Describe the path of ascent of sap due to transpiration pull
Describe the path of ascent of sap due to transpiration pull
This theory was originally proposed by Dixon and Joly (1894) and elaborated by Dixon (1914, 1924).
It is based on the following features:
(i) Cohesive and Adhesive properties of water molecules to form a continuous water column in the xylem.
(ii) Transpiration pull exerted on this water column.
Water molecules remain joined to each other due the H-bonds between them. Whenever a H-atom comes between two electro-negative atoms, a bond is established. In case of water, the electropositive H-atoms of one water molecule are connected with electronegative O-atoms of other water molecules by H-bonds.
Although H-bond is very weak, when they are present in enormous numbers, a very strong mutual force of attraction or cohesive force develops between the water molecules. The magnitude of this force is very high and so the continuous water column in the xylem cannot be broken easily by the force of gravity or the upward movement of water.
When transpiration takes place in the upper part of leaves, water evaporates from the intercellular spaces through the stomata. The mesophyll cells releases more water which is drawn from the xylem of the leaf. So, a tension is created in the water in the xylem elements of the leaves. This tension is transmitted downward to water in the root xylem through the xylem of petiole and stem. Then, the water is pulled upward in the form of continuous unbroken water column to reach the top.
This theory was originally proposed by Dixon and Joly (1894) and elaborated by Dixon (1914, 1924).
It is based on the following features:
(i) Cohesive and Adhesive properties of water molecules to form a continuous water column in the xylem.
(ii) Transpiration pull exerted on this water column.
Water molecules remain joined to each other due the H-bonds between them. Whenever a H-atom comes between two electro-negative atoms, a bond is established. In case of water, the electropositive H-atoms of one water molecule are connected with electronegative O-atoms of other water molecules by H-bonds.
Although H-bond is very weak, when they are present in enormous numbers, a very strong mutual force of attraction or cohesive force develops between the water molecules. The magnitude of this force is very high and so the continuous water column in the xylem cannot be broken easily by the force of gravity or the upward movement of water.
When transpiration takes place in the upper part of leaves, water evaporates from the intercellular spaces through the stomata. The mesophyll cells releases more water which is drawn from the xylem of the leaf. So, a tension is created in the water in the xylem elements of the leaves. This tension is transmitted downward to water in the root xylem through the xylem of petiole and stem. Then, the water is pulled upward in the form of continuous unbroken water column to reach the top.
