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Xylem Part - 2.pptx

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Xylem Part - 2.pptx

  1. 1. STRUCTURE OF XYLEM 1. TRACHEIDS 2. TRACHEA OR VESSELS 3. FIBRES CALLED XYLEM FIBRES OR WOOD FIBRES 4. PARENCHYMA, REFERRED TO AS XYLEM OR WOOD PARENCHYMA • Of all parenchyma is living while all other components are dead. Tracheids Trachea or vessels XYLEM OR WOOD PARENCHYMA
  2. 2. 1.Tracheids • A tracheid is a very much elongated cell occurring along the long axis of the organ. • The cells are devoid of protoplast and hence dead. • A tracheid has a fairly large lumen without any contents and tapering blunt or chisel-like ends. • The end walls usually do not uniformly taper in all planes. • Tracheids are round or polyhedral in cross section. The wall is hard, moderately thick and usually lignified
  3. 3.  The secondary walls are deposited in different manners, so that the tracheids may be annular, spiral, reticulate, scalariform or pitted.  Bordered pits are most abundant. Through these pits they establish communication with the adjoining tracheids and also with other cells, living or non-living. Pitted Spiral Annular
  4. 4.  Tracheids occur in both primary as well as secondary xylem.  A typical fibre differs from a tracheid in more pronounced thickening of the wall and correspondingly much smaller lumen, as well as reduction in the size of the pits.  An intermediate type of cell element, called fibre tracheid is found in some plants.
  5. 5. 2.Tracheae or vessels These are long tube-like bodies ideally suited for the conduction of water and solutes. Short cell, cell are shorter than tracheids Cell become dead at maturity. A trachea or vessel is formed from a row of cylindrical cells arranged in longitudinal series where the partition walls become perforated, so that the whole thing serves like a tube. Perforations are commonly confined to the end walls, but they may occur on lateral walls as well. The walls undergoing these perforations are called perforation plates, which are of two types – multiple and simple
  6. 6. • Multiple perforation plate - In primitive plants the end walls between the cells do not completely dissolve, but the openings or perforations remain either in more or less parallel series like bars called scalariform perforations, or in the form of a network known as reticulate perforation or a group of circular holes called foraminate perforations. • Simple perforation plate – In advanced plants, the dissolution of the end wall is more or less complete and the perforation occurs in the form of a large circle. Scalariform perforation plate
  7. 7.  The vessels are considerably long bodies. For e.g. in ash plant (Fraxinus excelsior) of family Oleaceae vessels have been reported to be as long as 10ft. Vessel elements are typically found in angiosperms (flowering plants) but absent from most gymnosperms such as conifers.  Vessel elements are the main feature distinguishing the "hardwood" of angiosperms from the "softwood" of conifers. Vessels are absent in most Pteridophytes except Pteridium and Selaginella and Gymnosperms except Gnetales.  They are present in all Angiosperms except some members of order Ranales. They are also absent in parasites and aquatic plants and secondary xylem of monocots (secondary growth is absent in many monocotyledons)  Vessels are the most important members of the xylem as they are primarily adapted for easy transport of water and solutes and secondarily for mechanical support.
  8. 8. 3.Xylem fibres • These are the sclerenchymatous cells which remain associated with other elements of xylem and provide mechanical support. • They are very much elongated and dead cells with thick lignified walls. • Xylem or wood fibres are mainly of two types: fibre tracheids and libriform fibres.
  9. 9. 4.Xylem parenchyma • Living parenchyma is a constituent of xylem in most plants. These cells are meant for storage of starch, fats and other matter like crystals and tannins. • In primary xylem they remain associated with other elements and derive their origin from the same meristem. • In secondary xylem parenchyma occurs in two forms: xylem parenchyma or wood parenchyma, where the cells are somewhat elongated and lie in vertical series attached end on end and ray parenchyma, where the cells occur in radial transverse series.
  10. 10. • Based on their association with vessels xylem parenchyma is classified as: Apotracheal, not in contact with vessels and Paratracheal, in contact with the vessels. • Parenchyma is abundant in the secondary xylem of most of the plants except few conifers like Pinus, Taxus and Araucaria.

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