Plants absorb water by two distinct and independently operating methods : Passive absorption and Active absorption.
(i) Passive absorption :
(a) In this process, water is absorbed through the roots hairs rather than by the roots.
(b) The root cells play a passive role or merely act as physical absorbing system.
(c) The force for this type of water absorption originates in the aerial parts of the plant due to transpiration. It creates a tension or low water potential in the mesophyll cells of the leaves.
(d) The latter, in turn, draw water from the xylem of the vascular bundle. Thus, water in the xylem is under pull or tension. It is called transpiration pull.
(e) Due to the presence of continuous column of water from leaves to roots through xylem channel, the tension is transmitted to the xylem of roots and finally to root hair through endodermis and cortex.
(f) Thus, a water potential gradient develops between the root hair cell and the soil solution. This results, movement of water across the root as a continuous stream.
(ii) Active absorption :
(a) Active absorption takes place by the activity of root itself, particularly root hairs. It utilizes metabolic energy.
(b) A root hair cell functions as an osmotic system. Water is absorbed by the root hair due to osmotic differences between soil water and cell sap.
(c) There exists a great difference in the osmotic pressure of the two sides or water potential gradient between the soil solution and cell sap.
(d) The soil solution having less OP, has higher water potential than the cell sap with more OP.
(e) Thus, water moves from the region of higher water potential towards the regions of lower water potential.
(f) Water continues to enter the root hair cell as long as the water potential of the root cell sap is more negative than that of the soil solution. Water moves from cell to cell along the water potential gradient and reach up to endodermis and pericycle.
(g) Finally water enters into the xylem.
