Ascent of sap MohanBio

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Ascent of sap MohanBio

  1. 1. Ascent of Sap • The upward movement of water from roots to aerial parts of plants through xylem is called ascent of sap. • Balsam plant experiment to demonstrate xylem is the pathway for ascent of sap.
  2. 2. Materials: • A beaker, Water, Eosin stain (red ink), stand, White balsam plant with roots. Procedure: • Put few drops of eosin stain in a beaker containing water. • Insert a balsam plant into it. Fix to stand. Keep it for few hours. • Red lines can be seen in the transparent stem and in the veins of leaves. • When a section of the stem is observed under the microscope, it shows that only the xylem vessels are coloured. • This expt confirms xylem is the pathway for ascent of sap.
  3. 3. Mechanism of ascent of sap. Transpiration pull and cohesion theory: • The theory was proposed by Dixon and Jolly (1894). This theory is based on 1. Cohesive and adhesive properties of water. 2. Transpiration pull force developed in leaves by transpiration. • Cohesion is the phenomenon of attraction between similar molecules. The water molecules remain attracted by the cohesive force. • There is attraction between water molecules and the inner wall of xylem by adhesive property. • Due to this water forms a continuous column in xylem vessel.
  4. 4. • During day time mesophyll cells looses water due to transpiration. • As a result turgor pressure and water potential of mesophyll cells decreases. • At the same time water potential of xylem vessels of leaf is high. • The mesophyll cells sucks the water from xylem vessels. • Due to this –ve force called transpiration pull force develops. • This force moves to the xylem vessels and pulls water on upwards.
  5. 5. experiment to demonstrate ascent of sap due to transpiration pull force Principle: • When water lost from aerial part of the plant by transpiration, the transpiration pull force develops. This force is responsible for ascent of sap. Materials: • Simple potometer, water, mercury, beaker, plant twig, stand, and wax.
  6. 6. Procedure: • Fill the potometer completely with water. • Fix the cut end of plant twig to the upper end of potometer using single holed rubber cork. • Immerse the lower end of potometer in a beaker containing mercury. • Make the setup air tight by applying wax. • Leave the apparatus in sunlight for few min and observe
  7. 7. • Observation: After few min raise in mercury level is seen in the tube. • Inference: the raise in mercury indicates the transpiration pull force in potometer is responsible for ascent of sap.

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