Plants absorb water through the entire surface - roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles. As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis.
Water in the roots move by two pathways. They can be classified as1) Apoplast pathway2) Symplast pathway
In this pathway the movement of water occurs exclusively through cell wall without the involvement of any membranes. Majority of the amount of water goes through the apoplast pathway. The cortex of the root does not oppose such movement of the water.
Symplast pathwayHere the movement of water molecules is from cell to cell through the plasmodesmata. The plasmodesmata forms a network of cytoplasm of all cells.
The Casparian strip separates the cortex and the endodermis. It is composed of a wax like substance called suberin, which blocks water and solute molecules through the cell wall of the endodermis. Now the water is forced to go through the cell membranes of different cells leading to a transmembrane pathway.
Areas of Root Involved in Absorption and Translocation of Water
Mechanism of Water AbsorptionWater can be absorbed by two methods:
- Active absorption
- Passive absorption