definition for trachelds,vessels, xylem parenchyma,xylem fibres
Answers
Explanation:
G Cell Types in Xylem Tissue
Xylem is a tissue that is comprised of four cell types (Table 14.1): tracheids and vessel members, which make up the tracheary elements, fibers, and parenchyma cells (Esau, 1977, p. 103). The tracheary elements are the most highly specialized cells of the xylem and are concerned with the conduction of water and substances in water. They are nonliving cells at maturity. They have lignified walls with secondary thickenings and a variety of pits.
TABLE 14.1. The four cell types in xylem tissue: tracheids, vessel members, fibers, and parenchyma cells. (From Esau, K., Anatomy of Seed Plants, 2nd ed., p. 106, ©1977, John Wiley & Sons, Inc.: New York. This material is used by permission of John Wiley & Sons, Inc.)
The two kinds of tracheary cells, the tracheids and the vessel members, differ from each other in that the tracheid is an imperforate cell whereas the vessel member has perforations, one or more at each end and sometimes also on a side wall (Esau, 1977, p. 106). The longitudinal series of vessel members interconnected through their perforations are called xylem vessels or simply, vessels. The perforated part of a wall of a vessel member is called the perforation plate. A plate may be simple, with only one perforation, or multiperforate, with more than one perforation.
The fibers are long cells with secondary, commonly lignified, walls (Esau, 1977, p. 108). The walls vary in thickness but are usually thicker than the walls of tracheids in the same wood (wood is secondary xylem). Two principal types of xylem fibers are recognized, the fiber-tracheids and the libriform fibers. If both occur in the same wood, the libriform fiber is longer and has thicker walls than the fiber-tracheid. Fibers give support to the xylem.
Parenchyma cells of the primary xylem occur in interfascicular regions and are considered to be part of the ground tissue (Esau, 1977, pp. 112–113). In the root, the primary xylem forms a core with parenchyma (as in some monocotyledonous roots) or a core without parenchyma (as in dicotyledonous roots). In secondary xylem, the parenchyma cells make up the axial and ray parenchyma. These parenchyma cells store starch, oils, and many other ergastic substances. Ergastic subtances are products of protoplasts such as starch grains, fat globules, crystals, and fluids. They occur in the cytoplasm, organelles, vacuoles, and cell walls (Esau, 1977, p. 509).
We can list the characteristics of tracheids, vessel members, fibers, and parenchyma cells as follows:
TRACHEIDS
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More primitive (in angiosperms, gymnosperms, and lower vascular plants)
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Tapered ends
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Long cells
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Thin cells
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Lignified secondary cell walls
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No protoplasts at maturity (dead)
VESSEL MEMBERS
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Present only in angiosperms (more evolved than tracheids)
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Short cells
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Broad cells
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Flat ends
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Perforated end walls with perforation plates
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Lignified secondary cell walls
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No protoplasts at maturity (dead)
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Several together form a continuous tube, which is called a xylem vessel
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A xylem vessel is a low-resistance circuit
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Width varies from 10 to 800 μm; an average diameter is about 40 μm
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Length of a xylem vessel varies from few hundred microns (μm) to a few millimeters. Long and wide xylem vessels occur in tropical plants such as vines (e.g., kiwifruit) and rattans (climbing palms). Fisher and colleagues (2002) of the Fairchild Tropical Garden in Miami, Florida, found a vessel in a rattan that was 3 m long and 532 μm in diameter.
FIBERS
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For structural support
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Long cells
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Thin cells
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Heavily lignified cell walls
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No protoplasts at maturity (dead)
PARENCHYMA CELLS
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For storage
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For lateral movement of water and solutes into and out of conducting cells (the ray system in secondary xylem) (Table 14.1).
Explanation:
coveringbor protective tissue in the animals body are ephitialial tissue