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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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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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  • $\begingroup$ 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. $\endgroup$
    – gremau
    Feb 8, 2013 at 20:39
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I don't buy this explanation. Trees don't have leaves in early spring and water comes up the tree with no problem. Also, if water was put under enough pressure to get sucked up a hundred feet, it would have so much heat that it would become steam. It's why buildings that are super high need pumps every so often to get it to go up.

I have a theory. In a tree you have different parts of the trunk. Heart wood, sap wood, cambium layer, and bark. I think because the heart wood and bark aren't taking up water, they aren't expanding like the sap wood is. The cambium layer is very slippery and let's the tension between the bark and sapwood to slip past each other. I think the expanding sapwood eventually gets squeezed and osmosis does the rest.

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    $\begingroup$ have any references to back up the claims you're making? $\endgroup$
    – Vance L Albaugh
    Jun 17, 2017 at 2:27
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    $\begingroup$ Trees don't have leaves in early spring and water comes up the tree with no problem you mean "no leaves = no gravity". buildings that are super high need pumps every so often to get it to go up so pumps don't use pressure to push water. Welcome to Biology.SE, but you seriously need references $\endgroup$
    – another 'Homo sapien'
    Jun 17, 2017 at 4:10
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Trees don’t use either adhesion or cohesion to move water against gravity. It is impossible to “suck” above 10.3 m, which is equal to our atmospheric pressure. Air Pressure is pushing up the xylem Micro-tubes - which have a lower pressure / vacuum at the top (created be evaporation from the leaf). This “siphon effect” is powered by Air Pressure, pushing up a closed tube.

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It has to do with heat transfer and Newton's Law of thermodynamics. Matter that is heated will always travel to a cooler location. The roots are like a sponge, soaking up water from the ground. The water moving through the capillaries causes friction = heat, These heated water molecules collide with carbon atoms and form CO2, which is expelled trough the leaves in summer, but is negligible in winter.

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I somehow discovered that in some trees with thin bark, I could place my ear on the bark and hear the movement of the water in the tree. It was not a continuous flow, but with hesitations like spurts or pauses. I thought there might be a valve system to keep the water from coming back down. But by saying it was pulled up by evaporation from the leaves, I can't explain the pauses.

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  • $\begingroup$ Hello Martha. Welcome to Biology. Your answer is not really addressing the asked question. Even if it did, your answer is based more on personal experience and not validated facts. You can edit your answer so that it provides relevant points that address the question or else it would be deleted. $\endgroup$
    – WYSIWYG
    Jan 23, 2019 at 11:06

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