Vascular tissue is composed of xylem and the phloem. Xylem, which is the material you think of as wood, conducts water and dissolved mineral salts forming a continuous system that runs throughout the plant body. Water molecules enter, or leave a cell by osmosis. This is the movement of water molecules across a semi- permeable membrane from an area of high concentration of water molecules to a law concentration of water molecules.
Plants take up water mainly through the root hairs. Each root hair is a single,
long cell with a structure similar to the palisade. Within the system, water
and minerals dissolved in water passes from the roots up through the shoot in a
continuous stream. When water reaches the leaves, much of it passes into the
air as water, through the stomata, a process called transpiration.
Phloem conducts food and other dissolved materials in
both directions along the length of the plant. Most of the cells that form
mature xylem are dead. They do not have cytoplasm.
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| Vascular tissue (Xylem and Phloem)Source: http:www.headlandamenity.com/products_Chem_Cabadex.hlml |
As water transpires, more is drawn from the xylem. This movement of water exerts ‘suction ‘on the water filling the xylem vessels of the stem. As the water is ‘sucked’ upward through the xylem of the stem, more water is supplied to the bottom of the xylem by the roots causing a continuous movement of column of water from the roots to the leaves. This is called the transpiration stream.
Protective tissues
The protective tissue that forms the outer layer on
leaves, green stems and roots is called the epidermis. The cells of the
epidermis secrete a wax substance called cutin. Cutin forms a layer over the
outer surface of the epidermis called cuticle.
This layer cuts down on water and protects against infection by microorganisms.
Cork protects woody stems and roots. Cork is produced
by the cells of the cork cambium and lives for only a short time. Actually
fully grown cork is dead cells which protect the underlying living tissue.
Transpiration in plants
Transpiration is the evaporation of water from plants.
It occurs basically at the plant’s leaves while their stomata are open for the
passage of carbon dioxide and oxygen during the chemical process of photosynthesis.
The diagram of a plant below shows the movement of water from the soil into the
leaves up to the leaves.
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| Transpiration in Plants |
This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem vessel of the roots and stem.
1. Importance of transpiration
Transpiration is the process that pulls water up from
the roots to:
1) supply
water for photosynthesis,
2) bring
minerals from the roots for biosynthesis within the leaf, and
3) Cool
the leaf.
2. Translocation
The phloem conducts carbohydrates (mostly sucrose) to
where they are needed by the plant as food. Food conduction in phloem is
carried out through two kinds of elongated cells, the sieve cells and sieve
tube members. The movement of sugars and other substances from one region
to another through the sieve cells is called translocation:
3. Factors affecting transpiration
Investigating
the factors affecting the rate of transpiration
Using a potometer (right), one can study the effect of
various environmental factors on the rate of transpiration. As water is
transpired or otherwise used by the plant, it is replaced from the reservoir on
the right. This pushes the air bubble to the left providing a precise measure
of the volume of water used.
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| The factors affecting the rate of transpiration |
4. Environmental factors that affect the
rate of transpiration
(i)
Light
Plants
transpire more rapidly in the light than in the dark. This is largely because
light affects the opening of the stomata. Stomata open fully when a plant
receives more light than when it is dark.
(ii) Temperature
Green plants transpire faster at higher temperatures
because water evaporates more rapidly as the temperature rises. For example, at
30°C, a leaf may transpire three times as fast as it does at 20°C. High
temperatures make more water evaporate from leaves.
1.
Humidity
The rate of diffusion of any substance increases as
the difference in concentration of the substances in the two regions increase.
When the surrounding air is dry, diffusion of water out of the leaf goes on
more quickly.
Wind
On windy days, transpiration increases as the wind
blows around the leaves, blowing away any water vapour that diffused out from
the leaf and could be on the surfaces When the air is still, the air
surrounding a leaf becomes increasingly humid thus reducing the rate of
transpiration.
Soil water
A plant cannot continue to transpire rapidly if its
water loss is not by replaced from the soil. When absorption of water by the
roots fail to keep up with the rate of transpiration, loss of turgor occurs,
and the stomata close. This immediately reduces the rate of transpiration (as
well as of photosynthesis). If the loss of turgor extends to the rest of the
leaf and stem, the plant wilts.
5. Control of transpiration in plants
Plants in areas where rain is seasonal have specially
developed features to overcome excessive loss of water by transpiration. We say
they have adaptations for conserving water. The following are some ways of
adaptations for plants:
(iii) They shed their leaves in the dry season to survive through the dry
season.
(iv) Some plants have leaves whose surfaces are hairy or leaves with sunken
stomata for reduction in transpiration.
(v)
Some trees such as mangoes, oranges,
avocado and pine are evergreen. These retain their leaves all year round. The
reduction in transpiration is because these leaves are covered with a layer of
wax.
(vi) Other plants such as pine and cactus have small and needle-like leaves
to 'allow very little transpiration.
(vii) Fewer stomata on the upper surface of leaves like mango plants.
(viii) The upper surfaces are shinny, thus reflects some of the direct light.
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