Question

What is ascent of sap? Explain the transpiration pull and cohesion theory.

Answer 1

The upward movement of water from roots to the aerial parts of a plant is known as an ascent of sap. Most accepted theory for an ascent of sap is cohesion-tension theory that was given by Dixon and Jolly in 1894. The mechanism of the ascent of sap is based on:

1. Continuous water column- From root hairs to the tip of the plant, water forms a continuous column in tracheary elements.

2. Cohesive and Adhesive forces- The water molecules remain joined to each other due to the force of attraction called Cohesion force. Water has a high tensile strength due to which the water column doesn't break in xylem vessels.

3. Transpirational pull- When transpiration takes place, water evaporates from the intercellular spaces of the leaves to outer atmosphere through the stomata. Due to water loss, the osmotic pressure of the leaf cells increases and as a result, the cells develop low water potential and water from the leaf veins (xylem) moves into leaf cells. The xylem vessel, in turn, draw water from the xylem of the main stem and thus, a pressure or pull is exerted by all the leaves on the stem. This combined pressure called transpirational pull is strong enough to pull up the column of water to a great height.