Biological Sciences
Xylem
Xylem is a type of vascular tissue in plants that transports water and nutrients from the roots to the rest of the plant. It is composed of specialized cells called tracheids and vessel elements, which are arranged in a way that allows for efficient water conduction. Xylem also provides structural support to the plant.
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7 Key excerpts on "Xylem"
eBook - PDFPlant Anatomy
An Applied Approach
- David F. Cutler, Ted Botha, Dennis Wm. Stevenson(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
CHAPTER 3 The structure of Xylem and phloem Introduction Xylem and phloem are the main tissues concerned with the movement of substances through the plant. Xylem transports mainly water and dissolved solutes usually in the form of minerals, and phloem translocates substances synthesized by the plant. Xylem and phloem are normally found together, and their functions are coordinated. Structure–function relationships are considered at the end of this chapter, but simply put Xylem conducts water and dissolved minerals from the roots to the aerial parts and phloem con- ducts assimilates from the leaves to the stems and roots. The Xylem The structure of primary Xylem is dealt with in Chapters 3, 4, 5 and 6 and will not be repeated or enlarged upon here. Instead, we will concentrate on secondary Xylem only, insofar as this is related to use and as an aid to classifi- cation and identification. The CD-ROM contains numerous additional images. Secondary Xylem construction Whilst primary Xylem consists of the axial cell system only, that is, Xylem cells that are elongated parallel with the long axis of the organ or vascular trace in which they occur, secondary Xylem, one of the products of the vas- cular cambium, is more complex. As we have seen in Chapter 2, the cam- bium is composed of two sorts of cells, the axially elongated fusiform initials, which give rise to the axial system of cells, and the short, more or less isodiametric ray initials giving rise to the radial system or rays. Figure 3.1 shows the axial and radial systems in Alnus glutinosa wood. The structure of Xylem and phloem 29 Ch 3 Xylem & phloem Because both axial and radial systems are present, the study of secondary Xylem can only be carried out properly by examination of three specific planes of section from a block of wood. These are the transverse section (TS) the radial longitudinal section (RLS) and the tangential longitudinal section (TLS).
eBook - PDF- Vanderlei Rodrigues da Silva(Author)
- 2013(Publication Date)
- IntechOpen(Publisher)
The issue of struc‐ tural basis for water flow in wood of woody plants and the methods of its measurement and assessment are presented and the adjustments to water stress in short- and long-time scale as well as at different levels of woody plant organizations discussed. 2. Structural basis for woody plants hydraulics Wood anatomy can be studied in the view of different disciplines e.g. technical applications, tree pathology, ecology, dendrochronology but also in terms of hydraulics as a specific hydrosystem based on cells with lumina filled with water. © 2013 Marciszewska and Tulik; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In perennial vascular plants the transport of water is to great extent determined by the hydraulic architecture. This term describes the Xylem network within the plant and its variations with wood type, plant age and growth form (e.g. liana, shrub vs. tree) [1]. The Xylem network integrates all main parts of the plant’s body, i.e. roots, branches and leaves. It means that any root in this system is more or less directly connected with any branch and not with a single one. Moreover, the Xylem network is redundant in two meanings: at a given level of the stem several Xylem element are present in parallel and they develop lateral contacts with other tracks of vessels or tracheids. The Xylem network is mainly represented by tracheary elements (conduits) that arise from vascular cambium periodically or continuously. Mature tracheary elements are dead cells with lignified walls. The lignin makes the Xylem cells strong and prevents them from collapsing. Moreover, due to lignification the cell walls are impermeable and in effect waterproof.
eBook - PDF- Octavian S. Ksenzhek, Alexander G. Volkov(Authors)
- 1998(Publication Date)
- Academic Press(Publisher)
To under-stand how water transport mechanisms function in plants, it is necessary to con-sider the arrangement of the Xylem and the forces acting on water and forcing it to move through the Xylem from roots to leaves. In its most simplified form, the scheme of a Xylem system can be represented as shown in Fig. 11.1. There is the input device (A), a long-distance transport channel (B), and the output device (C). The input device (A) is located in the plant's root. It provides the extraction of water from soil into the long-distance transport channel (B). This channel begins in a root, lengthens in the stem, and ends in the leaves of a plant. It represents a tube formed by a sequence of long narrow cells. Xylem consists of a set of such parallel tubes. The flow of sap in Xylem is set into motion by a pressure gradient quite similar to that of water flow in plumbing pipes. An output device (C) is located in the leaves. It provides for evaporation of water into the atmosphere. The pressure gradient, which is neces-Water vapor in the air B Figure 11.1 A diagram of long-distance transport in plants: (A) a lower device (a root pump); (B) transport channel; (C) an upper device (a pulling pump). Hydrostatics of Xylem 247 sary for tbe translocation of a liquid in a channel of Xylem, is formed by the combing action of the input and output devices. In this sense the input device may be termed the lower pump and the output device the upper pump. The first absorbs water at the beginning of a Xylem channel in the root, whereas the other pulls water through the channel and exhausts it out of the leaf. Mechanisms of function of both pumps will be discussed further. Now let us consider the forces that act on water during its way from the soil through plants to the atmosphere.
eBook - ePub- Horst Marschner(Author)
- 2011(Publication Date)
- Academic Press(Publisher)
Long-distance transport of solutes in the Xylem and phloem is important for shoot nutrition, the redistribution of essential mineral elements between tissues during ontogeny, the maintenance of charge balance in leaves of nitrate-fed plants, the removal of potentially toxic elements from leaf tissues, and the systemic signalling of plant nutritional status. This chapter describes the anatomy of the Xylem and phloem, the composition of Xylem and phloem saps, and the movement of Xylem sap from root to shoot in response to gradients of water potential generated by root pressure and transpiration and of phloem sap from source to sink tissues in response to osmotic gradients generated by differences in phloem sucrose concentration. Emphasis is placed on the pathways of solute movement within the plant and recent insight into the transport proteins catalysing the loading and unloading of elements to and from the Xylem and phloem.3.1. General
The long-distance transport of water and solutes – elements and low-molecular-weight organic compounds – takes place in the vascular system of Xylem and phloem. Long-distance transport from the roots to the shoots occurs predominantly in the non-living Xylem vessels. Coniferous trees lack a continuous system of Xylem vessels and depend on tracheides, which are non-living conducting cells ranging in length from 2 to 6 mm ( Tyree and Ewers, 1991 ). In annual plant species, long-distance transport in the Xylem vessels may also be interrupted by tracheides, for example at the root–shoot junction ( Aloni and Griffith, 1991 ) or in the nodes of the stem. These structures pose an internal resistance to Xylem volume flow but simultaneously permit an intensive Xylem–phloem solute transfer.Xylem transport is driven by the gradient in hydrostatic pressure (root pressure) and by the gradient in water potential. Pure free water is defined as having a water potential of zero. Accordingly, values for water potential are usually negative. The gradient in water potential between roots and shoots is quite steep particularly during the day when the stomata are open. Values become less negative in the following sequence: atmosphere>>leaf cells>Xylem sap>root cells>external solution. Solute flow in the Xylem from the roots to the shoots is therefore unidirectional (Fig. 3.1 ). However, under certain conditions in the shoots a counter-flow of water in the Xylem may also occur, for example, from low-transpiring fruits back to the leaves ( Lang and Thorpe, 1989
eBook - PDFPlant Science
Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro
- Ana Gonzalez, María Rodriguez, Nihal Gören Sa?lam, Nihal Gören Sağlam, Ana Gonzalez, María Rodriguez, Nihal Gören Sağlam(Authors)
- 2020(Publication Date)
- IntechOpen(Publisher)
Introduction Phloem is the vascular plant tissue responsible for the transport and distribution of sugars produced by the photosynthesis. Since the plant is a continuum, phloem will be found in the external part of root cylinders ( Figure 1a ), in the stem vascular bundles ( Figure 1b ) and in the abaxial part of the venations of every single leaf ( Figure 1c ). While the most common is to have the phloem external to the Xylem in roots and stems and abaxial in leaves, some exceptions exist and are usually taxon specific. The phloem found in the inside is named internal or intraxylary phloem ( Figure 1b ). As a constitutive tissue in the plant body, phloem functions extrapolate its main function of sugar transport, including transport of signalizing molecules such as mRNAs, hormones, defenses from biotic and abiotic agents, sustenance of the organs, gas exchange, and storage of many ergastic materials, such as starch, calcium oxalate crystals, and tannins. Parenchymatic cells of the phloem can also give rise to new meristems, such as the phellogen or cork cambium. All vascular plants have phloem, which typically includes specialized living conducting cells Plant Science - Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro 2 named sieve elements whose nucleus, ribosomes, and other organelles degenerate during maturation, making sugar transport more efficient. The life and function of these cells will then rely on closely associated parenchyma cells which support the physiological functions of these sieve elements [1]. Although typical phloem is exclusive of vascular plants, rudimentary phloem-like conducting cells are present Figure 1. Location of the primary phloem in different organs and its cell composition. (a) Ranunculus acris (Ranunculaceae). Root transverse section (TS), exarch structure, six strands of primary phloem alternating with the six protoXylem poles.
eBook - PDF- A. J. Peel(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
However, there has been a considerable amount of controversy generated by the question as to whether columns of water under tension exist within the Xylem of transpiring plants. Preston (1952) has produced evidence, both experimental and theoretical, against their existence, as has Greenidge (1957), employing a technique in which all the vessels of transpiring trees were opened to the atmo-sphere by means of double overlapping saw cuts. However, there is good evidence that high negative pressures can exist within the Xylem of transpiring plants (Scholander et al., 1966), and that water movement can occur through a system of microcapillaries when the Xylem vessels are blocked by air (Scholander, Love and Kan-wisher, 1955; Scholander, Ruud and Levestad, 1957; Peel, 1965a). Other writings on the subject of water movement in Xylem to which the reader is referred are those of Zimmermann (1965), Slatyer (1967) and Zimmermann and Brown (1971). Certainly, large quantities of water are moved in the transpiration stream at velocities up to 50 m/h. Moreover, the transpiration stream contains solutes, principally ions but also sugars and other organic solutes, albeit in low concentration (around 0.5% w/v). The pathways of long distance transport 23 detopped herbaceous plants, and analyses of sap extracted from lengths of the stem of woody plants. In the following chapter we shall be looking at the mobilities of substances in the main long distance transport channels, but it would seem to be of value here to examine the validity of the methods employed to obtain Xylem sap. Exudate may be obtained by boring holes into the wood of trees in the early part of the year before the leaves have expanded. The species most studied in this way has been Acer saccharum, no doubt because of the ease with which sap may be obtained during the bleeding season. Other woody species which have been examined include Betula, Vitis, Carpinus and certain gymnosperms.
eBook - PDFPlant Stems
Physiology and Functional Morphology
- Barbara L. Gartner(Author)
- 1995(Publication Date)
- Academic Press(Publisher)
II Roles of Stems in Transport and Storage of Water This Page Intentionally Left Blank 5 Limitations on Stem Water Transport and Their Consequences John S. Sperry I. Introduction Mechanical support and long-distance transport are two of the most ob- vious functions of stems. Vascular tissue, which makes up most of the stem in plants with secondary growth, plays a major role in both functions. Over- production of vascular tissue relative to requirements for support and trans- port represents a waste of resources; underproduction may place restric- tions on growth. The chapters by Mattheck [3], Gartner [6], and Givnish [1] in this volume discuss constraints of stem support on plant size and shape. Implications of stem phloem transport are discussed by Pate and Jeschke [8]and Van Bel [9] in this volume. This chapter considers interac- tions between stem water transport, Xylem structure, vegetative phenology, and stomatal regulation of gas exchange. II. Importance of Stem Water Transport The significance of stem water transport is apparent in its influence on leaf water status and, ultimately, in how leaf water status is linked to the regulation of gas exchange and other leaf-level processes affecting whole- plant carbon gain. The abundance of Xylem conduits specialized for longi- tudinal flow suggests an efficient transport system offering minimal fric- P~.t stems 105 Copyright 9 1995 by Academic Press, Inc. All fights of reproduction in any form reserved. 106 John & Sperry tional resistance. However, a number of studies have shown that a large fraction of the decrease in leaf water potential caused by transpiration re- sults from the hydraulic resistance of stem Xylem (reviewed in Tyree and Ewers, 1991).
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