TOPIC:

Dave:
Why is it impossible for leaves to produce the suction required? If you work out the osmotic pressure produced by just the sugar in orange juice (as an example plant fluid it is easy to get figures for) it comes out at about 9.8 atmospheres, enough to suck water up about 100m (about 300 feet), so as long as the water in the xylem doesn't cavitate it should work fine!

A large tree has tens, or hundreds, of thousands of leaves. So each leaf would only have to suck, by osmosis, a few militres a day to make up the 1000 litres a day you quoted earlier. Surely this is a perfectly reasonable rate?

I don't think the cavitation problem is as bad as it sounds - for a start we can observe that there are xylem in a tree that are 100m long and they don't cavitate, and in the link I posted earlier it says that branches have been spun in centrifuges so they are experiencing negative pressures equivalent to 92metres of water. So the question isn't "is it reasonable for a xylem not to cavitate?" but why isn't the xylem cavitating?

I would guess the answer to this is related to nucleation. It is possible to heat water above it's boiling point, without it boiling, if you do so in a very clean container. This is because although it is (free) energetically favourable for all the water to boil, to do so it would have to form a bubble. Creating a bubble is difficult because you have to fight against suface tension, and it turns out that to be stable the bubble has to be more than a critical size. Now if the xylem is smaller than this critical size it would be impossible for a bubble to form until the tension is so large that the critical size of the bubble is smaller than the diameter of the xylem.

I will do some calculations at some point but I guess this will be at considerable negative pressures.

Reply #20 on: 27/04/2005 12:41:00 »
Tubular Water
One way to envision water pulled into and up a capillary tube is to use a suspension bridge model. The column of water is suspended against gravity by its adherence to the walls of the tube. Cohesive force keep all the water molecules together. Capillary movement is greater as tube diameter decreases. Extremely small diameter tubes, pores, or spaces can attract water and move it a relatively long way.
Capillary movement is responsible for within- and between-cell water movement in trees, and small pore space movements in soils. Cell wall spaces are extremely small (interfibral) and can slowly wick-up water. The water conducting tissues of trees (xylem), does not utilize capillary movement for water transport. If xylem were open at its top, a maximum capillary rise of 2-3 feet could be obtained. Xylem transport is by mass movement of water not capillary action.
Capillary movement is a matter of inches, not dragging water to the top of a 300 feet tall tree. Capillary movement components can be seen where liquid water touches the side of a glass. The water does not abruptly stop at the glass interface, but is drawn slightly up the sides of the glass. This raised rim is called a "meniscus." The meniscus is the visible sign of adhesive forces between the glass and water pulled up the side of the glass. The smaller the diameter of the glass, the greater the adhesive forces pulling-up on the water column and the less mass suspended behind.
Tiny Bubbles
Gas bubble formation in water columns is called cavitation. As temperatures rise and tension in the water column increases, more gases will fall out of solution and form small bubbles. These tiny bubbles may gather and coalesce, "snapping" the water column. As temperatures decrease, water can hold more dissolved gasses until it freezes. Freezing allows gases to escape and potentially cavitates water conducting tissue when thawed. Trees do have some limited means to reduce these cavitation faults.
On The Move
Water movement and transportation of materials is essential to tree life. The three major forms of transport are driven by diffusion, mass flow, and osmosis forces.
Diffusion – Diffusion operates over cell distances. Diffusion is the movement of dissolved materials from high concentrations areas to low concentration areas. Diffusion can move a dissolved molecule in water across a cell in a few seconds. Diffusion does not operate biologically over larger distances. It would take decades to diffuse a molecule across a distance of one yard / one meter.
Mass Flow – Most movements we visualize are due to the mass flow of materials caused by pressure differences. Wind, gravity, and transpiration forces initiate and sustain small differences in pressure. These small differences drive water and its dissolved load of materials in many different directions. Because pressure is the driving force in mass flow, (not concentration differences as in diffusion), the size of the conduit is critical to flow rates. If the radius of the conduit is doubled, volume flow increases to the fourth power of the size increase (double conduit radius and flow rate increases by 16 times — 24).
Osmosis – Osmosis is the movement of water across a membrane. Membranes in living tree cells separate and protect different processes and cellular parts. Membranes act as selective filters, preventing materials with large hydration spheres or layers from passing through. Small, uncharged materials may pass freely. The driving force to move materials in osmosis is a combination of pressure and concentration forces called a "water potential gradient."

by Dr. Kim D. Coder
Daniel B. Warnell School of Forest Resources
University of Georgia
6/99

Dave:
Yep that all sounds about right.
As far as I can work out the presently accepted way water gets up a tree is:

The xylem are full of water from the beginning, as each year they grow up from the roots and are full of water from the start.
This column of water is essentially hanging from the top of the column, and is stable (despite being under considerable negative pressure) because the xylem is so small and covered with hydrophillic substances so cavitation is difficult as I described above (this is known as the cohesiveness of water).

Now the tissue around the xylem have more sugar and other salts dissolved in them than are in contents of the xylem so they suck water across the cell membranes surrounding the xylem by osmosis. You are right to point out that osmosis is a slow process, but this is happening over the whole area of the tree so it adds up.

Because the water is cohesive if you pull on the top the whole column moves up like a piece of string so it sucks water in at the bottom.

The water in the cells evaporates concentrating the salts and sugars in them, and allowing them to draw more water in by osmosis. So the energy to power the whole process comes from the sun evaporating water in the leaves.

Where is the problem with this picture

lets not forget the limit which suction can work under normal atmospheric pressure in physics. I.E. a pump/suction placed above a water source has a ceiling. Above 10 metres, the pump fails to work, and the water level remains at 10 metres, and the space above the 10 metres is vacuum, the limit was discovered by Galileo, while asked to work out why water at 40 feet below the surface could not be drawn up by a pump. Cavitations do occur and can be heard as cracking noises in a tree using a stethoscope.

Dave
As I mentioned earlier it is possible to get water below ther pressure it should cavitate and therefore be able to suck it up more than 10metres, (in the same way as you can super heat water) especially if you are in a very thin tube covered in hydrophillic substances (or a xylem as it is otherwise known). In fact looking at the web (and from a conversation we had in Brixham once) you have syphoned water 24m vertically using quite a large tube, so it must be possible to do better with this using a smaller tube.

Yes cavitation does happen, especially in drought conditions, but surely that shows that the water in the xylae is under tension and therefore unstable, so is evidence for the standard theory. There are a lot of xylae in a tree and it will be ok as long as the tree is growing the xylae faster than they are breaking due to cavitation.

the water at Brixham was not siphoned, as you well know a siphon will not work at those heights. In fact, to prove it was not a siphon that was taking place, I lowered one of the bottles in my experiment to see if siphon would occur, and because there was no saline solution at the centre of the loop of tubing no circulation took place, therefore disproving that we were looking at a siphon.

We can agree now on the fact that cavitations are known to occur. I believe that when a cavity occurs, the pressure changes reverse to a positive downward force, which has a direct influence on fluids in the rest of the tree, forcing the fluids in nearby tubes to rise higher and repair the cavitations, therefore enabling the bulk flow to continue.

Having said that, I am intrigued as to where and when we met, did you attend the demonstration in 1994?

Andrew

Rosy:
Hm, yes... I agree not exactly a siphon in the traditional sense... but i can't see why that matters. Whatever the source of the upward "pull" on the water at the top of the left hand column, it's still just a pull at that point, as would be the pull due to transfer of the water across a cell membrane by osmosis.
I really can't see where this tells us anything new.

BTW I think the guys are talking about this
www.the-tree.org.uk/TreeTalk/3Spring2003/Gravity/gravity1.htm
webpage, I googled it but I thought I'd link it to save others the bother (as I couldn't find it higher up the thread).

Thanks for agreeing with me on the non-siphon effect.
Strange that you cant find anything new in this? The flow rates observed within this simple paradigm parallel any observed rates in trees or plants, if not exceed them with ease. That being because of the obvious fluid friction within a tree or plant and the lesser degree of friction in the tubular models.

One should not jump to the conclusion that current understanding of osmosis is comparative to the efficacy of the new paradigm, without first testing the simple tubular experiments for oneself.

Andrew

Dave:
You had at in the experiment a loop of tube filled with water with a height difference from the top to the bottom of more than 10m? So you have prooved that a column of water more than 10m high is stable, so why shouldn't a xylem be able to do the same thing? Especially as it has had 200million years to optmise this process.

You are right what you were doing (injecting denser fluid near the top of the tube on one arm of a syphon loop) is not conventional syphoning, you are making one arm of the syphon heavier by using a higher density fluid rather than by using a lengthened arm, but if this works then a syphon will work.

If the water column has not cavitated there is no reason why it shouldn't syphon - the reason why it is often said that you can't syphon over 34 feet is that the fluid will have a tendancy to cavitate. How far did you lower your bottle, and for how long? If your tube was 150feet long a the system will have a resonant period of about 45 seconds (assuming that there is no damping, which would make this period longer), so to definitely see any effect you would have had to wait at least this long.

What you describe in your 'tubular experiments' sounds entirely reasonable to me and exactly what I would expect to happen from standard physics, but I don't see how it would apply to a tree.

Although you get a downward flow of sugars through the Phloem and an upward flow through the Xylem, as you mentioned earlier, 98% of the water that is lifted up is evapourated, so the less than 2% of water going down would have to lift 20 times that amount of water.
For the syphon device you describe in the link to work the weight on the downward side must be greater than the upward side or it obviously won't work. Unless the density of the sugar solution is 50 times that of the water coming up I don't see how this could work.

btw. You came to a hands on science event I was running in Brixham a couple of years ago.

You had at in the experiment a loop of tube filled with water with a height difference from the top to the bottom of more than 10m? So you have prooved that a column of water more than 10m high is stable, so why shouldn't a xylem be able to do the same thing? Especially as it has had 200million years to optmise this process.

*****Answer
A little more than 10 metres actually, 24 metres to be exact, as that was the length of tube I was using at the time.

The column of water is not stable in the tubes, cavitations is demonstrated as the stress on the water bead causes bubbles to form and the columns collapse eventually, just as they do in the tree.
*****



You are right what you were doing (injecting denser fluid near the top of the tube on one arm of a syphon loop) is not conventional syphoning, you are making one arm of the syphon heavier by using a higher density fluid rather than by using a lengthened arm, but if this works then a syphon will work.

*****Answer
Feel free to try your siphon at these heights. Ill bet you draw the same conclusion that many others have already observed as the accepted height at which a siphon will work.

Picture a loop of tubing suspended above the 10 metre limit, producing an unbroken bead of water, under the tension produced by the equal weight of the water on both sides of the tubes. Now initiate the lowering of one of the ground based bottles to try to cause a siphon. The result would be that the lowering of the one bottle would merely cause the bead of water to become elasticised and stretch to the point where it would collapse. But during the stretching process, we hypothetically inject a tiny amount of concentrated saline solution coloured, in one side of the loop at the top/upper most part of the loop. The result would be an obvious independent flow and return system, within the pre tensioned bead of water, flowing with total disregard to pressures, and creating its own pressure changes within the tension placed upon the bead of water.
This flow system does not require pressure in order to function, but delivers pressures as it functions.
*****


If the water column has not cavitated there is no reason why it shouldn't syphon - the reason why it is often said that you can't syphon over 34 feet is that the fluid will have a tendancy to cavitate. How far did you lower your bottle, and for how long? If your tube was 150feet long a the system will have a resonant period of about 45 seconds (assuming that there is no damping, which would make this period longer), so to definitely see any effect you would have had to wait at least this long.

*****Answer
Wrong, there is a fundamental reason why siphon does not occur as explained above.

The bottle was lowered 2 steps, presumably around half a metre, as I did not measure the steps, and remained for well over your 45 seconds without any evidence of siphon.

What you describe in your 'tubular experiments' sounds entirely reasonable to me and exactly what I would expect to happen from standard physics, but I don't see how it would apply to a tree.
*****Answer
According to the points you raise above, this is not quite correct, as your understanding of the siphon does not apply here.
*****

Although you get a downward flow of sugars through the Phloem and an upward flow through the Xylem, as you mentioned earlier, 98% of the water that is lifted up is evapourated, so the less than 2% of water going down would have to lift 20 times that amount of water.

*****Answer
This paradigm can lift many thousands of times the volume going up, and only requires a minute of solutes flowing down to cause the greater volume of less dense solution to flow up, giving the tree more than enough water to evaporate and produce a denser sap.
*****
For the syphon device you describe in the link to work the weight on the downward side must be greater than the upward side or it obviously won't work. Unless the density of the sugar solution is 50 times that of the water coming up I don't see how this could work.
*****Answer
This is where you go wrong David: imagine a 24 mil bore tube on one side and a 6 mil bore tube on the other side, blended seamlessly together to form a single looped open ended tube of different sizes immersed at equal levels in two bottles of water, suspended 24 metres vertically by the centre. The weight of the 24 mil bore side of the loop will be counterbalanced exactly by the 6 mil bore side of the tube, with no net movement either way. Now add the tiny amount of salt to the 6 mil bore side at the centre and circulation begins. In the case of the tree, the structure and size differences of the tubes compensates for the loss of moisture through the leaves and returns the resulting concentrates back towards the ground.


btw. You came to a hands on science event I was running in Brixham a couple of years ago.


I do remember popping in the town hall now you mention it, as you were closing your event I believe.

Thank you for remembering me.

Andrew

daveshorts
quote: imagine a 24 mil bore tube on one side and a 6 mil bore tube on the other side, blended seamlessly together to form a single looped open ended tube of different sizes immersed at equal levels in two bottles of water, suspended 24 metres vertically by the centre. The weight of the 24 mil bore side of the loop will be counterbalanced exactly by the 6 mil bore side of the tube, with no net movement either way. Now add the tiny amount of salt to the 6 mil bore side at the centre and circulation begins.

This system will produce a flow, but because the amount of water in the system is always the same, if you get 1 litre falling out of the 6mil tube, the 24mil tube will suck up 1 litre, however because the area of the bigger tube is 16 times larger the water you have sucked up will only go up 1/16th of the tube, you haven't pumped any water to the top.

quote: In the case of the tree, the structure and size differences of the tubes compensates for the loss of moisture through the leaves and returns the resulting concentrates back towards the ground.

But how are you getting the water out at the top? The water is at a negative pressure, this means that to get it out you have to pull, and pull very hard against a large pressure. Evaporation will do this, but if evaporation is doing the work you don't need the tube coming down and that is just the conventional model you are so dead set against.

In what way has your system produced a net flow of water to the top of the cliff? Overall you have moved water from one jar to another one next to it. If you had filled a bowl of water at the top of the cliff that would be equivalent to what the tree is doing, and I will believe it could be an issue when you can do that.

So would it break if you lift one of the jars, which will reduce the tension in the water column...?

Yes David, cavitation will inevitably cause the columns to collapse. The additional tension placed upon the bead by lowering the level of one side, merely serves to hasten the process of cavitation. Even if you raise a jar following initial lowering, the cavitation is already underway. In the link that Rosy put on her post, I have tried to address the way cavitations continually form and self repair within the multi conduit system of a tree. Cavitations do not interfere/interrupt the flow within the narrow tubes of the bench top model. In fact, the cavitations/bubbles behave oddly when sufficient saline solution is added. They are observed to flow down instead of up, and there is water flowing around the bubbles also.

Fascinating to see bubbles flowing down instead of up. Maybe you might want to test the simple bench top version for yourself?

Dave:

But there is no difference in the pressure of the water at the top of the tube, between your clifftop experiment and an equivalent syphon, so I don't see why you think one will work and the other won't. What sized tube did you use for your experiments?

Yes! Altering the heights of the 2 jars merely serves to place additional stress on the fluids within the unbroken bead of water. Therefore, the column is not permanently stable, as is so in the tree and plant. The tree gets around this problem by having an outer sleeve (bark) and a multi conduit system inside the outer sleeve. This enables the resulting pressure change when cavitation occurs, to gain height due to the resulting downward force on the broken bead, pushing up fluid under greater force to refill the broken bead.

quote:Originally posted by daveshorts

quote:quote]

This system will produce a flow, but because the amount of water in the system is allways the same, if you get 1 litre falling out of the 6mil tube, the 24mil tube will suck up 1 litre, however because the area of the bigger tube is 16 times larger the water you have sucked up will only go up 1/16th of the tube, you haven't pumped any water to the top.



The model is simple, I do not have the time nor the inclination to try to construct a perfect artificial tree.

I only have to show the driving force in this paper. The trees design takes care of evaporation as the water and minerals flow though its veins

quote: In the case of the tree, the structure and size differences of the tubes compensates for the loss of moisture through the leaves and returns the resulting concentrates back towards the ground.



But how are you getting the water out at the top? The water is at a negative pressure, this means that to get it out you have to pull, and pull very hard against a large pressure. Evaporation will do this, but if evapouration is doing the work you don't need the tube coming down and that is just the conventional model you are so dead set against.

Common sense should tell anyone that there is no attempt to extract water from the tubular models


In what way has your system produced a net flow of water to the top of the cliff? Overall you have moved water from one jar to another one next to it. If you had filled a bowl of water at the top of the cliff that would be equivalent to what the tree is doing, and I will belive it could be an issue when you can do that.



I have never seen a bowl of water at the top of any tree other than those left by the owners of apple trees to prevent scrumpers.

In the case of a tree, we could place a plastic bag over a branch and collect and extract the condensed water in its canopy.

It is possible to design a model that can lift sea water, extract pure water and return the denser ballast to the sea through a tube in order to provide the pumping for the desalination. But I have long since given up jumping though loops to amuse people.