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What is the definition of mass flow in plants?

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Mass flow in plants is the movement of water and nutrients from the root through the xylem and phloem vessels to all other areas of the plant such as leaves, by diffusion and active transport.
Movement of a fluid medium in one direction under a pressure gradient - example in plants would be movement of water in xylem and phloem


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Mass flow: movement of water due to a force. Concentration gradients don't matter here. Think of it as a river, that's flowing at high speed, it is going to carry everything along with it in the direction it's moving.

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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