SUPPORT AND TRANSPORT IN PLANTS

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SUPPORT AND
TRANSPORT IN PLANTS
ANATOMY OF DICOT PLANTS
The Root
• Two types of root systems
- tap root system
- adventitious root system
TAP ROOT SYSTEM
ADVENTITIOUS ROOT SYSTEM
- Arises from the radicle of
embryo
- Arises from any part of the
plant except from the radicle
the embryo
- Main root can be
- Main root cannot be
distinguished
Functions of the roots
• They anchor the plant firmly to the ground.
• They absorb water and mineral salts.
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.
Transverse section of young dicot root, in
plan
Transverse section of a young dicot root in
detail
Enlarged Root Hair
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
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
External structure of a dicot stem
Transverse section of a young dicot stem in
plan
Transverse section of a young dicot stem in detail
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
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
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.
Transverse section of stem after a complete ring of
cambium has been formed
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.
Annual Rings
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
Xylem Vessels
Pitted Thickening
Annular thickening
Spiral
Thickening
Scalariform
Thickening
Xylem Tracheids
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
Phloem
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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
Phloem tissue
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
Collenchyma and Sclerenchyma
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 SEMIPERMEABLE MEMBRANE
OSMOSIS: MOVEMENT OF WATER MOLECULES FROM
HIGH
WATER POTENTIAL TO LOW WATER POTENTIAL
THROUGH SEMI PERMEABLE MEMBRANE.
Lateral transport of water and mineral salts to the stele
of the root
Upward movement of water and mineral salts through
the stem from Roots to Leaves
Forces responsible for the upward
movement of water
• Transpirational pull
• Root pressure
• Capillarity
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
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
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
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
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.
Wilting Plant
Guttation
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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