Cohesion tension theory is the most widely accepted theory as to why water flows up through xylem. However, there are some other theories that try to explain this phenomenon, and they're not mutually exclusive with cohesion tension. It's important to point that out. However, given all of the evidence, it is most likely that cohesion tension is the dominant reason that water is flowing up. These other things might contribute, but they're probably not the main factor is the point. So I want to talk about one of these other theories, root pressure theory, which basically says that positive pressure builds up in the root xylem due to increased absorption of water relative to transpiration. So remember, I said that the idea of transpiration was going to come back in terms of water moving up through the xylem. Well, here's where it comes back to haunt us, or it's just the start of it really. Now the reason all of this water is going to build up in the root xylem is that ions are pumped into root xylem. And this creates a negative water potential relative to soil, so it's going to drive water into those root xylem. As we said before, water enters via osmosis, and the more water that builds up in the root xylem, the greater the positive pressure.
Right? So the basic idea is that more water goes into the root xylem here, then leaves the leaves through transpiration. And there is some evidence for this, for example, stomata close at night, but roots continue to absorb water and ions from the soil. And in fact, if you look at root pressure it is highest in the morning due to this, and in fact, you can even sometimes see this phenomenon known as guttation, where water is forced out of leaves due to all of this pressure. And you can actually see a picture of guttation happening here. These water droplets are being squeezed out of the leaves due to this super high pressure. And this sight is most common in mornings when pressure is the highest, and like right before the plants open their stomata and start transpiring again. You know, and they've had the whole night to suck up water and ions basically. So there are going to be some factors that we need to cover in order to understand cohesion tension theory, and the movement of water through xylem in general.
Water has some ability known as capillary action or capillarity. This is the ability of a liquid, in this case water, what we're talking about, to move through narrow spaces. And it's basically due to three factors. We see capillary action as a product of three factors. One of those is adhesion, and this is the attraction between unlike molecules. So in the case of capillary action, it's going to be the attraction between water and the different molecules that make up the tube. You can see an example of adhesion here, where these water droplets are clinging to the spider web. Cohesion is the attraction between like molecules. So in terms of capillary action when we're talking about water, it's going to be the attraction between water and itself. And you can see a nice example of cohesion here. The water beading up on the surface of these leaves is due to the fact that the leaves' surface is hydrophobic, and so water is going to want to cling to itself here. This is an example of cohesion because, you might have to look closely to see this, the water is actually forming orbs. Instead of the droplet having a flat bottom, the water is actually beaded up in an orb like that. So it's lifting off the surface because the molecules are being attracted to each other in that droplet.
Now, this cohesion will sometimes lead to what's known as a meniscus, which is a concave surface boundary, due to cohesion and adhesion. You can see an example of a meniscus here. That's the type of meniscus that water is going to form, where it comes up on the sides like that. There are some liquids that will form a convex meniscus like that. Those tend to be heavier liquids than water. For example, mercury forms a convex meniscus like that. Now, the last force that I want to talk about in terms of capillary action is surface tension, and you can see an example of surface tension, I'm just writing "ST" on this image for surface tension, right here with this paper clip that is seemingly floating, though it's actually being held up on the surface of the water. It's not actually floating because it hasn't broken the surface. And basically, surface tension is the force between the water molecules at the air-water interface. So the water is going to be attracted to itself at that air-water interface, and it's going to create a tension across the surface, which is why this paper clip can just sit there on the surface without breaking it.
So how does this all come into play in terms of capillary action, that you can see here, where the water has moved up the tube against gravity pulls up from the container wall? Right? So the adhesion between the water molecules and the tube wall is going to pull up. Surface tension is going to pull up from the very surface, and then cohesion basically transmits the pull between all the water molecules. So as surface tension pulls up from the surface, that meniscus, adhesion is going to allow pull from the walls, and cohesion is going to transmit that pull to all the water molecules in the tube. And that is how we get capillary action. And, with all of that in mind, let's actually flip the page and talk about cohesion tension theory.