Plants absorb water through their entire surface – roots, stems and leaves. However, the majority of water is absorbed by root hairs. Root hairs are thin-walled uni-cellular outgrowths of epidermis. They are in close contact with the thin film of water surrounding the soil particles.
The cell wall of the root hair is permeable to water and minerals, but its cell membrane and the membrane around the vacuole from semi-permeable membranes. Soil solution is a weaker solution as compared to the cell sap of root hair. Because of this osmosis occurs and the water is absorbed by the root hairs through cell membranes from the soil. Then the root hair cells become more turgid and their osmotic pressure falls. The cells of cortex have higher osmotic pressure which causes the diffusion of water from the root hair to cortical cells.
It is through this mechanism in plants that water moves into deeper cortical cells under an osmotic concentration gradient till it reaches the endodermis of the root. When it reaches the endodermis of the root it forces the water into the xylem tubes through the passage cells. Certain pressure or force is developed by which cortical cells push the water in the xylem tubes.
This pressure is called root pressure. The water column is sent and maintained up to certain height due to this pressure. The mechanism, of absorption of water from the soil by the activity of the root cells is called active absorption of water.
In tall trees, active absorption plays a minor role. The main and efficient mechanism by which most of the rooted green plants absorb water is passive absorption.
The passive absorption mechanism is carried out without utilization of metabolic energy. Here only the roots act as an organ of absorption or passage. Hence, sometimes it is called water absorption ‘through roots’, rather ‘by’ roots. It occurs in rapidly transpiring plants during daytime, because of opening of stomata and the atmospheric conditions. The force for absorption of water is created at the leaf end and is often referred to as the transpiration pull.
The main cause behind this transpiration pull, water is lifted up in the plant axis like a bucket of water is lifted by a person from a well. Transpiration pull is responsible for dragging water at the leaf end, the pull or force is transmitted down to the root through water column in the xylem elements. The continuity of water column remains intact due to the cohesion between the molecules and it act as a rope. Roots simply act as a passive organ of absorption. As transpiration proceeds, simultaneously water absorption also takes place to compensate the water loss from leaf end. Most volume of water entering plants is by means of passive absorption. Passive transport is nothing different from diffusion but just explaining its meaning “passive” refers to requiring no input of energy. There is a free movement of molecules from their higher concentration to their lower concentration.