TOPIC:

Dave:
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:
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.

@ 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.

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.

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.

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!

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.

Dave

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
www.chaosscience.org.uk/pub/public_html/...ry=20050301222247333
and 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 the leaf height, 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 consistent to me...

Rosy:

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.

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???

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).

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.

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.

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.

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.

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.

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.
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.

“Tree 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.”
www.fordfound.org/publications/ff_report...ges/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.

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 Rosy’s 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.

Dave:

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?

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 analogous would be having a 10+m pipe with the top end covered with a partially permeable membrane and strong salt solution on the other side . It will be horribly slow as the surface area of the end of the tube is very small.
eg

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

out of interest where are you getting the partially permeable 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.

@ 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.

Dave: 30/04/2005 14:27:22 »
You see I think your experiment proves 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.

Now your experiment has very cleverly shown that water will not always 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 probability of cavitation is low enough for you to do your experiment without it always 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 out 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:
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.

Rosy:
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!

@ 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.

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!

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.

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 constructive.

@ 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

Dave:
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?