Ascent of sap MohanBio
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The ascent of sap occurs through the xylem pathway in plants. A balsam plant experiment was conducted where eosin stain was added to water and the plant was placed in it. After hours, red lines appeared in the transparent stem and leaf veins, showing the stain traveled up through the xylem. The mechanism of ascent is explained by the cohesion-tension theory where transpiration pull develops a negative pressure in the xylem, overcoming the cohesive forces of water, pulling it up from the roots. A potometer experiment demonstrated this by showing a rise in mercury when a plant twig was placed in it, indicating water movement up the xylem due to transpiration pull.
Ascent of sap MohanBio
- 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. 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. 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. • 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. 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. 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. • 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.