TRANSPORTATION OF LIVING MATERIALS -2
TRANSPORT IN PLANT
Introduction:
The transport system in plants is
not as complex as that of animals. Materials are transported by vascular
bundles made up of xylem and phloem tissues. Xylem tissue transports
water and mineral salts from the soil to all parts of the plant. Phloem tissue
transports manufactured food from the sites of photosynthesis to all parts of
the plant. In between the xylem and phloem is cambium. The cambium
divides to form newxylem and phloem
Vascular bundle in a stem showing the position of cambium
Components of the vascular system
Xylem
Xylem tissue is made up of the xylem
vessels and the tracheids. Mature xylem vessels and tracheids are made up of
hollow and dead cells. Their walls are made of cellulose and lignin. Lignin
strengthens the cell walls and makes them rigid. Therefore, xylem has an
additional function of giving support to the plant.
Xylem
vessels
The movement of substances in the
xylem is always upward and is by conduction. A xylem vessel is made of
hollow cells without end walls. These cells are joined end to end to form a
pipe-like structure. See above Xylem vessels begin in the roots, go up through
the stem and branch into every leaf of the plant.
Xylem vessels have no cytoplasm and
nuclei. This enables them to transport a larger volume of water and mineral
salts.
Tracheid elements are elongated with pointed (tapering) ends (Figure. below)
they are also laid end to end to enable continuous flow of water. Their end
walls have perforations (pits) unlike in xylem where end walls are missing.
This makes them less efficient in conduction of water.
Phloem
The phloem tissue is made up of
sieve-tube elements and companion cells.
Phloem
tissue
Like xylem vessels, sieve-tube
elements are made of cells that are joined end to end. However, the end walls
of these cells are not completely broken down. They have perforations or pores
that form sieve plates. These cells contain cytoplasm but they have no
nucleus. Fibres run through the pores thereby connecting adjacent sieve-tube
cells.
Each sieve-tube element has a
companion cell; they are separated by a thin wall made up of parenchyma
cells with pores called plasmodesmata which allow exchange of materials
between them.
Companion cells have a high
concentration of mitochondria. They provide the sieve-tube elements with
energy.
The movement of substances in the
phloem is by translocation. It can be in any direction.
The distribution of vascular bundles
in plants
The way the vascular bundles are
arranged in the roots, stems and leaves of monocots and dicots differ. This
arrangement also differs in the roots and stems of the two categories of
plants.
Monocotyledonous root
The arrangement of vascular bundles
is as shown:
Dicotyledonous root
The xylem is centrally positioned
and star-shaped. The phloem is found between the extensions of the xylem as
shown in Figure below
Monocotyledonous stem
The arrangement of vascular bundles
is random. See Figure below
Dicotyledonous stem
The vascular bundles are arranged
around the central pith, See Figure below
Absorption and movement of water and
mineral salts
Plants absorb water and mineral
salts from the soil through root hairs.
Structure and functions of root
hairs
Root hairs are extensions of the
epidermal cells of the root. Figure below shows the structure of a root hair.
Structure of root hair
Root hairs are long and slender to provide a large surface area for
the absorption of water and mineral salts from the soil. The large number of
root hairs also increases the total surface area of the roots.
The root hair cell sap is usually
hypertonic to the surrounding. Hence, water enters the cell by osmosis.
Root hair cells have a higher
concentration of minerals than the surrounding. Mineral salts are therefore
absorbed by active transport.
The root hairs are very thin in
order to provide a short distance over which absorption of water and mineral
salts takes place.
Movement of water and dissolved
mineral salts
When water is absorbed by the root
hair, it dilutes the contents of the cell sap vacuole. As a result, the cells
of the cortex, which are adjacent to the epidermis, have less water than the
root hair cells. Water moves from the root hair cells to the cortex cells by osmosis.
It moves the same way into the cells of the endodermis, then into the pericycle
and then into the xylem.
Movement of water from root hair xylem
Once in the xylem, the water and the
mineral salts dissolved in it move up the xylem vessel by transpirational pull,
capillarity and root pressure.
Transpirational pull
Transpiration occurs when water
evaporates from the plant through the stomata in the leaves. As the water is
lost, the mesophyll cells draw water from the xylem in the leaf which hi turn
draws water from the xylem in the stem. This creates a tension called transpirational
pull which draws water from the roots.
This results in a continuous column
of water from the roots, through the xylem to the leaves. This column of water
is called transpirational stream
Transpirational stream
Capillarity
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