3 Biology -- Water Relation

Explain about transpiration pull and cohesion of water theory.

Explain about transpiration pull and cohesion of water theory.

Ascent of sap is the phenomenon which helps to uptake water from roots to tip of plants. Many theories have been purposed to explain ascent of sap.

Ascent of sap by transpirational pull was given by Dixon and Joly.

According to this theory the column of water is pulled upwards by the tension created due to the loss of water by transpiration in leaves.

Water forms a continuous column extending between the root and the leaf parenchyma cells through the xylem ducts. This is also called Hydrostatic system.

Evaporation from the surface of parenchyma cells of the leaf increases the water potential and they withdraw water from the xylem elements. This outs the column of water under tension.

The column resists breaking because of cohesion between the water molecules.

Furthermore, because of an attraction between water molecules and the molecules of the wall of the tube the water column is put under stress but doesn't result in outing the water away from the enclosing wall.

The water is pulled up from the root.

The upward moving water doesn’t breakup due to the cohesion force present between water molecules.

In this way the transpiration pull or tension is transmitted to the root of the plant and water rises up from root to top of plant.

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