SUPPORT AND TRANSPORT IN PLANTS

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SUPPORT AND TRANSPORT IN PLANTS

• ANATOMY OF DICOT PLANTS

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The Root

• Two types of root systems
• - tap root system
• - adventitious root system

TAP ROOT SYSTEM ADVENTITIOUS ROOT SYSTEM
- Arises from the radicle of the embryo - Arises from any part of the plant except from the radicle of the embryo
- Main root can be distinguished - Main root cannot be distinguished
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Functions of the roots

• They anchor the plant firmly to the ground.
• They absorb water and mineral salts.

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External Structure of Root Tip

• Root cap is made up of parenchyma cells. It
  protects the inner layers of the root.
• The growing point is made up of meristematic
  cells. Some of the new cells produced replace
  the parenchyma cells of the root cap which were
  damaged.
• Region of elongation the cells comes from the
  growing point which begin to elongate.
• The root hair region has root hairs, which absorb
  water and mineral salts.
• Internally the cells in this region begin to
  differentiate into epidermis, parenchyma,
  sclerenchyma, xylem and phloem
• Mature region has side roots and fully
  differentiated tissues.

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Transverse section of young dicot root, in plan
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Transverse section of a young dicot root in detail
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Enlarged Root Hair
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Functions of the various root tissues

• . The root hairs of the epidermis absorb water
  and mineral salts.
• The parenchyma of the cortex stores starch and
  the intercellular spaces allow water and mineral
  salts to pass through
• The passage cells of the endodermis direct water
  into the xylem of the stele.
• The pericycle gives rise to side roots
• The xylem transports water and mineral salts from
  the roots to the stem and leaves. It also
  provides strength to the plant.
• The phloem transports manufactured food from the
  leaves to the roots.
• The vascular cambium gives rise to additional
  xylem and phloem as the root grows in thickness

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The Stem

• Functions of the stem
• They hold the leaves in a favourable position to
  receive light for photosynthesis.
• They hold the flowers in a favourable position
  for pollination.
• Most stems contain chlorophyll and they therefore
  manufacture food by the process of
  photosynthesis.
• They store food and water
• They transport water and mineral salts from roots
  to leaves
• They transport food from the leaves to the roots

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External structure of a dicot stem
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Transverse section of a young dicot stem in plan
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Transverse section of a young dicot stem in detail
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Functions of the various stem tissues

• The cuticle reduces water loss through
  transpiration
• The cuticle is transparent allowing light to pass
  through for photosynthesis.
• The epidermis may have guard cells to allow
  gaseous exchange while at the same time reducing
  water loss through transpiration.
• The hypodermis of sclerenchyma or collenchyma
  strengthens the stem and helps keep it upright.
• The parenchyma of the cortex stores food.
• The intercellular air spaces between the
  parenchyma cells allow for transport of water and
  gases.
• The endodermal cells stores starch.
• The sclerenchyma fibres gives strength to the
  plant and help to keep it upright.
• Xylem transports water and mineral salts from the
  roots to the leaves
• Phloem transports manufactured food from the leaf
  to the roots

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Secondary growth of the stem

• Increase in thickness of the stem is called
  secondary growth or secondary thickness.
• Secondary thickness takes place in 3 steps
• Formation of secondary xylem and secondary phloem
• Formation of cork
• Formation of lenticels

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Formation of secondary xylem and secondary phloem

• Parenchyma cells between the vascular bundles
  begin to divide. This with vascular cambium forms
  a complete ring of cambium
• Entire cambium divides to form secondary xylem on
  the inside and secondary phloem on the outside.
• Every year a new ring of secondary xylem is
  formed. These are called the annual rings. (can
  tell the age of the tree by counting annual
  rings)
• The xylem cells making up the older annual rings
  becomes clogged and are no longer able to
  transport water and mineral salts. They become
  heartwood.

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Transverse section of stem after a complete ring
of cambium has been formed
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Formation of Cork

• Certain parenchyma cells in the cortex begin to
  divide and form cork cambium
• The cork cambium divides to form cork cells on
  its outside.
• The cork cells have suberin laid down on their
  cell walls.

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Annual Rings
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Formation of Lenticles

• In woody stems the stomata are blocked by the
  presence of cork cells
• The epidermis of woody stems breaks up to form
  tiny pores called lenticles which allow gaseous
  exchange.

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XYLEM

• Xylem tissue is made up of four types of cells
• - xylem vessels
• - xylem tracheids
• - xylem sclerenchyma
• - xylem parenchyma
• Xylem vessels and xylem tracheids are highly
  specialised cells.
• Xylem vessels are dead cells. They are long,
  cylindrical cells.
• Their cross walls are perforated or completely
  absent and form continuous tubes from roots to
  leaves.
• Xylem vessels have thick walls made up of lignin.
• Lignin is laid down in various patterns
• Annular thickening ( lignin laid in circles)
• Spiral thickening ( lignin laid in spiral)
• Scalariform thickening (laid in ladder form)
• Pitted thickening (laid in pits)
• Xylem tracheids are similar to xylem vessels
  except that the ends are tapered and cross walls
  are always present

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Xylem Vessels
Pitted Thickening
Scalariform Thickening
Annular thickening
Spiral Thickening
Xylem Tracheids
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Adaptation of xylem to transport

• Cross walls are perforated or completely absent
  this forms xylem roots forms continuous tubes
  with the xylem stems and leaves.
• Xylem vessles have no living contents allows
  the water to flow freely inside.
• The walls of the xylem vessels and tracheids are
  not completely thickened the unthickened
  portions and bordered pits allow water to move
  across the root and stem

• Adaptation of xylem to provide strength
• the vessels and tracheids are thick-walled
  they have a secondary wall of lignin
• the vessels and tracheids are round in
  cross-section round structures give additional
  strength

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Phloem

• Phloem tissue is made up of four types of cells
• Sieve tubes
• Companion cells
• Phloem parenchyma
• Phloem sclerenchyma
• Sieve tubes and companion cells are highly
  specialised cells
• Sieve tubes are living cells with long
  cylindrical cells.
• Their cross walls are perforated to form sieve
  plates
• They are thin walled cells
• They contain strands of cytoplasm which run
  through the sieve plates from one cell to the
  next
• Companion cells lie next to the sieve tubes
  they are thin walled, with cross walls and has a
  well-defined nucleus

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Phloem tissue
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The function of phloem tissue

• The sieve tubes transport manufactured food from
  the leaves to the all parts of the plant.
• The companion cells probably control the
  activities of the sieve tubes

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Collenchyma and Sclerenchyma
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Absorption and Transport of water and mineral
salts by plants

• Water enters the root hairs of the epidermis of
  roots by osmosis
• POINTS TO REMEMBER
• THE SOIL WATER IS THE DILUTE SOLUTION
• THE CELL SAP HAS CONCENTRATION SOLUTION BECAUSE
  OF HIGH CONCENTRATION OF SALTS.
• THE CELL MEMBRANE AND TONOPLAST IS SEMI-PERMEABLE
  MEMBRANE
• OSMOSIS MOVEMENT OF WATER MOLECULES FROM HIGH
  
• WATER POTENTIAL TO LOW WATER
  POTENTIAL
• THROUGH SEMI PERMEABLE MEMBRANE.

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Lateral transport of water and mineral salts to
the stele of the root
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Upward movement of water and mineral salts
through the stem from Roots to Leaves
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Forces responsible for the upward movement of
water

• Transpirational pull
• Root pressure
• Capillarity

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Forces responsible for the upward movement of
water

• Transpirational Pull
• Water evaporates from leaves
• Creates diffusion pressure gradient between
  outside air and xylem
• Tension exists in the water column extending from
  the roots to the leaves
• Adhesive and cohesive forces keep the column
  continuous
• Thus water is continually being pulled upward as
  water evaporates from the leaves
• Upward pull transpirational pull

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Forces responsible for the upward movement of
water

• Root pressure
• Cell sap of root hair always has low water
  potential when compared to soil water
• Water enters root hair by osmosis
• Continuous osmosis causes pressure forcing water
  a short distance up the stem
• Capillarity
• Spontaneous movement of water up narrow tubes
  because of adhesion and cohesion is called
  capillarity
• Lumen of xylem vessels and tracheids are very
  narrow therefore capillarity takes place
• Upward distance is very short

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Structural modifications of leaves to reduce
water loss(Internal factors limiting
transpiration)

• Sunken stomata In some plants stomata may be
  sunken or small.
• Thickened cuticle A thick cuticle on the surface
  of leaves reduces the rate of transpiration.
• Hair on leaves the sheen r shine of the hairs of
  some leaves reflect sunlight and reduces
  transpiration. Hairs also trap water vapour,
  restricting water loss.
• Shape, size and arrangement of leaves The shape
  and size of leaves determine the total surface
  area exposed to the environment, and hence
  influence the rate of transpiration. The leaves
  of some plants are arranged in such manner that
  they shade each other or overlap each other. In
  this water loss is restricted

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Environmental factors affecting the rate of
transpiration

• Humidity High humidity decreases transpiration
  rate
• Wind Increases evaporation and thus
  transpiration
• increases. Wind removes the water vapour
  around leaf thus increasing water vapour pressure
  gradient.
• Temperature Increases evaporation and thus
  transpiration
• Light generally causes an increase in the rate
  of transpiration

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Wilting Loss of turgidity of the cells results
in leaves and Sometimes the stems, becoming limp,
causing them to droop. Then the plant is called
wilting. Guttation Loss of liquid water through
tiny pores, called hydathodes on margin of leaf.
Guttation
Wilting Plant
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• Economic uses of plants related to their anatomy
• Paper
• Clothing
• Timber for furniture-making and building
  construction
• Basket-making
• Thatching, matting and broom-making

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