I once heard someone on the radio talking about trees and wondering how it is that water is able to make its way from the roots all the way to the very top branches and leaves. The person made it sound as if the answer is unknown. They also say that this is the reason trees do not grow larger (tallest tree is 364’!?!). Looking at a tree that is 100 feet tall and with a canopy a good 20 feet in diameter makes ya wonder how the water overcomes the force of gravity to reach such heights. Is it merely a whicking action in which the cells carry the water up? I could maybe see this working for a tree no taller than a few feet, but a redwood? There is no pumping action the tree is capable of. Some sort of pressure differential above and beyond whicking? I did a search when this first piqued my interest. I’m sure I’ll get a correct answer from all of you.
Thanks
The leaves continually lose water to the surrounding air. Water is a highly polar molecule. That is, one end is electrically + and the other end is electrically -. So the water molecules line up + to - and as a water molecule evaporates from the leaves the whole chain of water is pulled up.
No, it’s not unknown, it’s been well studied and largely resolved. However like almost all science there are a few minor points that don’t quite fit. But we can declare this to be less of a mystery than gravity. There’s a really complicated answer, and a really simple answer. We’ll stick to the simple answer. If you want a more complex explanation simply put the words “tension cohesion” into Google.
The simple answer is that the tree uses the equivalent of a siphon effect to lift the water. A siphon works because water is cohesive, IOW water sticks to water. That cohesiveness of water in a siphon tube means that as water at one end falls it is able to suck more water up from the other end. Provided that the fall is greater than the rise you are trying to overcome you can use a siphon to lift water to significant heights.
A tree trunk is basically composed of masses of siphon tubes filled with water, but it doesn’t have any physical ‘fall’ component. Instead what the tree uses is the force generated by evaporation. As water enters a leaf it is exposed to the air and allowed to evaporate. Because water is cohesive that loss of water creates a negative pressure that pulls more water into the leaf within the leaf, and that in turn creates a negative pressure that pulls water into the vessel elements of the branch and the trunk. That pressure is sufficiently high to lift water to the tops of the tallest trees.
Which is why trees use such massive quantities of water. Even a small tree will use over 100 litres of water a day and less than 1% of that will be used physiologically. The rest is simply evaporated off to draw more water up the stem. It’s an incredibly wasteful process.
This is also the reason why trees in dry environments are invariably shorter than plants in wetter environments. Because the system functions like a siphon the water column in the trunk can never be allowed to break or the pressure inside will immediately drop to zero. It’s like getting vapour lock in a fuel line and plants can’t re-prime the line. There has to always be sufficient water in the ground to feed the negative pressure required in the column. The taller the tree the greater pressure required and hence the more water required. As a result the maximum height of tree is determined in large part by the amount of water available.
A conventional syphon works because atmospheric pressure is pushing the water up the pipe. So there’s a limit (9.8 meters, I think) to how high it can lift water. I thought the long-standing mystery was that trees can be taller than this?
According to Cecil (and me too, but I don’t count) "The action [of a siphon] depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure–a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight." In other words, the water isn’t being pushed over the hump by atmospheric pressure behind it, it’s being pulled by the water ahead, as though it were (excuse me, but this is how I conceived of it) a giant stringy booger."
AFAIK any height limit on a siphon is dependant on the cavitation pressure of the tube and the fluid, nothing to do with air pressure. That limit decreases as the tube becomes finer. That’s the same limitation that applies to the vessel elements of trees. Having incredibly fine tubes trees can lift water to incredible heights.
So at the end of the day the tree is using something very similar to a siphon effect except instead of a gravitational fall pulling the water up the tube it’s using the loss of wateer through evaporation. Same principle though, and same limitations.
The maximum theoretical height of a water syphon at normal atmospheric pressure is about 34 ft. (10.3 m). The fact that this almost equals the acceleration of gravity is a coincidence. A syphon will only raise water to a height such that the water pressure equals the atmospheric pressure. If you take a syphon to the moon you have wasted the fuel required to get it there. It won’t work.
Water weights 62.4 lb/ft[sup]3[/sup] There are 144 sq in in a sq ft. So a water column one foot high exerts a pressure of 62.4/144 lb/sq in. Ergo:
X*62.4/144 = 14.7 and X = 33.9 ft.
The answer you’re looking for is a combination of capillary attraction and osmosis. Here’s a cite that addresses it:
http://biomechanics.bio.uci.edu/_html/nh_biomech/trees/trees.htm
Want more? Google "capillary attraction in plants and trees.
No. The answer we are looking for is NOT a combination of capillary attraction and osmosis, it is a combination of capillary action and evaporation. Your own reference quite clearly states this: “The water moves up the xylem via… a combination of capillary rise and evaporation through the leaves”.
Capillation itself plays a fairly minor, though crucial, role.
The only role osmosis plays in the process is that water can enter the water column in the roots via osmosis.
NO? But…well…maybe? Hmmmmmmm.
With all due respect, I hardly consider that answer adequate. After all the water in a 10, 000 gallon tank is composed of the same molecules and it also evaporates, yet water doesn’t get pulled up the sides of the tank. An answer needs to include the role of the vessel elements in allowing the maintenance of pressure to be even remotely adequate IMO.
By comparison, the capillary action of concrete is about 2 miles vert, IIRC.
True. The water in large diameter vessels is subjected to thermal agitation because the surface is continually cooled by evaporation and the sides are subjected to local cooling and warming. This thermal activity is usually large enough to negate, or greatly reduce the tendency of water molecules to stick together. And, of course, when a water molecule leaves the surface of a large vessel it quite possibly pulls another molecule to the surface. In thin tubes, capillary tubes that is, the temperture is quite uniform and the thermal mixing is small so that the electrical attraction of the molecules for each other dominates.
I do think that the term “capillary action” is merely giving a name to an effect without explaining how capillary action comes about. And saying that water is “sticky,” ergo capillary action, doesn’t at all account for why it is sticky.
And I must say that the explanation I cited is somewhat more accurate that invoking syphoning. 
Hence the reason why I never used the term capillary action anywhere in my explanation.
I believe that your answer is grossly over-emphasizing the role of cappilation in water movement. As I noted later capillary action plays a very small, though essential role. The main lift is provided not thorough capillary action per se but through cohesion within the vessel elements generated by evaporation. Were it not for the negative pressure generated by evaporation capillary action couldn’t provided lift of much over 10 metres and under most circumstances would restrict trees to less than 5 metres height.
I didn’t notice you citing anything. You presented an explanation. 
And that explanation is seriously lacking simply because it implies that the sealed tubes are the essential factor at play. Never mind thermal differences or other factors, water simply can’t be lifted to those heights in a water tank or even a capillary tube because the vessel is open and can never generate pressures significantly less than atmospheric. In contrast trees normally have xylem pressure of 15-20 atmospheres. That’s what allows the water to be lifted, and you can’t generate those pressure with cappilation no matter how fine the tube or how uniform the temperatures.
You can however generate those pressure through cohesion and evaporation coupled with cappilation. Hence the mechanism is “the equivalent of a siphon effect”.
What if the tree is on a treadmill?
Then obviously its quack doesn’t echo.
My post No. 2 stated that trees evaporate a lot of water and this evaporation pulls the water up the trunk because the electrically polarized molecules of water attract each other. What could be clearer and simpler than that? No pressures are used to push the water up the trunk. A lot of additional folderol is just superfluity.
Let’s not be silly. If superfluids were involved, trees would be much taller.
Sorry to ressurect this old thread, but Veritasium has a video that purports to answer the question on how trees can grow so tall. The mystery, of course, is this: what is the mechanism that allows water to travel to the top of a very tall tree? The video supports @Blake’s assertion (above) that it is not due to capillary action, nor is it due to a siphon mechanism. According to the video, water evaporates from the leaves, and creates tension in the water columns. This tension can be thought of as “negative pressure,” and has the ability to push water up hundreds of feet inside the tree.
But isn’t a siphon mechanism based on the same negative pressure idea?