). The localization of nicotine biosynthesis in the parenchyma cells surrounding the xylem may aid the loading of the xylem with nicotine. 

Plants are predominantly composed of parenchyma and woody tissues. Parenchyma cells dominate the green tissues in leaves and are composed of a protein-rich protoplast surrounded by a cellulose wall. Woody plant cells dominate all support (sclerenchyma) and transport (xylem and phloem) stmctures in a plant. They are composed of several layers (middle lamella, primary wall, secondary wall, and tertiary wall) with varying proportions of cellulose, hemicellulose, and lignin (Fengel and Wegener, 1984). 

Fig. 38.—Various forms of calcium oxalate crystals. A, styloids from the bark of QuiUaja saponaria B, rosette aggregate from rhizome of Rheum officinale C, raphide from the bulb of Urginea maritima D. crystal fiber as seen in longitudinal section in either the xylem or phloem regions of Glycyrrhiza E, microcrystals (crystal sand) isolated from the parenchyma of Belladonna root F, monoclinic prisms and G, twin-crystals from leaves of Hyoscyamus niger. All highly magnified.

Conducting Parenchyma.— This type of parenchyma functions in the rapid translocation of food materials to distant regions in. the plant. It includes the wood parenchyma cells of the xylem which convey a portion of the crude sap (water with mineral salts in solution) and the phloem parenchyma (soft bast) which transports the elaborated sap (carbohydrate and proteid material in solution). Conducting parenchyma cells differ from those of ordinary paren-Digitized by Microsoft ... 

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