How do Trees Really lift Water to their Leaves?

Lyner:
So you are saying that the force on the bottom link of a hanging chain is the same as that on the top link?
What about the weight of the chain? This is the most elementary mechanics and doesn't need to refer to Prof Hammel or osmosis. That is just a smokescreen.

What you say about tension is merely an unsubstantiated statement - not a reasoned argument. Solutes will tend to fall because they are more dense. A stone will do the same thing in water.

You have no proof that the U tube is any different from the single tube because you have not done a control experiment. Yes, it would be a pain but, without it, you have not proved a difference. You are the one who needs to prove it - not me; those are the rules, I'm afraid. Old ideas are "Innocent until proved guilty."

You can't seem to deal with my 'necking' / adhesion argument and what would happen at the top if it weren't for adhesion so I presume you have no answer.

I would have to decline your kind offer to demonstrate your ideas to a bunch of innocent School kids because your whole methodology is flawed. They could really do without that sort of influence until they are equipped with some logical thinking skills.
"In the name of Science"?? What Science? Science is consistent - or aims to be so. You have introduced an inconsistent idea which is not proven. You just get upset when it is not accepted.
Give me a good, logical, argument which refutes the logic of how the column of water would not stay up there unless stuck to the tube. And would it work in a metal U tube?

We have already accepted that your experiment worked. That is an interesting and surprising result. It's your explanation which is not acceptable because it does not stand the logic test. The one doesn't follow from the other.

And 'scaling down' is not valid because you have not scaled the ambient pressure. Can you argue with that?

Remember. The U tube experiment was relating to the shape of vessels in nature. Trees do not have tall tubes that flow up to the top and end, they have circular vessels that entertain a circulation, something worth remembering.

Can you repeat that in a way that makes sense, please? Tubes don't flow. Which are the circles? What does "entertain circulation" mean?

Put another way: The sap that flows in the tree is not on a one way trip to atmospheric liberation, it circulates around the tree evaporating water from it to the atmosphere, just the same as we do not excrete blood unless wounded, a tree does not excrete sap but evaporates solute free water from the sap.
Can see I missed some words from the last post now sorry.

I know this conversation has become heated at time. I just want to say that I am grateful for your thoughts even if at times I appear to be unhappy with some of your less than civil comments. The original explanation for the tubular experiment was written for students at Junior and secondary level in order to introduce the concept of a flow and return mechanism, without clouding it and making it too complicated for them to understand it. I still feel I did a good job regarding this and had no problem convincing both students and teachers with the experimental model and indeed the flow and return argument in trees.

I am beginning to take on board that a more detailed explanation is required for eventual publication, and as you say the need to test a single tube, a metallic tube, a plastic tube filled with mercury etc etc. I am also beginning to understand, thanks to you and others why a simple explanation may prove too problematic for people to grasp the behaviour of stretched fluids without seeing it for themselves, again thank you for enlightening me on how this is interpreted by others.

Lyner:
That's OK as far as it goes Andrew.
I see that you are totally convinced that your Science is correct.
It is not 'too problematic' for me to understand what you are saying. It is 'too problematic' for me to accept it. There's a big difference.
As I have said several times. Your experiment show evidence of a phenomenon which is novel but your explanation is just too naive. I don't need to see it for myself. I believe you saw what you saw.
I also believe in well founded Science. Results, in general, agree very well with the established theories. You, apparently don't understand these theories or you would be looking for an explanation for your phenomenon amongst them, rather than making up your own incomplete one.
Science tries not to be a matter of opinion; we try to base it on rigorous logic. When someone brings up an objection, for instance, based on vectorial addition of forces the objection has to be answered with rigour. You have chosen not to consider how this shows flaws in your explanation. If you explanation were correct, you could explain such an apparent anomaly.
You seem to be leaving this dialogue, assuming that you are, in fact right and ignoring objections.
Please don't have the temerity to object to conventional Science teaching on the grounds that we tell kids to believe things "because we say so". That is just what you have been trying to do -and with far less justification and track record. If a student objects to any of the standard Science I present them with, I am in a position to justify it right up to the wire. I would not dream of belittling them merely on the grounds that they have 'interpreted it' wrong. I will give them a full and rigorous explanation after having worked, if necessary, for a long time to reconcile their ideas.
I see you regard yourself as a heroic figure, battling against the massed ignorance of the Science establishment.In fact you are a Don Quixote, tilting at Scientific Windmills and failing to see what is actually going on around you.
Please try to read some established texts. They cannot all be wrong in every respect so they may just be more right than you are about this topic - if only you could understand what they are really saying.
It's never to late for some independent learning and self education. I do it every day.

Transpiration-Pull users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html

In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above.

As we have seen, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf.

But even the best vacuum pump can pull water up to a height of only 34 ft or so. This is because a column of water that high exerts a pressure (~15 lb/in2) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia (the tallest is 370 feet high)? Taking all factors into account, a pull of at least 270 lb/in2 is probably needed.

The answer to the dilemma lies the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form.


When water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. Tensions as great as 3000 lb/in2 are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires.

Because of the critical role of cohesion, the transpiration-pull theory is also called the cohesion theory.
Some support for the theory

* If sap in the xylem is under tension, we would expect the column to snap apart if air is introduced into the xylem vessel by puncturing it. This is the case.
* If the water in all the xylem ducts is under tension, there should be a resulting inward pull (because of adhesion) on the walls of the ducts. This inward pull in the band of sapwood in an actively transpiring tree should, in turn, cause a decrease in the diameter of the trunk.

* The rattan vine may climb as high as 150 ft on the trees of the tropical rain forest in northeastern Australia to get its foliage into the sun. When the base of a vine is severed while immersed in a basin of water, water continues to be taken up. A vine less than 1 inch in diameter will "drink" water indefinitely at a rate of up to 12 ml/minute.

If forced to take water from a sealed container, the vine does so without any decrease in rate, even though the resulting vacuum becomes so great that the remaining water begins to boil spontaneously. (The boiling temperature of water decreases as the air pressure over the water decreases, which is why it takes longer to boil an egg in Denver than in New Orleans.)
* Transpiration-pull enables some trees and shrubs to live in seawater. Seawater is markedly hypertonic to the cytoplasm in the roots of the coastal mangrove, and we might expect water to leave the cells resulting in a loss in turgor and wilting. In fact, the remarkably high tensions (on the order of 500–800 lb/in2) in the xylem can pull water into the plant against this osmotic gradient. So mangroves literally desalt seawater to meet their needs.

Problems with the theory

When water is placed under a high vacuum, any dissolved gases come out of solution as bubbles (as we saw above with the rattan vine). This is called cavitation. Any impurities in the water enhance the process. So measurements showing the high tensile strength of water in capillaries require water of high purity — not the case for sap in the xylem.

So might cavitation break the column of water in the xylem and thus interrupt its flow? Probably not so long as the tension does not greatly exceed 270 lb/in2.

By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding 225 lb/in2.

Sequoias can successfully lift water 358 ft (109 m) — which would require a tension of 270 lb/in2.
However, such heights may be approaching the limit for xylem transport. (The tallest tree ever measured, a Douglas fir, was 413 ft. high.) Measurements close to the top of the tallest living sequoia (370 ft high) show that the high tensions needed to get water up there have resulted in:

* smaller stomatal openings, causing
* lower concentrations of CO2 in the needles, causing
* reduced photosynthesis, causing
* reduced growth (smaller cells and much smaller needles).

Lyner:
Facts quoted above, from others, could well be true.
It is your interpretation of them that is flawed.
Merely posting yards of interesting botanical information does not detract from the fact that your Science is wrong.
As usual, you have no answers to the more searching questions.
Give my regards to Sancho Panza.

Whatever

Lyner:
Interestingly, nowhere in your recent long post do you quote anyone denying the effects of adhesion. Nor do you quote anyone even hinting at your U tube theory.
It's just a re run of the previous stuff about the cohesion in water being very high, on occasions. A bit pointless as I have already agreed that the evidence for cohesion is compelling.
It's your personal interpretation that needs explaining and justifying.

1. No tree to my knowledge has a cap on the top.
2. Circulation in a tree flows predominantly down in the phloem from source to sink. Source being leaves, sink being lower than the leaves.
3. Evaporation from the leaf cannot take place without a change in density in the sap that flows through the leaves. It is bad science to ignore this fact!
4. The model we are debating while interesting is merely an experiment that was designed to show the power of a flow and return system that benefits from gravity, not to reflect the perfect anatomy of a tree.
5. I have never denied adhesion is important. The fact that I never mentioned it by name, but did mention it by including that a soft walled tube necks as the tension is applied, based on observations with other experiments using soft walled tubes, and further mentioning it in the varicose veins study again a soft walled tube behaving in exactly the same way as my experiments is not the bad science you purport it to be. I have said that adhesion relies on cohesion but cohesion does not rely on adhesion, including that even when water is stuck to the side of a water filled conduit, it does not arrest the flow of water through the conduit. Yet in a single upright tube capped at the top this is what will happen and water inside the tube will be pulling down against the top of the rounded capped tube. Adhesion inside the vertical single tube will enable cohesion to be pulling inwards and downwards. Adhesion is not therefore a force, the force is the body of water on the molecular bonding between the water / tubular interface.
6. And what if? What if I jump through your hoop, conduct your experiment and prove it one way or another? What have I accomplished? Who is going to listen? Does this mean it will become accepted?
7. Or does it mean that you et al will find yet another reason to continue to believe in the leaf sucking cohesion tension impossibility of the first degree? Come on here, play devils advocate for an hour or so and see through their smoke and mirrors theory. Leaves cannot suck water up a tree, no matter what spin is put on the theory it is still completely and utterly flawed.

The longer last post did provide you with evidence towards answering your many questions. It relates to the strength of cohesion, includes adhesion in exactly the same way I included my mention in the necking of tubes, It includes the vine experiment showing water boiling at ground level due to the tension applied in the stem, proving that tension occurs throughout the column of water as I had said it would do. And observed it doing as tiny vapour cavitations formed along the length of suspended tube, and go back into the water when the tube is lowered.

It mentions also the spinning / centrifuging of branches to test the cohesion which reflects the tubular experiment I swung round carefully. Although this needs to be done again with a water filled bottle on the end of it to produce a compared tension. Remember the tube used will easily withstand the crushing force.

I do not see many people rushing forward to defend the existing theory. Many thousands of people have read this thread now. Why do you think this is so? And finally the required tension of a giant sequoia at 270 pounds per square inch. Impressive tension that. If it is in place someone should have measured it by now?


Ulrich Zimmerman.
The Cohesion Theory considers plant xylem as a 'vulnerable pipeline' isolated from the osmotically connected tissue cells, phloem and mycorrhizas living in symbiosis with plant roots. It is believed that water is pulled exclusively by transpiration-induced negative pressure gradients of several megapascals through continuous water columns from the roots to the foliage. Water under such negative pressures is extremely unstable, particularly given the hydrophobicity of the inner xylem walls and sap composition (lipids, proteins, mucopolysaccharides, etc.) that prevents the development of stable negative pressures larger than about −1 MPa. However, many plant physiologists still view the Cohesion Theory as the absolute and universal truth because clever wording from the proponents of this theory has concealed the recent breakdown of the Scholander pressure bomb (and other indirect methods) as qualified tools for measuring negative pressures in transpiring plants. Here we show that the arguments of the proponents of the Cohesion Theory are completely misleading. We further present an enormous bulk of evidence supporting the view that – depending on the species and ecophysiological context – many other forces, additional to low tensions, can be involved in water ascent and that water can be lifted by a series of watergates (like ships in staircase locks). www3.interscience.wiley.com/journal/1187...ct?CRETRY=1&SRETRY=0
I have asked Ulrich if he would like to take a look at this thread and add a comment.

Lyner:
In case you hadn't noticed, the reason I keep asking the same question is because you have not answered it yet.
You have just replied with more acres of stuff from other people and with second hand arguments which don't deal with the issue.
My question is, and has always been, to you personally.
Like a cabinet minister under tight interviewing, you have still not answered it. I have no strong opinions about the stuff you have just written about. I just want to know, in terms of the actual science, how you can still insist on the difference between your U tube, which you have tried and my single tube, which you haven't tried.
Not one of your references deal with that.
Your replies would only need to be a couple of paragraphs long if you were to stick to the issue.

Yes, there have been thousands of visits to this and the 'siphon' thread. The only recent contributions, however, have asked you to answer my question. You still haven't.

Perhaps an honest reply would be that you don't understand enough of it to explain the phenomenon thoroughly. There would be no shame in that.