Absorption of water
Land plants absorb water from the soil mostly by root hairs. Root hairs are unicellular, thin-walled outgrowths of epidermis. They are in close contact with the thin film of water surrounding the soil particles.
Cell wall of the root hair is permeable to water and minerals, but its cell membrane and the membrane around the vacuole (tonoplast) from semi-permeable membranes. Soil solution is a weaker solution as compared to the cell sap of root hair. Hence osmosis (endosmosis) occurs and the water is absorbed by the root hairs through cell membranes from the soil. Due to this, the root hair cells become more turgid and their osmotic pressure falls. Adjacent cells of cortex have higher osmotic pressure. This results in the diffusion of water from the root hair to cortical cells.
By this mechanism water moves into deeper cortical cells under an osmotic concentration gradient till it reaches the endodermis of the root which 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. In passive absorption forces are built up in the leaves due to rapid transpiration during day hours. Roots play only a passive role of serving as channel for water movement.
Absorption of Minerals
Mineral salts are absorbed by the root through the process of diffusion. Inorganic salts are absorbed in the form of ions. Each type of ion is taken up according to its requirement.
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