How do Trees Really lift Water to their Leaves?

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.

Your experiment idea is a test of adhesion, and yes I do understand why you feel this may be strong enough to support the water. But this experiment has been conducted my many people over many years using many different materials, all of who have failed just like the great philosophers of science did over 300 years ago. If it were to succeed what then? Would this change my own experiment one bit? Not a chance, because as I have said and will say again the experiment performed as it was expected to perform and water was observed to flow around it from one vessel to another. The fact that you have a problem with what goes on at the molecular level inside a U tube compared to the inside of a capped end tube is fascinating but hardly going to destroy my own observations whichever way it goes is it?

You keep trying to throw a protective shield around students steering them from what you see as heretical science. But then fail to address the flaws in the accepted science.

Adhesion inside the tube is an obvious stabilising force preventing the water from pealing away from the walls of the tube. The downward force of the column of water pulls on the water molecules in the opposite side of the loop of tubing, which in turn balances out the opposing force enabling the water to become stretched by the weight of the two columns of water. The water inside your capped tube can only be stretched to the point that the water is able to stick to the capped end of the tube. We are talking about 6 mil bore tubing here, not some micro-bore capillary tubing.

I have not tested the experiment with metal tubing, and do not have access to a 48 meter length of unblemished metal tubing do you know where one could be found? Remember soldered joints will provide nucleation sites so can’t be used.

You have failed to answer my questions on paragraphs six and seven. I would like you to answer them if you feel you can. Is there going to be an opened door at the end of it and if so can I have it in writing please.

Lyner:

Quote

Your experiment idea is a test of adhesion, and yes I do understand why you feel this may be strong enough to support the water.


"MAY BE"??? I am saying, with better logic than any single one of your own 'original' ideas, that the adhesion has to be equal or greater than cohesion for your U tube or a single tube to work. If you can't understand my reasoned argument for that then that is your problem.

Quote

You keep trying to throw a protective shield around students steering them from what you see as heretical science. But then fail to address the flaws in the accepted science.

The problem is that your 'Science' is not even complete enough to be heretical.
I would protect them from what you have said so far in the same way that I would urge them to take care when reading adverts for snake oil and lunatic political parties.

Yes, I would say that any theory which ignores the simplest bit of vectorial force addition was nonsense. You do not even realise that what you are saying fails in this respect. You are implying that
"Forces Add Vectorially Except at the Top of a U Tube. In this situation we can ignore that particular bit of Science".
Is that supposed to be serious Science?
Describing the adhesion as a 'stabilising force' is to show that you don't understand the whole basis of vector addition. Why not use the proper terminology - which keeps bridges and planes up in the air and allows people to do Physics, Engineering and Chemistry with great success. This is not new Science and it is really not open to debate - certainly not by someone who seems not even to understand it. (Do you understand the 'triangle of forces' and what it implies in this context?)


Paras 6 and 7: I would fully expect the single tube to behave the same as the U tube but, as you say, it is more difficult to implement. It is you who claim that it wouldn't work on grounds which just demonstrate that you don't understand the basics.

You can buy long rolls of annealed copper tubing (10 mm microbore) in very long lengths. You would find it difficult to do the same experiment, though. I guess you would have to detect the amount of water suspended in it by weighing it. Adhesion between water and metals being what it is, I should expect the 10m limit would apply, so you could prove it one way or another with 21m of tube.
Smoke and mirrors? Can you show in a rigorous way how any aspect of modern Science is just Smoke and Mirrors? This is another of your Windmills.
Where do you get your ideas about the Science establishment. You put me in mind of Jude the Obscure, who was rejected by the Establishment in the Distant University City because he had tried to educate himself and had actually GOT IT WRONG. He was the only one who was surprised at what happened.
Give a Scientist, who is worthy of the name, a good reason to think that an existing theory is wrong and he will bust a gut trying to PROVE it is wrong. He won't rant and rave that the system is flawed and that everyone is being horrible to him. He will scrutinise the new idea and try, in an informed way, to get the right answer.
Clearly, you think your education in Science gave you a bad deal; that's sad but, unlike after a war, it is the losers who try to write the history. Mine certainly gave me a very good deal and I learned a lot throughout my life; it continues. What I did learn at a very early age is that if you can't answer a question you admit it and ask for help.

Quote

The fact that you have a problem with what goes on at the molecular level inside a U tube compared to the inside of a capped end tube is fascinating but hardly going to destroy my own observations whichever way it goes is it?

Do you actually read what I have written?
My idea does not attempt to destroy your observations - it successfully destroys your INTERPRETATION of them. Can you find, anywhere in this or the Siphon thread, a comment of mine which casts doubt on your observations?
Do you really want to ignore what happens at the molecular level and how the most basic bit of mechanics applies? It's a bit too rigorous and hard-fact for you, perhaps.

What you are basically saying is that you are right because you say so and the whole of Science is out to get you and your teachers were rubbish.
That's an opinion but proving the Science is a bit more involved than that.
« Last Edit: 09/11/2008 10:56:14 by sophiecentaur »

Ive found a supplier of 50 meter micro-bore copper tubing, cost £88.62 including VAT.
www.mytub.co.uk/product_information.php?product=309455

Maybe your school might be interested in testing both models? I can supply the school with the plastic pipe for free.