2.2.4 Transport systems in plants
Two transport systems are required in plants, one for water and
Structure of minerals and the other for assimilates. Collectively these are known
vascular tissue as vascular tissue.
in plants
A source is where plant assimilates are synthesised through
photosynthesis and released into the transport system.
A sink is an area which takes up assimilates from the
transport system.
Vascular tissue
Consists of xylem and phloem tissue organised into vascular
bundles.
In addition there are groups of packing cells called parenchyma
cells, and also some cambium. This contains meristematic cells
capable of cell division.
Flowering plants are organised into monocotyledons and
dicotyledons. The classification is based on the number of primary
leaves that grow from their seeds.
The seed of a dicot grows two primary leaves whereas a
monocot only grows one.
Xylem structure
Xylem transports water (and soluble mineral ions) up the plant.
The main type of tissue in xylem is the xylem vessel element.
-The cells are long and thin with thickened walls.
-Walls contain lignin which is impermeable to water.
This means the cell contents of the vessel elements die
quickly when the xylem is first formed.
The cells are arranged one above the other in columns. This leaves
a long, thin column called a xylem vessel. This offers little resistance
to the passage of water.
Lignin
Function:
Keeps the xylem vessels open and prevents them collapsing
inwards.
Also provides support and strengthens the walls.
, Xylem vessel wall has pores where there is no lignin, called bordered
pits.
-This allows water to move horizontally between one xylem vessel
and another.
This ensures water movement can continue even when there
is a blockage in one vessel.
Phloem structure
Phloem transports assimilates and sugars up and down the plant.
Made up of two types of cells:
Sieve tubes
Long thin cells laid end to end.
End walls have many small pores to form a sieve plate.
This allows sugar sap to flow from one cell to the next.
Cells contain very little cytoplasm and have no nucleus or
tonoplast.
This also means that the sugar sap can move through
unimpeded.
Companion cells
Small thin cells, about the same length of sieve tubes but much
thinner.
-Contain lots of mitochondria to produce ATP needed for metabolic
processes in both the sieve tubes and companion cells.
Between the sieve tubes and companion cells are many
plasmodesmata (small gaps in the wall) which allow communication
and exchange of materials between the sieve tube element and the
companion cell.
Arrangement of vascular tissue
Monocots: Dicots:
Root Vascular tissue found Vascular tissue found in a
towards the centre of the central core.
root. Xylem forms the arms of an
Arranged as a circle of X and the phloem is found
xylem and phloem between the arms.
tissue. Around the whole vascular
tissue is the endodermis.
Stem Vascular bundles rarely Outermost layer is the
form a circle, instead are epidermis.
scattered evenly across Under this is the cortex,
the whole stem. which is a thin layer of
Xylem towards the inner parenchyma cells tightly
part of the stem and packed.
phloem tissue to the Below this the vascular
outside. bundles are organised in a full
Two transport systems are required in plants, one for water and
Structure of minerals and the other for assimilates. Collectively these are known
vascular tissue as vascular tissue.
in plants
A source is where plant assimilates are synthesised through
photosynthesis and released into the transport system.
A sink is an area which takes up assimilates from the
transport system.
Vascular tissue
Consists of xylem and phloem tissue organised into vascular
bundles.
In addition there are groups of packing cells called parenchyma
cells, and also some cambium. This contains meristematic cells
capable of cell division.
Flowering plants are organised into monocotyledons and
dicotyledons. The classification is based on the number of primary
leaves that grow from their seeds.
The seed of a dicot grows two primary leaves whereas a
monocot only grows one.
Xylem structure
Xylem transports water (and soluble mineral ions) up the plant.
The main type of tissue in xylem is the xylem vessel element.
-The cells are long and thin with thickened walls.
-Walls contain lignin which is impermeable to water.
This means the cell contents of the vessel elements die
quickly when the xylem is first formed.
The cells are arranged one above the other in columns. This leaves
a long, thin column called a xylem vessel. This offers little resistance
to the passage of water.
Lignin
Function:
Keeps the xylem vessels open and prevents them collapsing
inwards.
Also provides support and strengthens the walls.
, Xylem vessel wall has pores where there is no lignin, called bordered
pits.
-This allows water to move horizontally between one xylem vessel
and another.
This ensures water movement can continue even when there
is a blockage in one vessel.
Phloem structure
Phloem transports assimilates and sugars up and down the plant.
Made up of two types of cells:
Sieve tubes
Long thin cells laid end to end.
End walls have many small pores to form a sieve plate.
This allows sugar sap to flow from one cell to the next.
Cells contain very little cytoplasm and have no nucleus or
tonoplast.
This also means that the sugar sap can move through
unimpeded.
Companion cells
Small thin cells, about the same length of sieve tubes but much
thinner.
-Contain lots of mitochondria to produce ATP needed for metabolic
processes in both the sieve tubes and companion cells.
Between the sieve tubes and companion cells are many
plasmodesmata (small gaps in the wall) which allow communication
and exchange of materials between the sieve tube element and the
companion cell.
Arrangement of vascular tissue
Monocots: Dicots:
Root Vascular tissue found Vascular tissue found in a
towards the centre of the central core.
root. Xylem forms the arms of an
Arranged as a circle of X and the phloem is found
xylem and phloem between the arms.
tissue. Around the whole vascular
tissue is the endodermis.
Stem Vascular bundles rarely Outermost layer is the
form a circle, instead are epidermis.
scattered evenly across Under this is the cortex,
the whole stem. which is a thin layer of
Xylem towards the inner parenchyma cells tightly
part of the stem and packed.
phloem tissue to the Below this the vascular
outside. bundles are organised in a full
