How Can Trees Be Taller Than 10m?

[Applause] Now, in a previous video, I showed you that you can only suck up a straw that's 10.3 m long. And that's even if you can create a perfect vacuum inside your mouth. If you haven't seen the original video, check it out.

But that raises an interesting question: how can these trees, which are 100 m high, get the water all the way from their roots up to the leaves? The argument, as you're alluding to, is that if you're sucking water out through a drinking straw, for example, you can only have a straw 33 ft long. The reason for this height limit is because the weight of the water in a column must be supported by the pressure difference between the top and the bottom.

So at the bottom, you're going to have atmospheric pressure, and the lowest pressure you can produce at the top would be a vacuum, that is, zero pressure. So atmospheric pressure can support a column of water that is only 10 m high. What's worse is if you were able to create a vacuum, the water would start boiling spontaneously. That's called cavitation, and, uh, obviously that can't be taking place within a tree.

So how are they doing it? Well, we started to develop some different theories. My guess, though, is people talk about this being a continuous water column. And what they, I think when you say that, you think this big, like empty pipe, right? That's what we're picturing. Uh, but I think what's more likely to be the truth is this big tube, which you're saying needs to be filled with water, is actually made up of cells.

The tree effectively has valves in it. So you don't have a column of water that is much higher than 33 feet tall. So, the water is pumped up by, um, osmotic pressure due to differences in concentration of sugars and so on. But each individual stage is just quite a small one. My guess is that it's probably more like a bucket brigade where, once, you know, here we’re at the end, there's sunlight coming in and it heats up the water.

And the water evaporates, and so the water goes off as water vapor, vaporizing off. So now this guy's like, my bucket's empty, I want some more water. And this is a cell; this is a cell here, but this one can give the water there because locally, like, it's surrounded by water, and a little bit of water will go there via osmotic pressure.

Now, another theory is that osmotic pressure at the base could actually push the water all the way up the tree. If the solute concentration is different enough between the roots and the water in the surrounding soil, then water would actually want to push into the roots in order to equalize the solute concentrations. That could create a positive pressure, which would push the water up the tree.

And this hypothesis led me to being challenged to blow water up a tube. Now, the water, as you can see, is Veritasium color. Three, two, one, go! Unbelievable! Stop, stop, stop! Yeah, yeah, nailed it! I hate you, D!

Now, my ability to blow water up that tube was impressive, but I don't really think that a tree would be able to get so much osmotic pressure at the roots that it could push the water up 100 m. Well, some people may be wondering why we haven't talked about capillary action yet. That's due to the adhesion between the water molecules and the walls of a tube. So, you can suck water up through, uh, perforated materials.

Now I'm not sure that the tubes inside a tree are small enough for this effect to have a significant impact, but it may well. I don't want to give you the complete answer yet. Uh, I'd like you guys to tell me what you think and maybe post a video response.

I'll tell you that I had a significant misconception that was stopping me from working this out, so if you can spot what that is, uh, do let me know. And let me give you a summary of the ideas we came up with:

One, that the tree does not contain a continuous water column.

Number two, osmotic pressure at the roots may be pushing the water up the tree.

Number three, osmotic pressure throughout the tree, uh, helps pull the water up.

And number four, capillary action.

So let me know what roles you think those different factors play in allowing a tree to draw the water up 100 m. And if you don't want to do that work, then subscribe to the channel and I'll post the answer in a week.