Stems and Roots by sDuUx1i


									Stems and Roots

  Chps 9 and 10
              Chp 9 Vocabulary
•   Lignin
                       •   Secondary xylem
•   Tracheids
•   Vessel elements
                       •   Secondary phloem
•   Primary xylem      •   Cork
•   Primary phloem     •   Periderm
•   Vascular bundles   •   Suberin
•   Sieve elements     •   Pressure (mass) flow
•   Companion cells    •   Vascular cambium
•   Symplastic         •   Cork cambium
  Stems are Fundamental Plant Organs
• Vascular plants are those plants that have a conducting system
  composed of vascular tissue (xylem and phloem).
• Stems are indispensable organs for most plants. All other organs
  (leaves, buds, roots) are attached to stems. Stems enable plants to
  increase their height or length, mass, and surface by the activity of
  apical meristems.
• Plant stems are usually branched, which allows increase in mass
  and the amount of surface available for attachment of leaves and
  reproductive structures. The more leaves on a stem, the greater
  the amount of sunlight they can harvest in photosynthesis.
• Stems transport water and minerals collects by roots from the soil
  to the leaves where these materials are needed for photosynthesis
  (xylem), and conduct sugars produced in the leaves to roots and
  any other places where sugar fuel is needed (phloem)
• Xylem is defined by the presence of the tough, waterproofing
  compound lignin on walls of specialized cells – tracheids and vessel
  elements. Lignin also provides support to vascular tissue and thus
  the plant.
Structure and Function of Stems
        • In herbaceous (nonwoody) stems and the
          young stems of woody plants, xylem and
          phloem tissues differentiate from precursor
          tissue (procambium) formed by the apical
        • Mature conducting tissues formed in this way
          are known as primary xylem and primary
          phloem. These primary conducting tissues are
          located near each other within elongate
          vascular bundles.
        • The vascular tissue in a plant is interconnected
          – extending from the roots, through thte stem,
          into branches and leaves and other organs like
          the water pipes in your house.
Phloem Tissues
   • Phloem tissues include pipeline
     components known as sieve elements,
     which may consist of sieve cells or sieve
     tube members end to end.
   • Sieve elements possess pore-containing
     end walls known as sieve plates. Their
     perforations develop by expansion of
     plasmodesmata. Pores in the end walls of
     sieve elements allow phloem sap – a
     watery solution of sugars and other organic
     molecules – to move freely from one cell to
     the next.
   • Phloem sieve elements are alive at
     maturity, but the nucleus and some other
     cell components are degraded during sieve
     element development and are thus absent
     from mature cells. In order to function,
     sieve elements require the help of the
     adjacent companion cells, which have
     nuclei and provide materials to the sieve
     cells via plasmodesmata.
• Ex. When a plant is cut or wounded, P protein
  (phloem protein) masses along the sieve plate of
  sieve elements, forming a “slime plug”. Such plugs
  function to reduce the loss of phloem sap, much as
  clots reduce blood loss from the vascular system of
• Another wound response is deposition of the
  carbohydrate callose, which also helps to plug
  phloem sap leaks.
Phloem Conducts Sugars
     • Phloem provides plants with a long-distance
       transport system. The direction of transport in
       phloem is from the source of organic molecules
       to sites, known as “sinks”, where molecules are
       utilized like roots, flowers, etc.
     • Direction is “source to sink”
     • In some plants, sugars are loaded from the cells
       producing them either directly into sieve
       elements or indirectly, via companion cells
       through plasmodesmata. This process is known
       as symplastic loading.
     • Other plants have apoplastic loading of
       phloem, which occurs from intercelular spaces.
       This means that it will require active transport
       through the cell membrane.
     • The force that moves organic compounds
       within the phloem is known as pressure flow or
       mass flow and is based on osmosis and cell
       water potential.
  Water Moves bc of Transpiration
• With a few exceptions, plants obtain their water and minerals
  from the soil and move these materials via the root’s xylem
  into the stem. The stem’s xylem’s main function is to
  transport water and minerals to other organs.
• Water moves in the xylem as the result of transpiration, the
  evaporation of water from plant surfaces like the stomata. A
  stream of water rises through the plant as each water
  molecule lost from a cell at the surface is replaced by another
  from inside the cell, which in turn exerts an attractive force on
  nearby water molecules, causing the water to rise.
• Xylem will help new leaves and flower buds grow by placing
  sugar in a watery solution to flow up the tree (days are warm,
  nights are cold in spring). Maple trees are good sources of
  such xylem sap and are cultivated in large plantations called
  sugar bushes. With care, 150 L can be tapped per year
  without harming the tree.
             Wood and Bark
• Many plants produce no wood or bark, but
  woody plants produce wood tissue and bark
  by the action of two meristems (vascular
  cambium and cork cambium).
• The girth increase of a tree trunk is known as
  secondary growth. The cambiums are
  secondary meristems.
              Vascular Cambium
• Mature vascular cambium takes the form of a
  cylinder. It produces lignin-rich secondary xylem
  tissue on the inside (wood) and secondary phloem on
  the outside (inner bark).
• Addition of a thick cylinder of wood requires that the
  circumference of the vascular cambium must increase,
  necessitating the addition of new vascular cambium
  cells by cell division.
• Ray initials produce ray parenchyma cells and ray
  tracheids (together form vascular rays). Rays store
  things and transport food laterally across the stem.
• During each growing season, the
  vascular cambium produces new
  cylinders of secondary xylem,
  adding new wood and growth
  rings. Rings from previous years
  may still transport water, but
  really old rings (toward the
  center) can become clogged by
  tyloses from neighboring
  parenchyma cells.
• Innermost wood is heartwood:
  full of decay-resistant chemicals,
  good for furniture
• Phloem exists towards the
  outside, in the inner bark layer.
  They are susceptible to bark
  damage because it would cut off
  its food supply (girdling or
  ringing a tree will kill it).
Cork Cambiumbegin to enlarge, the
 • As young woody stems
     delicate epidermis eventually ruptures and its
     protective role is replaced by cork.
 •   Cork is produced to the outside of a secondary
     meristem called the cork cambium.
 •   Together, the cork, cork cambium and
     parenchyma cells make up the periderm.
     When periderm become worn out, they will
     be replaced by a new periderm on the inside.
     Eventually the old periderm will create outer
     bark. The outer bark is dead whereas inner
     bark is still living.
 •   Cork cell walls have layers of lignin and
     suberin. Suberin helps prevent microbial
     attack and also waterproofs the stem’s
 •   Lenticels are slightly raised patches of various
     shape that interrupt the bark’s cork layer to
     allow gas exchange for the inner stem tissues.
                Human Uses for Stems
• Paper: Paper started as papyrus, which was made from the stems of
  the papyrus plant. Papyrus also made rafts, sails, cloth, and cord. To
  make paper, the Egyptians peeled the outer layers of papyrus stemss
  off, exposing the pith. The pith was sliced into thin strips and laid
  across one another. Workers then pounded the layers making starch
  release from the cells and thus gluing the strips together and then
  dried in the sun. Today, most paper is made from wood pulp. Genetic
  engineers are working on making trees with more cellulose and less
  lignin for paper. Lignin byproducts of the pulp are toxic and can
  threaten water supply.
• Cork: The cork oak’s cork layer is several inches thick. It can be
  stripped without hurting the tree. Cork is able to float and is used as
  insulation, floor covering, shoe soles and bottle stoppers.
• Bamboo: used for housing in some areas. UK is working on creating
  earthquake-proof housing with bamboo.
• Wood: construction material, fuel, paper, furniture. Known for
  strength and beauty and makes up more than 1% of the world’s total
  economy. Species like redwood and white oak are desired for ship
  building. Basswood, yellow birch, and black cherry are valued for
  making musical instruments because their structure lends to a
  beautiful tone.
           Chapter 10 Vocabulary
•   Embryonic root
•   Storage root       •Feeder root
•   Prop root          •Root hairs
•   Aerial root        •Gravitropism
•   Buttress root
•   Lenticel           •Micorrhizal fungi
•   Epiphytic plants   •nitrogen-fixing bacteria
•   Taproot system
       Roots Play a Variety of Roles
• When seeds germinate, the first plant organ to emerge is the
  embryonic root, the radicle,– and a primary root is soon present on
  young plants.
• Shoot development depends on enlargement of cells by water uptake.
  And photosynthesis requires water to serve as the necessary electron
  donor. Both of these processes are highly dependent on an early
  water and mineral supply.
• Bryophytes do not have roots. Moss and bladderworts are examples.
  Because their leaves are so thin, they can directly absorb water and
  minerals from their very wet environment.
• Some roots store carbohydrates during the first year of growth of
  biennials. Carrots, sugar beets, parsnips, and rutabagas are biennials
  grown for their food-roots.
     Roots are Hormone and Secondary
              Compound Sites
• Roots produce the plant hormones cytokinins and
  gibberellins, which are transported in the xylem to
  the shoot, where they influence growth and
• Roots are also a site for producing protective
  secondary compounds. Ex. Nicotine is made in the
  roots of tobacco plants and moved to the leaves to
  act as a poison that helps prevent herbivore attack.
• The roots of an African tree has long been known to
  produce a yellow substance used by healers to treat
  syphilis and leprosy (both caused by bacteria).
  Recent studies showed that the yellow compound
  (identified as a terpene) does in fact kill bacteria and
Root Support
          • If you look closely at the
            base of corn plants, you
            may notice prop roots,
            growing from the stem
            into the soil. These
            specialized roots help
            the tall corn plants stay
            upright even though
            they lack woody tissue.
          • Some tropical trees grow
            in thin soil and use
            buttress roots to help
            keep from falling over on
            a windy day.
          • Aerial roots form from a
            stem and form massive
            columns to support the
            heavy branches.
                      Specialized Roots
• Pneumatophores (“breath bearers”) are specialized roots of some types of
  mangrove trees. They grow upward into the air, absorb oxygen rich air via
  surface openings – lenticels.
• When the tide is up, the lenticels are protected by waterproofing
  substances. When the tide goes back down, air is sucked into the lenticels.
• Some herbaceous plants like dandelions have contractile roots which
  shorted by collapsing their cells. This allows the root to pull deeper into
  the ground where it’s warm to survive changing early spring weather.
• Parasitic plants like the dodder obtain water, minerals, etc from host plants
  by producing rootlike organs that penetrate the host’s stem and tap into
  the host’s vascular system.
• Epiphytic plants grow non-parasitically on other plants and have
  specialized roots Their roots are aerial and are photosynthetic. Ants often
  form an association with such plants and provide nitrogen to the plant with
  their waste.
      Types of Underground Root Systems
• Plant roots differ in their external form. These differences
  result from variations in the fate of a seedling’s primary root.
  For example, in gymnosperms and eudicot angiosperms, the
  primary root generates a taproot system – single main root
  from which many branches emerge.
• In grasses and other monocots, the primary root lives for a
  short time and is replaced by a system of roots that develop
  from the bottom of the plant’s stem. Roots from a stem are
  called adventitious roots. Many adventitious make up a root
• If no single root is most prominent, then we
  say it’s a fibrous system (many branched
  roots). These are usually shallower in the
  ground than a taproot.
• Feeder roots, produced by both taproot and
  fibrous root systems are fine (<2mm in
  diameter) peripheral root that are most active
  in absorbing water and minerals from the soil.
  Feeder roots have limited lifespans and are
  continuously replaced.
• Knowledge of feeder roots is helpful in
  landscaping. When transplanting a plant, make
  sure you know where the roots end so you do
  not risk cutting them when digging the plant
  up, otherwise the plant may not be able to
  obtain nutrients from the injured roots and not
External Root
      Root Structure and Function
• Feeder roots are young branch roots. Soil texture influences
  root branching. Plants that must grown through hard, dry soil
  have fewer branch roots than those growing in moist, loose
• Branch roots and the main root axis are covered by an
  epidermis, which is sometimes covered by a cuticle.
• A region closer to the root tip is fuzzy with countless root
  hairs – fingerlike extensions from some epidermal cells. For
  most roots, these hairs are the main location of water and
  mineral absorption, and root hairs are a major site of uptake
  selectivity, the ability of plant roots to discriminate between
  useful and harmful soil minerals.
• At the cone-shaped tip, there is a root apical meristem
  (RAM). This region of meristematic cells, which divide rapidly,
  increasing the number of cells in the main portion of the root.
• Protecting the RAM is a root cap, whose cells are also generated by
  the apical meristem. Cells in the center of the root cap contain
  starch-rich plastids, amyloplasts. Some experts think that
  amyloplasts operate as gravity sensors since they are heavy enough
  to fall as the root grows, thus signaling the downward growth path
  normal to most root cells.
• Other experts believe there are different mechanisms for
  gravitropism, a root’s growth response to gravity. Plant biologists
  do not fully understand why plants know which way is down.
• Root cap cells slough off the root tip a few days after being created,
  so they must continually be replaced. These dispersal cells, known
  as root border cells, do not then just die, but apparently help
  modify the external root environment in ways that prevent attack
  by microbes and tiny soil worms.
• The tips of roots are embedded in a blanket of mucigel, a gluey
  substance secreted from the Golgi apparatus of root tip epidermal
  cells. Mucigel lubricates the root and helps in water and mineral
  absorption and creates a favorable environment for beneficial
            Root Mineral Absorption
• Root xylem obtains minerals and water in one of two ways
   – Water and minerals are selectively taken up by root hairs and
      transmitted via plasmodesmata
   – Water and minerals that penetrate root tissues within intercellular
      spaces and cell walls are selectively absorbed at the cell membrane
      of nonsuberized surfaces of endodermal cells and released on the
      other side.
• Mineral passage from root hairs through the cortex and endodermis via
  plasmodemata is known as symplastic transport. This allows beneficial
  minerals to be absorbed and harmful ones to be excluded.
• When minerals dissolved in water diffuse from the root’s environment into
  epidermal cell walls, then through walls of cortical cells to the
  endodermis, such movement is known as apoplastic transport. In
  apoplastic transport, harmful minerals are unable to be excluded which
  could cause the plant to be injured.
 Root Hairs Have Selective Absorption
• Epidermal root hairs and cells of the endodermis
  filter out mineral content of water.
• Many metal ions (iron, copper, manganese, and
  magnesium) are needed by plants for the proper
  functioning of enzymes and other complex molecules
  in plant cells. Magnesium is used in chlorophyll and
  iron is an electron carrier in photosynthesis and
• Aluminum is abundant in soil but toxic to plants. It
  can bind to things like proteins and nucleotides
  causing disruption in membrane function. The first
  symptom of aluminum toxicity in plants is that roots
  stop elongating within 5 minutes of exposure.
• Aluminum toxicity is a major
  limitation in growing crops. Its
  more prominent in acidic soils in
  high industry areas.
• Acid rain can turn soil acidic.
  When soil gets below a pH of 5,
  positively charged metals stick to
  the soil releasing aluminum ions
  that are then dissolved in the soil
  water and available for
  absorption. Plants can bind the
  aluminum by releasing organic
  acid, but the best way to avoid it,
• Phosphate is needed to construct phospholipid
  membranes, ATP, and DNA/RNA. Phosphate is one of
  the most important components for plants.
• Phosphate forms strong chemical bonds with iron
  and aluminum oxide minerals in solid, reducing its
  availability. The organic acids mentioned earlier can
  help dissolve the aluminum, freeing phosphate.
• Plant root cell membranes contain transporter
  proteins whose shapes enable them to bind even
  small amounts of soil phosphate and move it into the
  cell. When soil phosphate is low, root cells increase
  the number of phosphate transporter proteins
    Roots Need Food and Oxygen
• Plant root cells are efficient at mineral absorption,
  but they use a lot of ATP. ATP is also required for cell
  division at the root tip.
• The most efficient mode of ATP production is aerobic
  respiration (uses O2). Roots cannot photosynthesis,
  so the phloem must bring them sugar sap.
• Roots of a plant are called heterotrophic because
  they (like animals) must consume their food (don’t
  make it).
• Root produced carbon dioxide dissolves in soil water
  to produce carbonic acid which will cause weathering
  of the soil. This is another way plants help reduce
  carbon dioxide.
                Beneficial Microbes
• Beneficial microorganisms can form symbiotic relationships
  with plant roots.
• These include mycorrhizal fungi and nitrogen-fixing bacteria,
  which live within roots of legumes and some other plants.
• These microbes help plants obtain the large amouts of
  minerals needed for growth (this growth would otherwise be
  limited). This allows for a better competing plant and a higher
  crop yield.
• Mycorrhizal fungi are important in providing phosphate.
  Almost all vascular plants have mycorrhizal fungi.
• Nitrogen-fixing bacteria supply the nitrogen compounds
  required by plants to produce amino acids and proteins.
  Legumes have much closer associations with such bacteria,
  producing special root nodules whose tissues harbor
  nitrogen-fixing bacterial partners.
• Legume-bacterial relationships begin with a
  chemical conversation.
• Legume roots secrete flavonoids into the soil
  (secondary compound)
• Legume-root flavonoids signal to soil N-fixing
  bacteria, which respond to the flavonoid signal
  by secreting small organic molecules into the
• Legume-root epidermal cell membranes contain
  receptor molecules that recognize and bind
  molecules excreted from specific bacteria.
  These bacterial compounds cause the root hairs
  to curl within special root cells.
• Root nodules contain both infected and
  uninfected cells and mature nodules possess
  vascular tissue that connects with the root
  vascular system that distributes compounds
  containing nitrogen throughout the plant.
• The bacteria are plant specific due to the
  secondary compounds released.

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