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Author Topic: How do Trees Really lift Water to their Leaves?  (Read 244960 times)

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #50 on: 29/04/2005 16:36:54 »
quote:
Yes even if its on top of something else! Take a look at the flow of dense rock pulled towards the Earths core

But that is because the thing the rock is sitting on is acting as a fluid (over the timescales involved anyway). From my experiments it doesn't matter how long I stand on the floor (from observation of wardrobes up to a period of a couple of hundred years) I am not going to fall through the floor.

quote:
To state that solutes and sugars stay put and are not acted upon by gravity is absurd! How do we tap rubber, harvest amber and maple syrup?????? There is an obvious downward flow!!!!

Surely you say later that maple syrup is liquid flowing upwards (through the xylae) powered by Carbon di-oxide.
quote:
And for every action there must be a reaction !!!!!

Indeed this is a fundamental part of physics, however this relates to forces not flows. Eg if I push the wall it pushes back, if the rocket pushes hot gas downwards the rocket gets pushed up, if the earth's gravity pulls something down then the earth gets pulled up slightly.

 It doesn't necessarily relate to fluid motion in a tree - this is obviously the case as there is 50 times more fluid going up than down

here is an interesting link:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phloem.html

The Phloem are at a positive pressure, even near the top of the plant - if they weren't aphids would have to suck the sap out, as it is even if just their mouthparts are left in the stem sap will come out. According to your theory the Phloem must be at a negative pressure (to pull the water up the xylem) so if there is a positive pressure there must be something else going on.


quote:
"The explanation requiring the fewest assumptions is most likely to be correct."

There is a slight caveat to this, the explanation has to explain the observed phenomena - microscopically as well as on the large scale.

If I were a tree designer and I could come up with a way of absorbing carbon dioxide without loosing water I would try and design my tree using something like your principle, especially if I were designing my tree to work in arid conditions as you would avoid having to lift huge amounts of water just to get a few minerals. However if you look at a tree microscopically it isn't cosistent with the structures you find there.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #51 on: 29/04/2005 17:38:51 »
quote:
Originally posted by rosy


If the primary system for moving water up trees is this convection type system you're proposing, how do the sugars get *up* the trees to the ends of the branches for leaf formation in the spring? According to your model, if there aren't any leaves yet how does the flow get started and worse how does it draw more sugars (and amino acids and whatever else it needs) up than it drops down (which it must in order to construct new leaves)? It's got to use active transport in the phloem.


Firstly, it is not a convection system. It is a flow and return system which operates when concentrations of denser solutes occur above less dense solutes due to evaporation.

In the spring, there is an initial temperature change, which initiates the flow and return system, causing the circulation inside the leafless tree to flow, and to generate both positive and negative pressures within the moving fluids as it goes.

 
quote:
sugar is produced and water is lost by evaporation from the leaves.
Sugars are transferred by (mainly passive) transport (depending on the concentrations) into the phloem. Given the sugars are already (since they're made in the leaves and moved to other parts of the plant) moving down a concentration gradient, there is no reason for more water to follow them across the cell.


Actually there is a very good reason for more water to follow, that being the cohesiveness of water molecules adhering to water molecules. This is precisely why I keep asking you to repeat the experiments.

 
quote:
Loss of water from the leaves results in water being drawn up from the roots via the xylae(osmosis).


Absolute nonsense, osmosis requires the belief that water can attract water up a tree and out through the leaves? I cannot see any logic in your argument here.
quote:

The sugars want to move to a position of lower energy/higher entropy (and so to places where there is less sugar already). There *is* an energy gain in going downwards, yes, but as Dave points out it isn't actually very big if you're losing a whole load of water at the top. Your Brixham experiment depends on using the weight of the water coming over the top of the loop to draw the water below it up. In order to produce any energy at all the salt/sugar solution has actually to move


Sorry, I disagree with you, the weight of the water in the opposing side is irrelevant, however, the density of the fluid in the opposing side can counterbalance the flow, much the same as overfeeding a plant can cause it to whither and die, or the same as acid rain can alter the density of the soil water by dissolving minerals too effectively.

This flow has nothing to do with the weight of the water in the opposing side of the loop. Let me try to clarify what is now known in relation to your argument. Picture a loop of tubing causing a slow but steady siphon effect. Now picture a small amount of saline solution injected into the top of the rising side of the siphon. Result: The coloured solution will flow in the opposite side to the siphon, and can be clearly seen in the turbulence of the coloured solution as it interacts with the clean solution. But the saline solution will not flow down without dragging on the water causing a two directional flow. I have seen this and you too can see it if you conduct the experiments for yourself!


 
quote:
, which in your model it can't do unless the water which it pulls up follows it straight back down the opposite tube. Indeed, as was pointed out by EL Hemetis, there is before us the evidence of plants quite happily growing with their leaves below their roots. I'm far more convinced by the idea that that concentration effects dominate.



This flow system will always run in the path of least resistance, be it horizontal, down or up, it makes no difference. But somewhere within the plant or tree, there is a pathway for gravity to draw solutes down, be it in the branches, the trunk, or the roots.
Picture a u shaped branch on ivy, eventually roots will begin to form at the bottom of the u branch. In fact some of the tallest trees on earth have u shaped branches.

quote:

Amber really is just the fossilized stuff. The "amber" people harvest is probably copal, which I *think* is a form of resin.
http://www.emporia.edu/earthsci/amber/copal.htm[/quote]

Ok I will concede that it may not be true amber in the sense of fossilized resin, but the stuff did come from a tree all said and done [:I]
Andrew


"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #52 on: 29/04/2005 18:21:42 »
quote:
Absolute nonsense, osmosis requires the belief that water can attract water up a tree and out through the leaves? I cannot see any logic in your argument here.

You may find this stuff interesting about osmosis
http://www.chaosscience.org.uk/pub/public_html//article.php?story=20050301222247333
and
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ospcal.html
has a nice program for calculating osmotic pressures

If evaporation keeps the concentration of the liquids in teh leaf hight, osmosis can generate the appropriate pressures to suck water out of the xylem, and the column of water in the xylem behaves like a wire (because it is cohesive) so if you pull at the top the whole column moves up, it all sounds consitent to me...
 

Offline rosy

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Re: How do Trees Really lift Water to their Leaves?
« Reply #53 on: 29/04/2005 18:56:06 »
Yeah, I saw that.
Could you have a look at this (mainly for tone) before I post it?

quote:
Firstly, it is not a convection system. It is a flow and return system which operates when concentrations of denser solutes occur above less dense solutes due to evaporation.

Yes, OK...  I know it's not really convection, but it's still a "water goes up, gets heavier, comes down" system. I was being lazy.
quote:
In the spring, there is an initial temperature change, which initiates the flow and return system, causing the circulation inside the leafless tree to flow, and to generate both positive and negative pressures within the moving fluids as it goes.

How???
quote:
Actually there is a very good reason for more water to follow, that being the cohesiveness of water molecules adhering to water molecules. This is precisely why I keep asking you to repeat the experiments.

I don't *think* this applies in the phloem. The water has to cross cell barriers, which it has to do by moving through pores which as I understand it would tend to disrupt the intermolecular forces holding water molecules together. The rules applying to tubes have to be looked at very carefully before we can apply them to the phloem cells which (as I keep saying) are split up by cell membranes. I don't have time to look up the (1st year undergrad cell biology textbook) information on this. I'm assuming you've looked at some texts at a higher level than GCSE, because although my biology mostly isn't up to much I do know that the GCSE chemistry syllabus is so much oversimplified as to be largely meaningless (I only ask because you quote so extensively from a GCSE text further up the thread).
quote:
Absolute nonsense, osmosis requires the belief that water can attract water up a tree and out through the leaves? I cannot see any logic in your argument here.

WHAT??????????
Of course water has to cohere in the xylae for osmaosis to work. The osmosis occurs at the cell membranes of the leaf cells there has to be a column of water sustained in the xylae the height of the tree. That relies on cohesion.
quote:
Sorry, I disagree with you, the weight of the water in the opposing side is irrelevant

Um, the weight of the solution is relevant. I know you don't accept it but what you've got there is fundamentally a syphon. Look up how syphons work and do the maths.
quote:
The coloured solution will flow in the opposite side to the siphon, and can be clearly seen in the turbulence of the coloured solution as it interacts with the clean solution.

You're introducing a coloured solution with a sideways velocity into a stream of water with a pre-existing velocity upwards. Sure you're going to see turbulance.
And, until the solution has largely diffused out into the rest of the water the water with the salt/dye in it will tend to go downwards, being denser than the water around it. That's all completely standard.
However, I would expect, if you add a significant quantity of salt solution that the rate of flow of water over the top of the loop to decrease measurable (you'd have to add the solution part way up the "up" tube rather than at the top or some of the solutes would be drawn over to the "down" tube and mess up the experiment before the system stabilised.
quote:
But somewhere within the plant or tree, there is a pathway for gravity to draw solutes down

Sure about that? I'm absolutely convinced I've seen plants in hanging baskets etc in which most of the leaves are below the basket and consequently below the roots.
quote:
This flow system will always run in the path of least resistance, be it horizontal, down or up, it makes no difference. But somewhere within the plant or tree, there is a pathway for gravity to draw solutes down,

So, like a syphon, then ;)
Only applies if the roots are, overall, above the leaves. As they aren't in hanging baskets.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #54 on: 29/04/2005 21:57:01 »
No not like a siphon at all, but I guess you will never know and have no inclination in exploring the experiments for yourself. What you have there is a fundamental flaw in your analogy of this beautifully simple flow and return system. There is an excellent attempt at explaining a siphon on the link below.
http://www.straightdope.com/columns/010105.html

Within my system, the two ends of a single tube can be pre-filled with water, and a small amount of saline solution added at one end of the tube, which is then joined together. Lift / elevate the saline contaminated end of the loop and the flow system works perfectly. If soft wall tube is used, the saline flows rapidly down causing that side to dilate and the opposing upward travelling side to be sucked in, indicating a negative and positive pressure caused by the action of gravity on the saline solution.
NOT, as in the siphon, pressures causing the flow. This is the flow causing the pressures and that is a monumental difference between the two methods.

RE Hanging baskets: Roots above the leaves. And leaves above leaves, it makes no difference where the roots are. Absolutely no difference to this system whatsoever, because as the stem bends over the basket, the loop still enjoys exactly the same forces from gravity.

ďTree Logic was created by Natalie Jeremijenko. It is made with 6
Flame Maple trees which have been hanging in MASS MoCA's Courtyard B.
Tree Logic at MASS MoCA was sponsored by the Sterling and Francine Clark Art Institute will, over time, vividly demonstrate the obvious fact that plants grow toward the sun. Showing that trees are dynamic natural systems, and Tree Logic reveals this dynamism.Ē
http://www.fordfound.org/publications/ff_report/images/03_sp_films1.jpg
The trees have been at Mass MoCA long enough that the branches have begun to turn and travel upward. The idea that Jeremijenko uses here in this exhibit is the idea of showing simple and obvious facts of life in biology in an extreme way.
Walker Metcalfe


I am surprised that you think that a mere cell wall, or pit, or an osmotic membrane for that matter could tear water apart. The bonding between water molecules is phenomenal, withstanding very high tensile stress. This force can easily pull water through the obstacles you mention.


"The explanation requiring the fewest assumptions is most likely to be correct."
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Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #55 on: 29/04/2005 22:12:30 »
Thoroughly enjoyed the animations, which do not represent the movement of osmosis, having been exaggerated to stress a point, nevertheless, we are dealing with a non-living force, in very simple apparatus, so in the morning, I will set up a hundred metre example of this model, and we should see water flowing effortlessly out at the top of it.

But this would not fit with the xylem, as there is no concentration gradient in the xylem, where the water flows up. However, there is a concentration gradient in the phloem where the water flows predominantly down, sometimes horizontal and occasionally up (for Rosyís benefit) J

Tell you what Dave, conduct the U experiment shown in the theory with salt solution one side and clean water the other, no membrane. Leave it stand for a week if you like suspended by both open ends and report back how much water has flowed out the top of either side of the tube.



quote:
Originally posted by daveshorts

quote:
Absolute nonsense, osmosis requires the belief that water can attract water up a tree and out through the leaves? I cannot see any logic in your argument here.

You may find this stuff interesting about osmosis
http://www.chaosscience.org.uk/pub/public_html//article.php?story=20050301222247333
and
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ospcal.html
has a nice program for calculating osmotic pressures

If evaporation keeps the concentration of the liquids in teh leaf hight, osmosis can generate the appropriate pressures to suck water out of the xylem, and the column of water in the xylem behaves like a wire (because it is cohesive) so if you pull at the top the whole column moves up, it all sounds consitent to me...



"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #56 on: 29/04/2005 22:51:01 »
Both a syphon and what you are describing work because one arm is heavier than the other

Both are analagous to a piece of rope over a single pulley - if one side has more weight on one rope it will move down. In your system the extra weight is caused by the extra density of the salt, in a conventional syphon this is produced by an extra length of water.

If water had no cohesion it will only work if the water is kept in compression by atmospheric pressure so neither system would work above 10m as the pressure would become negative. However with a very small clean tube you can produce a negative pressure at the top without it cavitating, due to the cohesion of water.

The cell membranes will not rip the water molecules apart, but they will stop the solutes form moving, and if the salt in your system can't move then your circulation will not happen.

What did you think about the links on osmosis?
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #57 on: 29/04/2005 23:08:20 »
Sorry I started writing that last post before you did.

There doesn't have to be a concentration gradient within the xylem, you just have to have a concentration gradient between the xylem and the cells in the leaf.

What would be more analagous would be having a 10+m pipe with the top end covered with a partially permiable membrane and strong salt solution on the other side . It will be horribly slow as the suface area of the end of the tube is very small.
eg
Code: [Select]

     _____   <-  membrane
     |   |
     |   |
     |   |
     |   |   <- code
     |   |
  |__|   |__|
  |  |   |  | <-resivoir
  |_________|


It may actually be easier to see an effect if you just put dry salt on the membrane as then it will be more sovious if it is getting wet.

out of interest where are you getting the partially permiable membrane from in the morning?

Also if you want to produce a very large pressure difference you will have to support the membrane very well as over an appreciable area the pressure will build up to a large force and rip the membrane.

I will see if I can find some tube and try it out on a small scale to test the feasibility of it.
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #58 on: 29/04/2005 23:09:57 »
in the diagram where i wrote code I meant tube... I think I need to eat supper ;)
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #59 on: 30/04/2005 09:10:43 »
Dave, Sorry, I was being sarcastic about trying it in the morning and you do not deserve it, and I appreciate your replies to my posts.

I really do know that it won't work, any more than a barometer will raise water above the 33-34 feet limit, water cannot be drawn that high by suction, using the most carefully maintained pump, let alone a semi-permeable membrane and some salt. We would both be on a hiding to nothing. It has long been known that there is a limit to the height that water can be drawn up using a capped ended tube. Get to 33 feet and the water simply tears away from the capped end of the water filled tube as it is raised above the reservoir, shown in your drawing. I have observed this many times. In fact, when the bead of water cavitates in the Brixham experiment, the water in both sides falls to the 33 feet limit, leaving vacuum above the limit and water below it.

If it were that simple, I would never have bothered to come up with a new theory.
The problem is that the membrane is also permeable to air, and the water will not rest in the tube. However, capillary action will work to a short distance using very fine tubes, however, to get it to raise to any appreciable heights, the capillary tubes used are often finer than those found in the xylem.

To my knowledge there is no working model other than mine that can demonstrate water circulating as effectively without a pump, not even a tree comes close. This is obviously due to a trees internal structure and the friction caused by it. Which incidentally must generate some heat, and is probably why many trees do not freeze in freezing temperatures.

Using dry salt on the membrane as you suggest may get wet, even if it is not in contact with the water, due to its ability to pull water from the air when it is in its dry state. Mangroves, leaves often show salts crystallized on them, but this is evidence of the evaporation and resultant salt build-ups.



quote:
Originally posted by daveshorts

in the diagram where i wrote code I meant tube... I think I need to eat supper ;)



"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #60 on: 30/04/2005 14:27:22 »
You see I think your experiment prooves that it is possible.

Why isn't the water cavitating at the top of the tube in your experiment? It is the pressure that drives cavitation, if it is below the vapour pressure of water the water will prefer to be a gas than a liquid, so it ought to be boiling.

If you consider the case of your experiment and a sealed tube (made out of the same substance as your tubes were) the pressure at the top will be identical, so they should behave similarly. If the water is over 10m high the pressure will be negative and it should be cavitating in both cases.
Code: [Select]
    _________                __
    /   ___   \              |  |
    |  /   \  |              |  |
    |  |   |  |              |  |
    |  |   |  |              |  |
    |  |   |  |              |  |
    |__|   |__|              |__|
    |  |   |  |              |  |
Now your experiment has very cleverly shown that water will not allways cavitate (this is something I didn't know before talking to you and is very interesting) as although on balance it would prefer to be a gas, breaking the surface tension is difficult so the probablility of cavitation is low enough for you to do your experiment without it allways cavitating. If the surfaces of the tube are covered with something hydrophillic (water loving) and made very small, the probability of cavitation will be lower.

Now I think this means that the single tube should be similarly stable.

I think you are right putting dry salt on the top of the column would probably mean that air can get to the membrane and you would get cavitation (This may be why trees tend to cavitate more in droughts as the membranes at the top dry aout allowing air in at the top -> a cavitation), but if we slightly alter the design to be more like a real tree and have the top of the membrane covered with a strong solution it may well work eg:
Code: [Select]
       |__________|
        |__________|  <- concentrated solution
            |  |
            |  |
            |  |
            |  |
          ..........
            |  |
            |  |
         |__|  |__|
         |  |  |  |  <- water
         |________|

As now the concentrated solution at the top will act as a seal over the membrane stopping gasses getting in.

I think I will try a small scale version of this at home using some of the thin stuff you get between onion layers as the membrane- I think making the membrane strong enough to support 10m of water will be difficult but that is a purely engineering problem.
 

Offline rosy

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Re: How do Trees Really lift Water to their Leaves?
« Reply #61 on: 30/04/2005 16:55:00 »
Andrew
I don't understand in what way the link to the "straightdope" site supports your argument. Cecil Adams describes it what happens beautiffuly, and then rejects the idea based on the fact that the water will cavitate above 10m.
As your experiment establishes that it will *not* necessarily cavitate that problem with the syphon vanishes.
The reason why a barometer can only go up to 10m, driven by atmospheric pressure, is because the water, whilst it will cohere to itself, cannot stick to the top of the barometer tube so a vacuum forms against which 1 Atm supports a 10m column.
In a loop of water, if there is no cavitation then there can be a syphon. If cavitation occurs, as you say, the water on both sides falls back to form two 10m columns.
The syphon works because the pressure at the top of loop is due to the weight of the water in the columns of either side. Since this weight is greater in the longer column there is a net force over the top of the loop towards the longer column.
Your system, likewise, has a greater weight of water/solution in the "down" column.
The pressure change in the soft tube on injection of an amount of concentrated solutiondoesn't seem to me to be particularly surprising. The soft tube absorbs the energy of the change in height of the extra weight and then transfers it back to the water by evening out the flow.
I don't think this is particularly relevant... the weight is still providing the energy, there's just then a delay in transferring it back into the system.

And the water in two different cells, when separated by a cell membrane, isn't in contact in the first place to it isn't "torn apart" by the cell membrane.
In order for the water to pass between cells it has to pass through a *very* small pore... too small for a glucose molecule to pass through. This has got to involve breaking some water-water hydrogen bonds!!
This process is due to two factors, osmotic potential and pressure. There'll be an effect due to the weight involved here (a pressure factor), but the osmotic effect works the other way as that's how the concentration gradient runs.
I don't know how much water actually gets out of the bottom of the phloem. Anyone (Andrew? Dave?) want to find some figures? Is it actually known?

Maple trees growing upside down... well, they don't seem to have died of all their solutes falling to the leaves. And of course the branches turn upwrds again as they grow, that's where the sun is!
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #62 on: 30/04/2005 19:02:29 »
Rosy, the concentrated sap in the phloem that is not actually taken up by the trees increasing growth cycle, is rediluted by incoming water from the soil, under a negative pressure generated by the falling sap.

Rosy
Be my guest and try to make a siphon work over the 10 metre limit. It does not work!, the weight of the water in the lowered side of the inverted U tube, merely stretches the water until it breaks and a cavity forms, causing the water to fall to remain at the 10 metre mark. As the Brixham experiment demonstrates, it is possible to raise the water in excess of 14 metres, but eventually the water bead will break. When I say a siphon will not work, it is because I have actually tested it!

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« Last Edit: 30/04/2005 19:04:32 by Andrew K Fletcher »
 

Offline rosy

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Re: How do Trees Really lift Water to their Leaves?
« Reply #63 on: 30/04/2005 19:26:50 »
quote:
Rosy, the concentrated sap in the phloem that is not actually taken up by the trees increasing growth cycle, is rediluted by incoming water from the soil, under a negative pressure generated by the falling sap.

OK, I have absolutely no idea what this is supposed to mean.
How on earth can the falling sap generate negative pressure in the roots? I think I must have misunderstood what you mean by this.
quote:
As the Brixham experiment demonstrates, it is possible to raise the water in excess of 14 metres, but eventually the water bead will break. When I say a siphon will not work, it is because I have actually tested it!

I'm afraid I simply don't believe that this is because it's impossible.
If you can (1) have a column of water 14m high, which you've shown you can and (2) you can exert a downward force on one side of the loop by introducing the extra weight of a few ml of salt solution.
I'm very interested to know how you conducted your syphon experiment (whether you set it up with two containers at different levels then raised the syphon tube, or whether you established the raised loop then raised/lowered one of the containers... I'm interested as which you did will influence rises/drops in pressure (I wonder because you refer to the "lowered side" of the inverted U.
Of course, I'm aware that you're in the very difficult position of attempting to prove a negative, and so you can win only by reasoned argument... and I'm not making your life easier by not being convinced ;)

Sadly I'm not currently in a position to carry out the experiment to my own satisfaction at present (or any time between now and mid-June) as I'm in Cambridgeshire (no cliffs) and don't have the kind of access to a high level window/the ground below it I'd need to do anything contructive.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #64 on: 01/05/2005 17:14:59 »
Rosy
Ive thought of a way to explain what happens when you try to siphon higher than 33 feet limit.

Picture some play slime /goo stuff that kiddies (and me) play with, when you pinch a little between your finger and thumb and try to lift the rest of the mass, it stretches until it snapps off. This happens with water inside the tube also.

With regards to the falling sap generating a negative pressure at the roots, relates to the suction at the lower part of the tree that draws water in. Even if the roots are removed, the suction is still there, rulling out root pressure as a driving force. What I should have said to be more accurate is; the falling sap generates a positive pressure in the phloem in front of the falling sap, and a negative pressure is caused behind the falling sap.

Hope this helps

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Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #65 on: 01/05/2005 17:51:24 »
Just to clear up a quick point that may have been causing confusion:
When me and Rosy are talking about negative pressure, we mean negative absolute pressure - where a vacuum is zero pressure.

The reason why GCSE textbooks tell you that a syphon will not work above 33 feet is that atmospheric pressure is enough to lift water 33feet, so up to this point the water is under compression by the atmospheric pressure. If you go above 33feet the water is in tension (a negative pressure) and should therefore boil or cavitate or something breaking the syphon.

Now as you have found out real life is rarely as simple as GCSE textbooks, and water can actually survive a negative pressure if it is continuous, there are minimal dissolved gasses (which you removed by boiling) because of the cohesiveness of the water. It is not stable like this and a small bubble will cause it to cavitate. However if there are no gasses this is unlikely enough for you to do your experiment in Brixham.

Now if you are using the same liquid in both tubes the pressure in the tube is only dependent on how much weight there is pulling on it, and the chance of cavitation is just dependent on the pressure. So the only difference between a normal syphon and your syphon is that the extra weight is provided by an extra length of water rather than salt.

Would your syphon work if you added the salt near the bottom of the system? if so how is this different from adding an extra weight of water by lengthening the tube?
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #66 on: 01/05/2005 22:00:03 »
Originally posted by daveshorts[/i]
 
quote:
When me and Rosy are talking about negative pressure, we mean negative absolute pressure - where a vacuum is zero pressure.


Is there any other kind of negative pressure?

 
quote:
The reason why GCSE textbooks tell you that a syphon will not work above 33 feet is that atmospheric pressure is enough to lift water 33feet, so up to this point the water is under compression by the atmospheric pressure. If you go above 33feet the water is in tension (a negative pressure) and should therefore boil or cavitate or something breaking the syphon.


Not sure that Iíve read anything about siphons not working at 33 feet in a gcse biol book? Iíve read it on plenty of other places on the web though. You do appear to be saying the GCSE Biol book is wrong, and that I something we can agree on at least.

Pascal demonstrated that the siphon worked by atmospheric pressure, not by horror vacui, by means of the apparatus shown.  The two
beakers of mercury are connected by a three-way tube as shown, with the upper branch open to the atmosphere. As the large container is filled with water, pressure on the free surfaces of the mercury in the beakers pushes mercury into the tubes. When the state shown is reached, the beakers are connected by a mercury column, and the siphon starts, emptying the upper beaker and filling the lower. The mercury has been open to the atmosphere all this time, so if there were any horror vacui, it could have flowed in at will to soothe itself.
source: http://www.du.edu/~jcalvert/tech/fluids/hydstat.htm#Siph

 
quote:
Now as you have found out real life is rarely as simple as GCSE textbooks, and water can actually survive a negative pressure if it is continuous, there are minimal dissolved gasses (which you removed by boiling) because of the cohesiveness of the water. It is not stable like this and a small bubble will cause it to cavitate. However if there are no gasses this is unlikely enough for you to do your experiment in Brixham.


Agreed

 
quote:
Now if you are using the same liquid in both tubes the pressure in the tube is only dependent on how much weight there is pulling on it, and the chance of cavitation is just dependent on the pressure. So the only difference between a normal syphon and your syphon is that the extra weight is provided by an extra length of water rather than salt.


Dave, I believe there is something else at work in this model, I believe the molecules of the dissolved salts align in conjunction with gravity as they mix with a greater volume of clean water in the same side. I.E. the more dilute the saline becomes and the greater the distance it spreads out, the greater the flow rates achieved, say thrice times the normal rate of decent and accent accordingly, depending on the height. It appears that the higher the experiment goes the greater the flow. Still trying to figure out how to set a scale of flow so that everyone will agree on the formula :)

On occasions, the saline flow has triggered a very rapid flow, as opposed to the normal stable flow. It canít be a siphon effect that it is triggering because there is no additional weight or density to the downward flow.   When a small amount of saline solution is added in a way that it can flow in both directions over the inverted u centre, you can clearly see the flow at work and the turbulence it causes in the ascending side as well as the descending side. The best place to view this is on a spiral staircase. Iíve often used the one in the local car park for my experiments.

Siphoning is used on a large scale to move huge volumes of water for irrigation from one reservoir to another. However, they find that this does not work if the height is too great and have to install a pump to maintain a positive pressure.

Even the smaller bench top model of the Brixham experiment reveals some amazing properties.

For example: cavitation can be observed, even at the low level. The syringe body filled with saline remains stable while the tube is in the elevated position. After injecting a small amount of coloured saline solution in at the top, the syringe begins to self-empty as the plunger is pulled up against gravity by the descending saline solution. This is pretty amazing when you consider that say 1 mil of saline solution is injected and 5 mils of saline solution are drawn up as the plunger rises from a near vertical down position and joined to a small length of the same tube to the T junction.

I have seen a siphon work on many many occasions. This simply is not a siphon at work here.

Yes the saline solution can be injected into any point on the descending side and the flow will occur. But as I stated earlier, the higher up you add it, the greater the flow rate.

What would you expect to happen to the water levels in the tube, when you remove the both ends of the tube from the bottles, while it remains suspended above the 33 feet limit?  


"The explanation requiring the fewest assumptions is most likely to be correct."
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Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #67 on: 02/05/2005 02:47:42 »
I am not quite sure what you mean by "the molecules of the dissolved salts align in conjunction with gravity". Gravity is my molecular standards an increadibly weak force the energy released by rotating all the water molecules to align with water would release less than a nano joule of energy - in comparison heating it up by a degree centigrade would require over 4kJ a difference of a trillion times. So on this scale thermal and intermolecular forces will hugely dominate any gravitational effects.

Gravity only becomes important at larger scales, so we can model water as a fluid affected by gravity rather than worrying about what happens on the molecular scale.

I think you would expect the syringe to be pulled in if you plug it into something with a pressure below that of a vacuum. It may stick in the beginning though. which is why it wasn't moving to start with. Have you tried a similar experiment injecting pure water into the system, and found out what happens to the syringe?

Yes the cohesiveness of water is an effect that is stronger the smaller the tube you use, and the cleaner the water, so syphoning huge amounts of dirty irrigation water is unlikely to work at over 10m.

Just a thought, have you taken into account the momentum the water in the tube will have once it starts moving. (This will not be insignificant if my experiments using a hosepipe to move a level around my parents barn are anything to go by)

Once the water starts moving through the tube it will tend to keep moving. I think this is why you get more flow when you put the saline in at the top, than when you put it in the bottom. When you put the saline in the top it will accelerate under gravity pulling the rest of the water with it. The further it drops the faster the saline, and the whole water column will be going. When it gets to the bottom, now the column is moving it will want to keep on moving because it has inertia, so it will keep syphoning(ish) until it slows down due to friction.

quote:
What would you expect to happen to the water levels in the tube, when you remove the both ends of the tube from the bottles, while it remains suspended above the 33 feet limit?

I would guess that the water would start to fall out of the bottom of the tubes as what is known as a slug bubble goes up them (basically what happens if you cover the top end of a tube and lift it out of the water - air goes up the middle and water comes down the sides.

Now through random factors like the exact sizes of the tubes and which one you took out of the bottles first one tube will empty a bit quicker than the other. This will mean that there is more weight on the side of the slower tube, which should start a syphon going which will pull the fast emptying side up faster and faster as the pressure difference between the two sides gets bigger. This would mean that you get more water out of one side than the other, and the difference between the rate of flow out of the tubes should get larger with time.

I am not sure how strong the syphon effect will be relative to them just emptying as I think this is dependent on the diameter of the tube. This is assuming that the fiddling with it doesn't trigger a cavitation.

How close am I? I am really interested to see how good my physical intuition is.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #68 on: 02/05/2005 09:21:06 »
Well, I did have a rather long conversation with a huge group of people on a physics newsgroup about whether gravity is actually a weak force or a strong force, and I did get quite a few people on to my way of thinking eventually, showing that you simply cannot take one pre-defined unit of gravity and measure it against 1 pre-defined unit of say EMF, as the comparison should have been measured against the total pull of the mass of the whole planet, because isolating any part of the mass does indeed result in the measurement of only one tiny part of the mass. Therefore collectively, the mass will always logically be greater than the sum of the smaller part of the mass.

But thatís another argument for another day.


I think you would expect the syringe to be pulled in if you plug it into something with a pressure below that of a vacuum. It may stick in the beginning though. which is why it wasn't moving to start with. Have you tried a similar experiment injecting pure water into the system, and found out what happens to the syringe?

Good, at least we now agree on the negative pressure issue. Actually, I have tried it at the same height of elevation without the solute, and the syringe body remains unaffected, as does the flow. However, as you correctly state, if we go substantially higher with the inverted U tube, the syringe will become sucked in by the negative pressure/tension placed upon the water.

Now using a complete loop of water filled soft walled tube (used to demonstrate how this flow could affect fluids in the body) once the saline solution starts to flow down one side, the turbulence becomes obvious as some of the coloured solution is pulled up one side and down the other side, proving complete rotation / circulation of the loop is taking place.

An obvious and very significant narrowing of the upward flowing tube takes place, and an equally obvious and significant bulging of the opposing downward flowing side of the tube takes place. Indicating the presence of both a negative and positive pressure, generated by the falling salt solution.

I have obviously thought about the momentum of the water. But you have just highlighted something very significant for me, which I believe has just explained my observations on the sudden acceleration of the water in the tubes during flow. Thanks Dave, youíre a star!

What Iíve just gleaned from our conversation is: The water is very elastic and stretches substantially when placed under tension, as does the analogy of using the play slime, mentioned earlier to Rosy.

The sudden acceleration is due to the sudden release of the built up elastic tension, caused by the falling salt solution on the opposing clean water-side of the loop. Just like releasing a stretched elastic band. This fits exactly with my observations when removing the two ends of the water filled tubes out of the two bottles and the water level rises up the tube by half a metre! Proving the amazing elasticity of water. The water remains suspended, even if we blow up one side of the tube, it temporarily alters the level in the side you are blowing up, but the water stays in the tube suspended almost like two weights linked by an elastic band and hung over a wall. More like two weights on an equal length of wire, joined by an elastic band in the middle and hung over a wall. Picture lifting one of the weights gently releasing the tension on the elastic band, but not sufficient as to over balance the weight in the other side. Amazing!
If there is some salt left inside one side of the inverted loop when both ends of the tubes are removed together, the saline side begins to draw up the water in the adjoining side, accelerating as it goes, until all the water flows out of the one side only!  6 mil bore tubing, hard nylon, which resists the negative tension more than adequately.


 But you were pretty close Dave. Do you live close to me in Paignton? Perhaps we could meet up and to give you a demonstration of the exp?


"The explanation requiring the fewest assumptions is most likely to be correct."
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Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #69 on: 02/05/2005 12:51:35 »
l_kryptonite
I disagree; your contribution is as valuable as anyone elseís. 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.




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Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #70 on: 02/05/2005 13:56:51 »
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.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #71 on: 02/05/2005 14:43:04 »
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.


 
quote:
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?


Yes, itís 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.


 
quote:
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!

 
quote:
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!

 
quote:
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 disconected, and thes 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 surounding soil into their roots, or into a cut stem or trunk, with no roots.



 
quote:
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

"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #72 on: 02/05/2005 15:23:43 »
quote:
Yes, itís 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.

quote:
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 disconected, and thes 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 surounding 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?


quote:
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.
 

Offline Andrew K Fletcher

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Re: How do Trees Really lift Water to their Leaves?
« Reply #73 on: 02/05/2005 18:00:39 »
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.


 
quote:
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,Ē 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.

 
quote:
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.

quote:
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.


 
quote:
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.
[/quote][/quote]

ho∑me∑o∑sta∑sis (hm--stss)
n.

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

"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
« Last Edit: 03/05/2005 16:16:28 by Andrew K Fletcher »
 

Offline daveshorts

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Re: How do Trees Really lift Water to their Leaves?
« Reply #74 on: 03/05/2005 16:58:02 »
quote:
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.

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 are 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?

quote:
ho∑me∑o∑sta∑sis (hm--stss)
n.

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?


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...
 

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Re: How do Trees Really lift Water to their Leaves?
« Reply #74 on: 03/05/2005 16:58:02 »

 

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