Water moves from the roots to the leaves through a continuous system of xylem vessels in a process called the transpiration stream. This movement happens mainly due to three forces — root pressure, capillary action, and transpiration pull.
Here’s how it works step by step:
1. Absorption by Root Hairs
- The process begins in the roots. Tiny root hair cells absorb water from the soil by osmosis (movement of water from a region of higher to lower concentration).
- The absorbed water then passes through root cortex cells to reach the xylem in the root.
2. Root Pressure
- When minerals are actively transported into the xylem, water follows by osmosis.
- This creates a small upward pressure in the xylem, known as root pressure, which pushes water upward.
- Root pressure is most effective at night or in small plants.
3. Capillary Action
- Water can rise in narrow tubes (like xylem vessels) due to capillarity, which occurs because of cohesion (water molecules stick to each other) and adhesion (water molecules stick to the walls of xylem).
- This helps move water upward through the plant stem.
4. Transpiration Pull (Main Force)
- The evaporation of water from leaf surfaces (mainly through stomata) creates a negative pressure (suction force) in the leaves.
- This pulls more water upward from the xylem in the stem to replace the lost water — like water being pulled up through a straw.
- This is called the transpiration pull, and it is the most powerful force in moving water to great heights in tall trees.
5. Continuous Water Column
- Due to cohesive forces between water molecules, the water forms a continuous column from roots to leaves, allowing smooth flow without breaking.
In simple terms, water moves from roots to leaves because root pressure pushes it up, capillary action helps it climb, and transpiration pull draws it upward, forming a steady stream that delivers water and minerals to all parts of the plant.