Xylem

Lab 5 Xylem Slides Prepared 1-Helianthus stem c.s 2- Tilia stem c.s 3- Quercus wood three sections 4- Pinus wood three sections 5- Pinus (pine) macerated wood 6- Zea stem c.s Fresh specimens Pelargonium leaves petioles. 1 Lab 5 Xylem -Xylem is the water conducting tissue in the vascular plants, function also in mechanical support. -It is a complex tissue that contains parenchyma cells + fibers + water conducting cells (Tracheary (conducting) elements: vessels and tracheids) - Tracheary elements are characterized by lignified secondary cell wall. -During the primary growth of the plant xylem differentiates from procambium. -Protoxylem is the first elements of the primary xylem to differentiate and become mature. -Metaxylem is the elements that mature later. -Plants that undergo secondary growth (Most gymnosperms and Dicots) the secondary growth are added from vascular cambium. *Elements of xylem Tracheary elements:  Vessel Members: -elongated,non-living cells. - their wall is lignified with secondary thickenings and variety of pits. - the vessel is made of several vessel elements connected by perforations found on perforation plate (simple or multiperforated) -vessels are found in xylem (wood) in nearly all angiosperms and absent from xylem of gymnosperms.  Tracheids: -Elongated, non-living, with pointed ends -imperforated cells with secondary cell wall with pits (simple, bordered pits (scalariform, opposite or alternate) Fibers: - Cells with thick secondary lignified wall, more thick than that of tracheids but shorter. 2 -Two types present: (fiber tracheids with bordered pits with slit aperture and Libriform fibers: longer with thicker wall and simple pits with slit aperture) - function in support and sometimes storage. Parenchyma: -are of two types ray parenchyma and axial parenchyma. -are living with thick secondary cell wall which may be lignified - function in storage and lateral transport Exercise 1 The cell wall of tracheary elements in primary xylem is thick lignified secondary cell wall, but the thickening occurs in different forms: annual, helical, scalariform, reticulate and pitted, to allow these cells to stretch during growth.  To identify tracheary elements (vessels and tracheids) with annular. Spiral (helical) the most common, scalariform, reticulate and pitted secondary cell wall -Separate a few vascular bundles from boiled (weak KOH solution) 1-Pelargonium, stain with toluidine blue and squash them between the slide and cover slip. (Maceration: artificial separation of cells of tissue by disintegration of the middle lamella) 2- prepared slide Pinus macerated wood, notice tracheids and bordered pits on their wall, observe the aperture. Excercise 2.(to study primary xylem)  A) Study prepared slide Helianthus (sunflower) stem c.s. Observe the vascular bundles and the perivascular bundle caps of fibers, and then carefully examine the vessel elements in the primary xylem (protoxylem and metaxylem), tracheids and fibers. Notice also the interfascicular parenchyma between the vascular bundles  B) Study prepared slide Zea (Corn) Stem c.s, observe the scattered vascular bundles as a monocot stem, the perivascular fiber sheath and the elements of primary xylem. You will be able to notice the the protoxylm represented by lacuna and metaxylem 3 Excessive 3 (to study secondary xylem in angiosperm)  Study prepared slide for Tilia (angiosperm) stem c.s , observe 1)secondary xylem in a form of annual (growth rings), notice 2) the scattered distribution of vessels with various widths through out the ring (Diffuse porous) 3) the distinct boundary between annual rings (early wood and late wood) 4) ray parenchyma 4) axial parenchyma Exercise 4 (to study secondary xylem in angiosperm)  Study prepared slide for Quercus (angiosperm) wood three sections (c.s, tangential longitudinal section TLS: and Radial longitudinal Section RLS)  In c.s notice 1) the annual rings with distribution of vessels (Ring porous) (early wood and late wood) 2) the distinct boundary between annual rings 3) ray parenchyma 4) try to differentiate between vessel , tracheids and fibers in c.s.  In TLS notice vessels, tracheids, fibers in LS notice their pattern in wall thickening and pits , observe the shape of ray parenchyma (uniseriate or multiseriate) also determine if wood is storied or nonstoried depending on horizontal arrangement of ray cells, and axial parenchyma  In RLS notice vessels, tracheids, fibers in LS notice their pattern in wall thickening and pits , observe the shape of ray parenchyma (this section can tell about height of ray parenchyma) and axial parenchyma  Notice the presence of tyloses (outgrowth of parenchyma into a vessel) Excersise 5 (to study secondary xylem in gymnosperm)  Study a prepared slide for Pinus (gymnosperm) three sections Observe that tracheary elements of xylem in this plant as a gymnosperms lack the presence of vessels so it is only made of tracheids (uniform in size in c.s). Examine the annual rings, components of secondary xylem (no fibers, axial parenchyma may not present, resin ducts, uniseriate ray parenchyma, notice bordered pits on tracheids wall). Compare between the two types of .ls. 4 5 Late wood Ring Porous Diffuse Porous 6 Early wood 7 Wood in angiosperms Tylose Cross section Multiseriate ray parenchyma Vessel Axial parenchyma Ray parenchy ma Radial Longitudinal Section 8 Multiseriate ray parenchyma Tangential Longitudinal Section Non storied ray parenchyma Wood in gymnosperms Cross Section 9 Radial Longitudinal Section Tangential Longitudinal Section 10