Stems and Roots

1
Stems and Roots

• Chps 9 and 10

2
Chp 9 Vocabulary

• Lignin
• Tracheids
• Vessel elements
• Primary xylem
• Primary phloem
• Vascular bundles
• Sieve elements
• Companion cells
• Symplastic loading

• Secondary xylem
• Secondary phloem
• Cork
• Periderm
• Suberin
• Pressure (mass) flow
• Vascular cambium
• Cork cambium

3
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.

4
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 meristem.
• 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.

5
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.

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• 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 animals.
• Another wound response is deposition of the
  carbohydrate callose, which also helps to plug
  phloem sap leaks.

7
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.

8
Water Moves bc of Transpiration

• With a few exceptions, plants obtain their water
  and minerals from the soil and move these
  materials via the roots xylem into the stem.
  The stems xylems 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.

9
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.

10
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.

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• 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).

12
Cork Cambium

• As young woody stems begin to enlarge, the
  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 stems surface.
• Lenticels are slightly raised patches of various
  shape that interrupt the barks cork layer to
  allow gas exchange for the inner stem tissues.

13
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 oaks 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 worlds 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.

14
Chapter 10 Vocabulary

• Embryonic root
• Storage root
• Prop root
• Aerial root
• Buttress root
• Lenticel
• Epiphytic plants
• Taproot system

• Feeder root
• Root hairs
• Gravitropism
• Mucigel
• Pericycle
• Micorrhizal fungi
• nitrogen-fixing bacteria

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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.

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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
  development.
• 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 fungi.

17
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.

18
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 its 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 hosts stem
  and tap into the hosts 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.

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Types of Underground Root Systems

• Plant roots differ in their external form. These
  differences result from variations in the fate of
  a seedlings 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 plants stem. Roots from a stem are called
  adventitious roots. Many adventitious make up a
  root system.

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• If no single root is most prominent, then we say
  its 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 (lt2mm 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 survive.

21
External Root

22
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
  soil.
• 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.

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• 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 roots 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 microbes.

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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
  roots 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.

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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 respiration.
• 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.

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• 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, is to uh, avoid it! ?

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• 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

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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
  (dont 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.

29
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.

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• 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
  soil.
• 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.