How do Trees Really lift Water to their Leaves?

@ l_kryptonite
I disagree; your contribution is as valuable as anyone else’s. New-ground in science renders everyone on equal terms. Children being in the best position of all to take on board a new paradigm, as they have not been corrupted by erroneous literature and retain the capacity of having an open mind.

I agree with your statement that: A leaf cannot possibly suck water from the soil because it is not in contact with the soil. I assumed incorrectly that you were talking about osmosis. Although not the main method of fluid movement, it is still an important factor in a plant's survival.

quote]Originally posted by l_kryptonite

Or you could get involved in the incredibly complex discussion being held in the general science section. I need to do about 3 years of study before I get back into that one though. Way out of my league.[/quote]

Dave:
Thre are two things that could be causeing the effect you are referring to as the elasticity of the water. The stretching of the water or the deforming of the tube.

If we start on the stretching of the water. I have found a list of how the volume of water changes as you change the pressure.

temp F(C) 0 atm 500 a 1000 a 2000 a 3000 a
32 (0) 1.0000 0.9769 0.9566 0.9223 0.8954
68 (20) 1.0016 0.9804 0.9619 0.9312 0.9065
122 (50) 1.0128 0.9915 0.9732 0.9428 0.9193

so at 68 farenheit it takes 500 atmospheres to change thevolume by 2%, so 1 atmosphere will change the volume by about 2%/500 = .004% or about .4mm over 10m

Ok tension will be slightly different but probably not hugely so and most of the water in the column is at a positive pressure anyway so I don't think that the expansion of water will be producing a major part of the effect.

Your tube is pretty rigid, but if you squeeze it really hard I expect it will deform a little bit, you would only need a 5-10 percent deformation to cause a .5m movement in the water. Are you using the flexible clear PVC tube or the translucent white much more ridgid stuff?

Hang on a minute - do I understand you correctly in that the bottom half metre of the tube empties and is full of air, and then stops? Have you done anything else other than removed the demijohn? because just removing the demijohn will not alter the pressures at all - so you haven't done anything to the water column apart from let water fall out of the bottom, eg you shouldn't have altered the tension in the elastic band (whether the elasticity is due to water stretching or the tube deforming) as you haven't changed the size of the weights on each end... did you pull the ends out of the demi-johns by lifting the whole apparatus or just the ends of the tubes?

ps. by the way my calculation above was considering the force from the whole earth on the hydrogen atoms in a water molecule. Perhaps I should have said that on a molecular scale the earth's gravity is a very small force.

So even if we calculate the whole length of the tube at 48 metres, that still does not address the half metre rise in the water level on both sides of the tube, when they are lifted out of the bottles containing the water.

Zero water runs out of the tubes at the bottom, as you suggest might be the case. The water is sucked up inside the tube equally by the negative tension placed on the bead of water.

Your tube is pretty rigid, but if you squeeze it really hard I expect it will deform a little bit, you would only need a 5-10 percent deformation to cause a .5m movement in the water. Are you using the flexible clear PVC tube or the translucent white much more rigid stuff?

Yes, it’s the rigid translucent stuff! The softer walled tube will simply neck )( under the negative tension. This rigid stuff does not neck and therefore the diameter internally will not reduce as a result of the negative tension. If I were able to squeeze it and alter its shape, it still would not alter the volume, as in order to do this one would have to compress the tube equally from all directions and this would take a huge force.

Hang on a minute - do I understand you correctly in that the bottom half metre of the tube empties and is full of air, and then stops?

It empties, but empties upwards!

Have you done anything else other than removed the demijohn? because just removing the demijohn will not alter the pressures at all - so you haven't done anything to the water column apart from let water fall out of the bottom,

No water falls out of the bottom until the bead of water cavitates!

eg you shouldn't have altered the tension in the elastic band (whether the elasticity is due to water stretching or the tube deforming) as you haven't changed the size of the weights on each end... did you pull the ends out of the demi-johns by lifting the whole apparatus or just the ends of the tubes?

Just the ends of the tubes!
Not quite correct Dave, there has been a reduction in the weights, because the water in the two bottles has been disconnected, and these do have considerable weight. Consider the water in the bottles as part of the mass of water inside the tubes and you begin to understand how trees draw water and mineral from the surrounding soil into their roots, or into a cut stem or trunk, with no roots.

ps. by the way my calculation above was considering the force from the whole earth on the hydrogen atoms in a water molecule. Perhaps I should have said that on a molecular scale the earth's gravity is a very small force.

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

If there are any lurkers, please feel free to join in with this conversation.

Andrew

Dave:
quote:Yes, it’s the rigid translucent stuff! The softer walled tube will simply neck )( under the negative tension. This rigid stuff does not neck and therefore the diameter internally will not reduce as a result of the negative tension. If I were able to squeeze it and alter its shape, it still would not alter the volume, as in order to do this one would have to compress the tube equally from all directions and this would take a huge force.


An ellipse will have a smaller area than a circle of the same perimeter (think about how a toothpaste tube works), so you can reduce the volume of a tube by squashing it slightly, without compressing it from all directions.

:Just the ends of the tubes!
Not quite correct Dave, there has been a reduction in the weights, because the water in the two bottles has been disconnected, and these do have considerable weight. Consider the water in the bottles as part of the mass of water inside the tubes and you begin to understand how trees draw water and mineral from the surrounding soil into their roots, or into a cut stem or trunk, with no roots.

I am afraid you can't consider the water in the bottles as hanging off the tubes, the way fluids behave is to do with pressure. so as long as the demijohns are not sealed the pressure at the surface of the water is atmospheric. The pressure will increase as you go down the demijohn, but it will reduce as you come back up the tube, so inside the tube, at the bottle water level the pressure will be atmospheric.

So if you pull the tube out of the bottle, unless the level of the end of the tube is different to the level of the water in the bottle nothing has changed.

Did you lift the tubes up or down when you removed the tubes?

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

I am not sure what you mean by homeostasis, as it is not in the oed and the only definition I can find is that it is a biological system that is stable due to negative feedback. Some systems acting under gravity are stable due to negative feedback - eg water in a glass is stable, but to say everything acting under gravity is under negative feedback is ridiculous - there is no way that a cricket ball in the air is going to be held in position... I am confused by what you mean.

Sorry, I did not make myself clear about the requirement of compressing the whole tube equally. I was relating to the negative water causing the tube to collapse equally, as this would be the case with a liquid under tension. Not at all like a finger and thumb compressing it.

Did you lift the tubes up or down when you removed the tubes?

An ellipse will have a smaller area than a circle of the same perimeter (think about how a toothpaste tube works), so you can reduce the volume of a tube by squashing it slightly, without compressing it from all directions.

If the tube was collapsing,” as you state the case might have been", then there should have been a noticeable rise in water level of the two bottles as the tube was hoisted up the cliff. As the bottles were filled almost to the brim on elevating the tube, the only bottle to begin overflowing was the one with the saline solution in it. When no salt is added and the loop is raised, there is almost no alteration in the bottle levels, indicating tube collapse to be minimal if any.

So if you pull the tube out of the bottle, unless the level of the end of the tube is different to the level of the water in the bottle nothing has changed. Did you lift the tubes up or down when you removed the tubes?.

Lifted them up and out of the bottles and let them dangle in the air.

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

I am not sure what you mean by homeostasis, as it is not in the oed and the only definition I can find is that it is a biological system that is stable due to negative feedback. Some systems acting under gravity are stable due to negative feedback - eg water in a glass is stable, but to say everything acting under gravity is under negative feedback is ridiculous - there is no way that a cricket ball in the air is going to be held in position... I am confused by what you mean.

ho·me·o·sta·sis (hm--stss)
The ability or tendency of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes.
The processes used to maintain such bodily equilibrium. Fits ok with this paradigm and discussion on trees and plants?
Your cricket ball is in the air, because gravity holds the atmosphere in place, and it will eventually come back to earth and its ultimate resting place, due to the inevitable effects of gravity, no matter how hard you throw it.
Just like a nuclear explosion is brought back under control by gravity

Dave:
Think back to how the soft tube collapsed when you first tried it, it will neck - go flat. This is because the cross section can reduce in area by changing from a circle to an ellipse - Under a vacuum the stiff pipe will turn into an ellipse slightly, but be strong enough to not actually collapse. However if the level of water didn't change in the bottles the collapse wasn't very significant.

However if the water was stretching then the water level in the bottles should increase as you pull up the tubes too... I am not sure what is happening with the water going up the tubes, I expect that there is something subtle going on with exactly how you are doing the experiment as I don't think either of our explanations work.

Out of interest what happened when you took only one tube out of the bottle?

Not really, a force doesn't necessarily move a system towards an equilibrium (look at how the moon keeps falling towards the earth), a system may be designed to maintain an equilibrium.
You have to look at how the system is set up to find out whether it is stable or unstable

ps The cricket ball is in the air because I threw it there not because of the air - if I throw a ball up in the moon it will be up...

The soft tube collapsing at relatively low elevation levels during the bench top experiments does not reflect the properties of the tough nylon tube now used. It simply will not collapse unles it is heated, which it is not during the experiments.

RE: taking one tube out of the bottle. There is a huge difference in taking the tubes out at a relatively low height. I.E. below the 33 feet limit. When tubes are removed at this height there is a siphoning effect that can be initiated. When over the 33 feet limit, the whole experiment behaves differently, in the the water will now remain suspended in the tubes, rather than flowing out as one would expect to happen. So I guess the elasticity of water does not significantly increase under the 33 feet limit.

Incidentally, I cant remember what happens when one tube is removed as it is usually removed when the bead of water has broken and the levels returned to 33 feet with vacuum above the levels. So I can't honestly answer this question, but will test it next time I use the experiments.

with regards to your other point on the levels not changing significantly as the tube is raised.
One possible explanation for this is that the tube elongates, due to the weight of water plus the weight of the nylon tube maybe.

www.bio.net/mm/ag-forst/2000-March/015058.html

Dave:
If that is the case the stability of the bead is an interesting effect to do with syphonish things, as this would require the bead of water to be somehow attached at the top... although I would like to see the exact setup before being sure.

Maybe next time I come home to visit my parents I could pop over to Paignton on my way back to the train and have a look?

However even if this is the case your setup isn't a good model of the structures in a plant and even if it was it wouldn't explain the majority of what is happening due to energy concerns.