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I have an AP Bio class where I have to name 3 properties of water and I chose adhesion and cohesion for one of them. I'm having trouble finding out how exactly trees use adhesion and cohesion to move water. There is a lot of different answers out there on the net. How do trees use adhesion and cohesion to move water against gravity?

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2 Answers 2

up vote 5 down vote accepted

In high school, we did an experiment that showed this.

Basically, if you take a glass of water, and let it sit out, perhaps in front of an open window, it will eventually lose water due to evaporation. It may take a few days/weeks to really see a large difference, but the level will go down. But, if you take a few flexible straws, put them in so the bendy part is submerged, the water level will drop much more quickly. (I'm not exactly sure if the bendy straw part is really that important here. Logically, I don't think it is, but I haven't done the experiment so I can't really say.)

The reason the straws cause evaporation to happen more quickly is because as wind blows across the top of the straws, it pulls some of the water with it. And because, as the last person said, of cohesion and adhesion, the column of water is pulled up with it and thus it evaporates faster.

Similarly, these fluid mechanics are why toilets and siphoning work. When you siphon, you create a low pressure area inside the tube, like you are sucking out of a straw. Because of this low pressure, liquid is pulled into the tube. If you fill the tube with liquid, and turn the tube down toward the ground so the liquid starts coming out the end, the force of the liquid coming out of the tube creates low pressure in the tube again, thus causing more liquid to be sucked up.

This is very similar to how trees work. In the stomata, or the pores in the leaves that allow the leaves to "breathe," wind helps to pull the water out of the pores. But because of the decrease in pressure cause by the liquid being sucked out of the pore, water gets pulled up the tubes in the tree (xylem).

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The mechanism is called "capillary action". It requires a tube of a small diameter and happens because of the adhesion of water to the walls and the cohesion within the water (=surface tension).

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Its important to add that the driving force for the actual water movement is the gradient in water potential in the system (atmosphere is dry, while the mesophyll of the leaf is wet). The plant expends no energy to move the water. –  gremau Feb 8 '13 at 20:39

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