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Examine some mustard seedlings which have been grown on wet filter |
paper. Observe the mass of fine threads coming from the seed by hand |
lens. These are roots. They have fine microscopic structures called root |
hairs. These are root hairs through which water enters the plant. Gently |
squash a portion of the root hair between slide and cover slip in a drop of |
water and examine under a microscope. Note the thinness of the walls of |
root hairs. It is not completely understood how the water enters the root |
hairs and passes inwards from cell to cell until it gets into the xylem vessels, |
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64 X Class Transportation - The circulatory system |
but there is no doubt that osmosis plays an important role.(IX class |
plasmamembrane) |
Every living cell acts as an osmotic system, the cytoplasm lining of |
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the cell wall acts as the semipermeable membrane. Observe the following |
figure, notice how do roots penetrate into soil? You will find that the root |
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hairs grow out into the spaces between the soil particles and that the hairs |
are surrounded by moisture. |
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epidermal cell |
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soil particles |
soil water |
xylem vessel |
nucleus |
air spaces |
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cells of cortex cell wall of root hair |
LA vacuole |
cellsap |
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fig-16: L.S of root showing relationship of root hair and soil water |
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Note: In fig-16 Arrow marks shows the direction of flow of water. |
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The soil water is an extremely dilute solution of salts. Soil water |
concentration is more dilute than that of the cell sap in the root hair; |
therefore water will pass into the vacuole of the root hair by osmosis. |
Recall the process of osmosis that you have learnt in the chapter “moving |
of substances through plasma membrane” in class IX. The entry of water |
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dilutes the contents of the root hair vacuole so that it becomes more dilute |
than it’s neighbouring cell. |
So, water passes into the neighbouring cell which in turn becomes |
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diluted, finally water enters the xylem vessels. As there are vast number of |
root hairs and root cells involved, a pressure in the xylem vessels develops |
which forces the water upwards. This total pressure is known as root |
pressure. Root pressure is not the main cause of movement of water in |
xylem but it is certainly one of the factor. The other factors are also there. |
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You will learn about those reasons in higher classes in detail. |
Activity-6 |
What is root pressure |
Take a regularly watered potted plant and cut the stem portion 1 cm |
above the ground level. Then connect a glass tube by means of strong rubber |
Free distribution by T.S. Government 2019-20 65 |
tubing as shown in the figure. The size of |
clamp |
glass tube should be equal to the size of the |
glass tube stem. Take care while joining tube and stem |
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water level being bound tightly, so that water cannot |
strong rubber tubing escape from the tube. Now, pour some water |
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