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karthikmns
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eval_testing_mns

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cut stem portion in the glass tube until water level can be seen
soil above the rubber tube. Mark the level of
water (M 1 ) in the tube. Keep your
arrangement aside for 2 to 3 hours. Then
water observe and mark the water level (M2) in the
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tube.
Is there any increase in the water level?
fig-17: Root pressure What is the role of xylem in this action?
The difference between M2 and M-1 indicates the level of water raised
in the stem. Because of the root pressure, the water level increases in the
tube.
LA
The mechanism of water movement in plants
We have seen that there is a push from below due to root pressure on
the columns of water in the xylem vessels, but this is seldom high and in
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some seasons it is nil. How does the water reach 180 metres high to the
top of a tree like a eucalyptus?
Let us recall the activity that you performed in lower classes. Why
inner sides of cover become moist? Where do these water droplets or
water vapour come from?
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We know that this type of evaporation of water through
leaves is called transpiration. Water evaporates through
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stomata of leaves and lenticels of stem.
When the leaves transpire, there is a pulling effect on
the continuous columns of water in the xylem vessels. The
top ends of these vessels are surrounded by the leaf’s
mesophyll cells which contain cell sap, so the water is
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continuous from the xylem vessels to the walls of the
mesophyll cells from which it evaporates into the air spaces
causing the pull. The water column does not break because
of its continuous molecular attraction. This is a property of
water you demonstrate every time you drink through a straw.
Now we have a picture of the water-conducting system
fig-18: Transpiration of a tree. Water is absorbed by osmosis from the soil by
66 X Class Transportation - The circulatory system
the root hairs. This is passed into the xylem vessels which form a continuous
system of tubes through root and stem into the leaves. Here the water
evaporates and releases into the atmosphere. The evaporation creates the
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main pull of water above root pressure which gives a variable and minor
push from below. This results in a continuous column of moving water,
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the ‘transpiration stream’.
Is there any relation between transpiration and rain fall?
The amount of water passing through a plant is often considerable.
For example, an oak tree can transpire as much as 900 liters of water per
day. It follows therefore that areas of forest significantly affect the degree
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of saturation of the air above them, so that when air currents bring air
which is already nearly saturated to a forest area, it becomes fully saturated
and comes down as rain; this is why forest areas often have a higher rainfall
than areas nearby. LA
Do you know?
How much water is transpired by plants? Each fully grown maize plant transpires 15
liters per week. One acre of maize may transpires more than 13,25,000 liters of
water in a hundred day growing season. A big mango tree will transpire from 750 to
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more than 3500 liters of water per day during growing season.
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Transport of mineral salts
You know that mineral salts are necessary for plant nutrition (micro
and macro nutrients) and that they are obtained from the soil solution
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