IGCSE Biology Paper-3: Specimen Questions with Answers 317 - 317 of 358

Explanation

Xylem: The term xylem was introduced by Nageli (1858) . Xylem is a conducting tissue which conducts water and mineral nutrients upwards from the root to the leaves.

Xylem is Composed of Four Types of Cells

• Tracheid՚s: Term “Tracheid՚s” was given by Sanio (1863) . The tracheid՚s are elongated tube-like cells with tapering or rounded or oval ends with hard and lignified walls.
• The walls are not much thickened. The cells are without protoplast and are dead on maturity. The tracheid՚s of secondary xylem have fewer sides and are more sharply angular than the tracheid՚s of primary xylem. The cell cavity or lumen of a tracheid is large and without any contents. Tracheid՚s possess bordered pits. Maximum bordered pits are formed in gymnospermous tracheid՚s. They also possess various kinds of thickenings, e. g. , annular, spiral, scalariform, reticulate or pitted tracheid՚s. All the vascular plants have tracheid՚s in their xylem. The main function of tracheid՚s is to conduct water and minerals from the root to the leaf. They also provide strength and mechanical support to the plant.
• Xylem vessels or Tracheae: Vessels are rows of elongated tube-like cells, placed end to end with their end walls dissolved. Vessels are multicellular with wide lumen. The vessels may be classified into several types according to the thickening developed in their wall. They may be annular, spiral, scalariform, reticulate or pitted. Vessels are absent in pteridophytes and gymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium) . In angiosperms (porous wood) vessels are always present (Vessels are absent in family - Winteraceae, Trochodendraceae and Tepacenpaceae of Angiosperm i.e.. Lotus, Wintera, Trochodendron) . Vessels along with tracheids forms the main tissue of xylem of vascular bundles of the angiosperms and help in conduction. It also provides mechanical support to the plant.
• Wood (xylem) parenchyma: These are the living parenchymatous cells. As found associated with xylem they are known as wood parenchyma. They serve for the storage of reserve food and help in conduction of water upwards through tracheid՚s and vessels.
• Wood (xylem) fibers: The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibers. They possess mostly thickened walls and few small pits. These pits are found abundantly in woody dicotyledons. They said the mechanical strength of xylem and various organs of plant body.