Xylem primary

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). 

If the iron-efficient roots were given iron as FeHEDTA for 20 hr, then taken out of the nutrient solutions, rinsed free of FeHEDTA, and placed in nutrient solutions containing K3Fe(CN)6, Prussian blue formed throughout the protoxylem of the young lateral roots up to the meta-xylem (13). Ferrous iron was continuous in these areas of the roots and in the regions of root elongation and maturation of the primary root (13,19). 

Accuracy of measurements in the middle lamella depends not only on Nb, but also on the value of Nd for this region. Donaldson (1985a) measured the refractive index of the middle lamella in differentiating tracheids before lignifica-tion and in primary xylem, where the middle lamella is unlignified, and obtained... 

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). 

Venation is the pattern of veins in the blade of a leaf. The veins consist of vascular tissues which are important for the transport of food and water. Leaf veins connect the blade to the petiole, and lead from the petiole to the stem. The two primary vascular tissues in leaf veins are xylem, which is important for transport of water and soluble ions into the leaf, and phloem, which is important for transport of carbohydrates (made by photosynthesis) from the leaf to the rest of the plant. 

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-... 

Pig. 60.—Cross-section of a young root of Phaseolus muHi-florus. A, pr, cortex m, pith X, stele or central cylinder—all tissue within the pericycle, inclusive g, primary xylem bundles b, primary phloem bundles. B, cross-section of older portion of root lettered as in A b, secondary phloem, k, cork. (Stevens, after Vines.)... 

Pig. 61.—Photomicrograph of a transverse section of a California Privet root of primary growth showing epidermis (e) hypodermis (A) cortex (c) endodermis (en) pericambium (p) a xylem arm of the radial bundle (f) and pith (m). 

Fig. 63.—Transverse section of California Privet root made about an inch and a half above the section shown in Pig. 6l and showing secondary structure. Note that epidermis, primary cortex and endodermis have completely disappeared. Cork (cfe) phellogen -ph) secondary cortex (rc) protophloem p ) secondary phloem cambium (c) secondary xylem ( ) and protoxylem ( ).

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.)...

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. 

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.)...

Even in its first few months of growth, a seedling already has well-developed tissues that function in translocation and physical support. These early but critically important tissues are called primary xylem and primary phloem and are produced by derivatives of apical meristems they are longitudinally arranged into tissue zones called vascular bundles (J, 2). These structures are the only circulatory... 

During the division and enlargement phases of wood cell development, the cell wall is a thin, deformable, and extensible envelope of material referred to as the primary wall. Near the cessation of cell enlargement, however, a secondary wall may begin to be manufactured to the lumen side of the primary wall. Wood fibers, vessel elements, and certain other xylem or phloem elements that function in passive conduction and/or support normally develop a secondary wall (Figure 5). 

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