Plant systems A: Transport and support
Most large land plants belong to the Phylum Tracheophyta which have
a system of internal tubes to transport water and nutrients around the
plant . These tubes - called vascular bundles - also provide support
for the plant, allowing the green leaves maximum exposure to sunlight.
Xylem
The tubes which form a pipeline for water and minerals from the
soil are called xylem. The water is transported through the tiny root
hairs, up through the roots and stems to the leaves where it is required
for photosynthesis (see Plants
systems B: Photosynthesis, nutrition and respiration). Much of this
water evaporates through small openings (stomata) on the underside of
the leaves. This process, called transpiration, helps draw more water
up the plant against gravity, because of the cohesive force between
the water molecules. As it is the Sun which evaporates the water creating
the "pull", the Sun is therefore the energy source for water
uptake.
A medium-size tree (e.g. beech) can lose about 500 kg of water
a day through transpiration. Desert plants reduce this loss by closing
off the stomata in the heat of the day, having smaller leaves, or having
the stomata embedded in pits, or displaying the leaves vertically to
reduce the amount of sunlight falling directly on them.
The
xylem vessels become thickened and woody as the plant grows, thus providing
support for the stem so that maximum display of leaves to sunlight is
possible.
The bundles of xylem tissue can easily be seen in a cut stick of celery
placed in coloured water.
Phloem
As all living cells need energy, food made in the leaves by photosynthesis
must be carried to all parts of the plant, both downwards to the roots
and upwards to the growing shoots.
Alongside the xylem, in the vascular bundles, is the phloem tissue.
The cylindrical-shaped cells of the phloem lie end-to-end forming long
tubes transporting sugars and proteins.
"Ringbarking" large trees involves cutting through the bark
deep enough to damage the phloem tissue preventing food transport throughout
the plant. The tree therefore slowly dies so it can no longer take up
water which is then available for use by surrounding crops. The tree
does, however, remain intact providing hollows and nesting sites for
animals.
Living in water
Because water provides buoyancy, large marine kelp do not need
vascular tissue (xylem and phloem) to support the plant or hold the
photosynthesising parts up to the light. Water and minerals can pass
directly into all parts of the plant as it is in direct contact with
the surrounding water. Therefore complex transport and support systems
are not necessary.
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