Plant Stem

7 Key excerpts on "Plant Stem"

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  eBook - ePub

  Functional Biology of Plants

  • Martin J. Hodson, John A. Bryant(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Chapter 6 Stems

  We have seen that the roots anchor the plant in the soil and are the main means of absorbing water and mineral ions. The main functions of most stems are to support the leaves and flowers of the plant (e.g. holding the leaves in positions where they can obtain maximum light) and to provide transport systems connecting the roots with those leaves and flowers.

  6.1 Structure of the Stem

  Stems consist of an alternating system of nodes, the points where leaves are attached and internodes, the stem segments between the nodes. In the angle between each leaf and the stem is an axillary bud, which has the potential to form a branch shoot. Most of the axillary buds of a young shoot are dormant, and most of the growth occurs at the terminal bud. We will first consider the structure of the young stem, and so-called primary growth.

  6.2 The Young Stem

  A plant anatomist interested in the internal structure of a particular stem will invariably start by taking a transverse section. Here, even more so than in the root, the anatomy depends on the phylogenetic group investigated. A transverse section of a typical young eudicot stem (alfalfa) at the primary growth stage is shown in Figure 6.1 .

  Figure 6.1 Alfalfa (Medicago sativa) stem in cross section. Key: Epidermis (Ep), Vascular Bundle (VB), Pith (Pi).

  Photo: Prof. Thomas Rost

  Working from the outside, stems have the following structures:

  1. The epidermis, which may be single layered or multilayered. The outer surface of the young stem is covered by a waxy cuticle and may be ornamented. Stomata also occur on the surface of some stems, as can trichomes and hairs.

  2. The hypodermis is the layer of cells beneath the epidermis. Sometimes, but not always, it has a structure distinct from the cortical cells inside it.

  3.

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  eBook - ePub

  The Handy Biology Answer Book

  • Patricia Barnes-Svarney, Thomas E. Svarney(Authors)
  • 2014(Publication Date)
  • Visible Ink Press

    (Publisher)

  PLANT STRUCTURE, FUNCTION, AND USE

  PLANT STRUCTURES

  How are plants defined?

  Plants are defined in many ways due to their great number. In general, a plant is a multicellular, eukaryotic organism with cellulose-rich cell walls and chloroplasts that has starch as its primary carbohydrate food reserve. Most are photosynthetic and are primarily terrestrial, autotrophic (capable of making their own food) organisms.

  What are the major characteristics of vascular (tracheophyte) plants?

  The majority of vascular plants consist of roots, shoots, and leaves. The root system penetrates the soil below ground and anchors the plant; there, the roots absorb water and various materials necessary for plant nutrition. The shoot system consists of the stem and the leaves and is the part of the plant above ground level. The stem provides the framework for the positioning of the leaves; the leaves are the sites of photosynthesis. Growing plants maintain a balance between the size of the root system and the shoot system. The total water- and mineral-absorbing surface area in young seedlings usually far exceeds the photosynthesizing surface area. As a plant ages, the root-to-shoot ratio decreases. Additionally, if the root system is damaged, shoot growth is reduced by lack of water, minerals, and root-produced hormones.

  What are some specialized cells in vascular plants?

  All plant cells have several common features, such as chloroplasts, a cell wall, and a large vacuole. In addition, a number of specialized cells are found only in vascular plants. The following lists the main plant cells (for more information about cells, see the chapter “Cellular Basics ”)—also called ground tissue (that functions mainly in support, storage, and photosynthesis):

  Parenchyma cells —Parenchyma (from the Greek para , meaning “beside,” and en + chein

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  Scientific American Science Desk Reference

  • (Author)
  • 2008(Publication Date)
  • Trade Paper Press

    (Publisher)

  lignin, except for numerous small rounded areas, or pits, through which water and dissolved minerals pass from one cell to another. Once mature, the cell itself dies and only its walls remain.…

  transpiration loss of water from a plant by evaporation.

  tree perennial plant with a woody stem, usually a single stem (trunk), made up of wood and protected by an outer layer of bark.

  tropism , or tropic movement , directional growth of a plant, or part of a plant, in response to an external stimulus such as gravity or light. If the movement is directed toward the stimulus it is described as positive; if away from it, it is negative. Geotropism for example, the response of plants to gravity, causes the root (positively geotropic) to grow downward, and the stem (negatively geotropic) to grow upward.

  tuber swollen region of an underground stem or root, usually modified for storing food. The potato is a stem tuber , as shown by the presence of terminal and lateral buds, the “eyes” of the potato. Root tubers , for example dahlias, developed from adventitious roots (growing from the stem, not from other roots) lack these. Both types of tuber can give rise to new individuals and so provide a means of vegetative reproduction.

  turgor rigid condition of a plant caused by the fluid contents of a plant cell exerting a mechanical pressure against the cell wall. Turgor supports plants that do not have woody stems. Plants lacking in turgor visibly wilt. The process of osmosis plays an important part in maintaining the turgidity of plant cells.

  vascular bundle strand of primary conducting tissue (a “vein”) in vascular plants, consisting mainly of water-conducting tissues, xylem, and nutrient-conducting tissue, phloem.

  vascular plant plant containing vascular bundles. Ferns, horsetails, club mosses, gymnosperms (conifers and cycads), and angiosperms (flowering plants) are all vascular plants.

  vegetative reproduction type of asexual reproduction in plants that relies not on spores, but on multicellular structures formed by the parent plant. Some of the main types are stolons and runners, sucker shoots produced from roots (such as in the creeping thistle Cirsium arvense ), tubers, bulbs, corms, and

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  Science and the Garden

  The Scientific Basis of Horticultural Practice

  • David S. Ingram, Daphne Vince-Prue, Peter J. Gregory, David S. Ingram, Daphne Vince-Prue, Peter J. Gregory(Authors)
  • 2015(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Salvinia spp.). Water stress has been the principal selective force in the evolution of most such modifications (see Chapter 8). They are summarised in Table 1.4.

  The stem: reach for the sky

  The stem or shoot is the main axis of the plant, generating at its apex the leaves, which it holds aloft to collect the sun’s energy, and providing at its base a vital link to the roots, and the water and mineral nutrients of the soil.

  The growing point

  To find the growing point of the shoot, the so-called apical meristem (Fig. 2.10 ), it is necessary to strip away the immature leaves that surround and protect it from damage. It is usually dome-shaped, a millimetre or so in diameter, and very delicate. In flowering plants (angiosperms) it usually comprises an outer layer, the tunica , which is one to several cells thick, enclosing an inner mass of cells, the corpus . The cells of the apex are packed with cytoplasm and are constantly dividing and differentiating to produce new tissues. Such centres of cell division and growth are called meristems , in this case the apical meristem.

  As the apex grows, the cells of the tunica divide in one plane only, at right angles to the surface of the plant, which means that the tunica remains as a discrete layer. It ultimately gives rise to the epidermis of the shoot and the leaves. The cells of the corpus divide in a more irregular way and by expansion and division give rise to the internal tissues of the stem and leaves. In the stem apex of conifers and their relatives (gymnosperms) the apex is not visibly differentiated into tunica and corpus, although the epidermal tissues of shoot and leaf still arise from the outer cells.

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  BIOS Instant Notes in Plant Biology

  • Andrew Lack, David Evans(Authors)
  • 2021(Publication Date)
  • Taylor & Francis

    (Publisher)

  Herbaceous stems are green and nonwoody. They have a range of growth forms generated by an apical meristem that produces leaf and bud primordia. The shoot is surrounded by an epidermis, within which lies the cortex. Vascular tissues occur as a ring of separate bundles towards the outside of the stem in dicotyledons and in a scattered pattern in monocotyledons. The center of the stem is the pith parenchyma.

  Meristems

  The shoot meristem has a tunica producing the epidermis and the layers of cells beneath it while the corpus produces the cortex, pith and vascular tissues. The meristem can also be divided into the central zone where cell division occurs, the peripheral zone where leaves, shoots and new meristems form, and the rib zone where stem growth occurs.

  Vascular tissue

  Vascular bundles containing phloem and xylem form a ring or a complex array throughout the inner part of the stem. In some dicotyledons the ring is almost continuous.

  Architecture

  The shape of the stems is governed by the controlled positioning of leaves and buds by the meristem. Their positioning may be spiral, distichous, opposite, decussate or whorled.

  Related topics

  Meristems and primary tissues (D1) Features of growth and development (J1) Woody stems and secondary growth (D4)

  Shoot structure

  Herbaceous stems are green throughout without deposition of lignin that would make them woody. Shoots are more complex than roots and take on a wide range of growth forms reflecting function. Like the root, the shoot has an apical meristem. Unlike the root (where lateral branching only occurs in mature tissue), the shoot meristem produces lateral shoots and leaves (

  Fig. 1

  ).

  The apical meristem produces leaf primordia (which will form leaves) and bud primordia (which will form shoots). These are produced in position and order which gives rise to the characteristic form of the shoot which is recognizable for each species (see Architecture, below).

  The shoot is surrounded by an epidermis. This outer layer provides the protective barrier between the stem and its environment and is covered in a lipid-based protective substance, cutin (Topics E1 and M4). Within the epidermis, cells of the ground tissues, the cortex, may be photosynthetic and occupy the space surrounding the vascular bundles. In some species, the cortex is only a few cell layers, in others many more. Within the cortex, bounded by the vascular bundles, the center of the stem is occupied by pith (

  Fig. 2

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  eBook - PDF

  Introductory Horticulture

  • Carroll Shry, H. Reiley(Authors)
  • 2016(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Water moves through Plant Stems by being pulled from above, not pushed from below. Two forces, cohesion and adhesion, are responsible. Water molecules are attracted to each other like magnets with a force known as cohesion . As water evaporates from leaves, more water is pulled up through microscopic xylem tubes in a continuous column as the force of cohesion acts to keep water molecules stuck to one another. Water also clings to the walls of the xylem tubes via a force known as adhesion . The second function is the support of the leaves and reproductive structures ( flowers and fruit or seeds). Stems are also used for food stor-age, as in the Irish potato, and for reproduction methods that involve stem cuttings or grafting. Green stems manufacture food just as leaves do. External Stem Structure The outside of the stem consists of lenticels, or breathing pores, bud scale scars, and leaf scars. Bud scale scars indicate where a terminal bud has been located; the distance between two scars represents 1 year of growth. Leaf scars show where leaves were attached (Figure 3–7). Some plants, such as the Irish potato and the gladiolus, have stems that differ greatly from the stems of other plants. The stems in these plants are used for food storage and plant reproduction. A detailed discussion of this type of stem and how it differs from others is given in Section 3, Plant Propagation. PITH LENTICEL (Breathing pore) TERMINAL BUD AXILLARY BUD ONE YEAR OF GROWTH FROM ONE BUD SCAR TO ANOTHER LEAF SCAR BUD SCALE SCAR ▴▸ Figure 3–7 Parts of a stem. © 2017 Cengage Learning ® Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience.

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  eBook - ePub

  Wine Science

  Principles, Practice, Perception

  • Ronald S. Jackson(Author)
  • 2000(Publication Date)
  • Academic Press

    (Publisher)

  3 Grapevine Structure and Function

  Vegetative Structure and Function

  The uniqueness of some aspects of plant structure is obvious, even to the casual observer. However, many fundamental features become apparent only when studied microscopically. Unlike animal cells, plant cells are enclosed in rigid cell walls. Nevertheless, each cell initially possesses direct cytoplasmic connections with adjacent cells, through thin channels called plasmodesmata. Thus, embryonic plant tissue resembles one huge cell, divided into thousands of interconnected compartments, each possessing cytoplasm and a single nucleus. As the cells differentiate, many die and the plant begins to resemble longitudinal, semi-independent cones of tissue, connected primarily by specialized conductive (vascular) tissue.

  Most vascular cells elongate longitudinally, permitting the rapid movement of water and nutrients between superimposed sections of root and shoot tissue. The conduction of water and nutrients sideways between adjacent cells is limited, and direct translocation between tissues on opposite sides of shoots and roots is nonexistent.

  The vascular system consists of two structurally and functionally distinct components. The main water and mineral conducting elements, the xylem tracheae and tracheids, become functional on the disintegration of their cytoplasmic contents. The empty cell walls act as passive conduits. The primary cells translocating organic nutrients are the sieve tube elements of the phloem. They become functional only after their nuclei disintegrate.

  Plants also show a distinctive growth habit. Growth in length typically occurs behind special embryonic (meristematic) cells located in the shoot and root tips. Growth in breadth is initially limited to that resulting from the enlargement of cells produced in the shoot or root tip. Further growth in diameter occurs when a circular band of cells, the vascular cambium, becomes active and produces cells both laterally (to the sides) and radially (around the circumference). In addition, plants show distinctive growth patterns that generate leaves and their evolutionary derivatives, the flower parts. In these latter plant organs, sites of growth (plate meristems) occur dispersed throughout the young leaf or flower part. Nonuniform rates and patterns of growth of the plate meristems generate the characteristic shape of the respective plant part.

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