What is the definition of mass flow in plants?Watch
In plants, movement of water through the stem is mass flow. When the water enters the xylem vessel, only mass flow occurs as the xylem vessel is just a tube with no ends (so it cannot be diffusion). For example lets use the straw analogy, when we suck, water moves up the straw. The straw is the stem, us sucking the straw is transpiration (evaporation of water from the leaves, so water is sucked up to replace the lost water. Transpiration is the force). Water moves up the straw because of the cohesion-tension theory. Water molecules are cohesive, they stick together, so when water is sucked up the stem, the water molecules stick together and go up. The water molecules are under tension. Therefore it's called cohesion tension theory.
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