[Bored chemist]

this glass-sheet demonstration would work in a vacuum.

That, again, is merely a statement. Have you any evidence of this.
You might bear in mind that the  force which has been measured can be explained by atmospheric pressure.
What value do these inter molecular forces have, btw? Can you quote a value from somewhere. You imply that it has been measured. It would have huge implications on things like the boiling point of water, for instance.

The value has been measured- it's about 2.5KJ/g

[lyner]

The value has been measured- it's about 2.5KJ/g

KJ/g is not a unit of force.
Do you have a reference so that I could look at your source? It may make more sense than the bald statement.

[lyner]

The strong intermolecular forces between (polar) water molecules are why water has higher boiling point than similar sized molecules, e.g. water (H20) is liquid at room temperature, whereas ammonia (NH3) and methane (CH4) are gases at room temperature.
Hydrogen cyanide (HCN) boils at 26oC.

But water boils at room temperature at an ambient pressure at about 0.1 Atmospheres.  A volume of water inside the cylinder would boil once the pressure difference was reduced to that amount - with about 90% of the force which would be needed to separate the Magdeburg Hemispheres under the same conditions.

It is very easy to show the effect of depressing the boiling point of water under reduced pressure - I have done it in a bell jar with lukewarm water. It just boils when you reduce the pressure with a vacuum pump. If you conduct your experiment at room temperature, why would the same thing not happen at an appropriate pressure?

[RD]

 A 1966 paper on the tensile strength of water...

 [ Invalid Attachment ]


http://resources.metapress.com/pdf-preview.axd?code=q221256030087858&size=largest


http://www.springerlink.com/content/q221256030087858/

[Bored chemist]

The figure I gave is the latent heat of evaporation. It's a measure of the energy required to separate the molecules against the force that holds them together. As such it is an indirect measure of that force.
To convert it to a force is tricky because you need to know the form of the force/ distance curve so you can differentiate it.
However you can convert the figure from KJ/g to KJ/mol then to J/ molecule and, knowing how many molecules there are in a given area (from the density etc) you can work out the energy required to separate the molecules in a given area. If you assume that the potential is roughly linear over some small distance you can get an estimate of the force required.

I gave that figure just to show that the force and be calculated (it will be very large).
RD has cited a direct experimental value.
The point is that the force can be (and has been ) measured and calculated.

The other point- that the boiling point of water is dependent on atmospheric pressure is a red herring. The other liquids' boiling points also depend on pressure. However water will always have a much higer boiling point because it has strong bonds holding the molecules together whereas things like methane don't.

[lyner]

That's fair enough.  I was trying to relate it to a possible 'tensile strength' so the number would be handy.
I don't think that pressure vs boiling point is irrelevant. Doesn't it relate to the energy needed to make a surface molecule break free? If there is even a hint of a surface anywhere within the liquid bulk then it can form a bubble at a low enough pressure and any tension you might have had will not count. This argument would not prohibit dynamic tension, as long as the load is applied briefly enough.

But, if a normal lift pump will not operate  over a greater height than that which corresponds to atmospheric pressure for the liquid density, then how can an inverted u tube support a greater head?
I appreciate that, in a small bore tube, the effects of the tube surface could make a difference but, in a bulk liquid, what can keep a column above that which is supported by the AP difference?
The paper quoted above agrees with my point - it doesn't work for static pressures below the saturated vapour pressure.

[Andrew K Fletcher]

6 mil bore tubing was used. The reason the inverted tube works is the bead of water remains unbroken s0 at no point is water relying on adhesion to a surface other than the walls of the tube which does not affect the tension applied to the water inside it.
The water has no surface to be pulled from. Having a pump at the top would require the adhesive properties of water to stick to the diaphragm or piston, which in effect is the same as capping one end of a water filled tube and raising it vertically. Adhesion in water while very strong does not come close to the cohesive properties of water molecules.

The inverted tube worked as it was envisaged to do and supports the weight of two columns of water suspended vertically in the smooth bore tough nylon tubing.

The bead of water remains intact for a long time until water vapour forms into bubbles, these join together and cause the beads of water to separate. Now both levels fall back to the 10 meter limit at sea level and the space above is vacuum. As was the case in the original water pump problem that Galileo and Toricelli were faced with at the castle of the Grand Duke of Tuscany. The barometer was born later from trying to resolve this problem, but the problem to this date stands unless we change the parameters a little by using an inverted single open ended tube.

Andrew

That's fair enough.  I was trying to relate it to a possible 'tensile strength' so the number would be handy.
I don't think that pressure vs boiling point is irrelevant. Doesn't it relate to the energy needed to make a surface molecule break free? If there is even a hint of a surface anywhere within the liquid bulk then it can form a bubble at a low enough pressure and any tension you might have had will not count. This argument would not prohibit dynamic tension, as long as the load is applied briefly enough.

But, if a normal lift pump will not operate  over a greater height than that which corresponds to atmospheric pressure for the liquid density, then how can an inverted u tube support a greater head?
I appreciate that, in a small bore tube, the effects of the tube surface could make a difference but, in a bulk liquid, what can keep a column above that which is supported by the AP difference?
The paper quoted above agrees with my point - it doesn't work for static pressures below the saturated vapour pressure.

[lyner]

I wish you could tell me the difference between the top of a U tube and the top of  a single tube. Particularly if the tube had a 'domed' top.  Assume that the vertical height is greater than the 10m , conventional, limit.
The molecules need to stick to the top surface whether it's a U or just the top of the tube.
On the attached diagram, the region in the upper section of both the single vertical and the U tube are under exactly the same conditions. How is the water supposed to stick to the top of one yet not to the other? How do the molecules 'know' that they are in different bits of apparatus so that they can behave differently? A loop of string stuck to the top of either curve would pull away from the upper surface just as easily.  Whatever the tension may have been, the liquid would not 'stick' to the top any easier for either case.
Can't you see my problem?

[RD]

I wish you could tell me the difference between the top of a U tube and the top of a single tube...

With reference to my chain-in-a-bucket-on-a-table analogy...

If an end of the chain is lifted to the brim of the bucket and released it will fall back into the bucket.

If an end of the chain is pulled over the brim and out of the bucket and below the level of the table then released, it could set off a process which pulls the entire chain out of the bucket.

The top of a siphon's U tube is analogous the brim of the bucket, and the water, (which can have tension), analogous to the chain.

[lyner]

RD. All you have done is to repeat the 'analogy'. That is not an explanation in any sense. Of course what you say about a chain is correct. We can all measure the tensile strength of a chain; it's what we call a solid and the molecules are very well stuck together. If the tensile strength of water was as high then it would behave like a solid in many ways - it wouldn't flow, for a start. Or perhaps you would like to reclassify materials in some way.

Siphons don't work in a vacuum, in any case. You can't ignore the way a mercury barometer functions; it actually tells you about how the atmospheric pressure is acting. Without pressure pushing on the reservoir surface, the column collapses. Are you suggesting that a siphon would work at the same rate irrespective of the ambient atmospheric pressure? Is there something about the conditions in the tube leading up to a lift pump which is different from the conditions in the 'up' tube of a siphon? I ask again "How do the molecules 'know' what they are supposed to do in each case?" Do they know where they are? Can they ignore the atmospheric pressure in one case and yet totally rely on it in another case?
Dip your hand into a bowl of water. Can you draw up a thread of water by 'tension?

If you want to justify / explain how this phenomenon is going to work as you predict then you have to say, not only how these intermolecular tensions work to keep the column of water stuck together but what happens on the TOP surface / interface with the material of the tube?  Are the molecules stuck to that too? If not, why won't they drop off due to the low (negative?) hydrostatic pressure in that region and form a void? Forces act in three dimensions and all directions count.

You could, perhaps, also tell me what the limit to this effect could be. You could imagine making the top section tube wider and wider  until the width of the top channel was almost as great as the total height of the loop. Your 'tension' should then also be present. When would the effect stop? At the very last instant when the lower face of the horizontal section dips under the level of the water in the reservoir?

AKF, at least, has seen a phenomenon which needs some explanation and which doesn't confirm normal textbook models. All that you are doing, RD, is to assert a naive idea which is not in accord with the overwhelming opinion and a huge body of evidence.

BTW, there is a great little demonstration to show that air pressure is needed in order for water to be 'drawn' up a tube. If you try to suck up a drink through a straw when an airtight lid (sealed round the straw too, of course) is put on the beaker. If you didn't need atmospheric pressure, then you could suck all the water up with no problem. Try it. Try getting your outboard / lawn mower motor to operate without undoing the breather into the fuel tank. Air has to get in or the reduced pressure will soon stop liquid flowing.

[lyner]

AFK. Re your Brixham experiment. Did you, at the same time, do a control experiment in which you lifted a single sealed, water-filled tube up to the same height? It would be interesting to have compared the two situations.

[Andrew K Fletcher]

Sophiecentaur

Your domed tube is still a capped end tube. This still relies on the ability of water molecules to adhere to the dome. The weight of the column of water will tear it away at the 10 meter limit. There is no point repeating what is known.

There is adhesion to the top of the U tube, but it equates to the same adhesion the water has on all of the internal wall of the tube. We know that if the top end is open the water flows freely out so adhesion to the walls of the tube is not required to hold the water up. However adhesion is important to prevent the water from necking as it is placed under tension. in that I mean water molecules being pulled away from the inside of the tube so that vacuum can replace it. Adhesion here is important. What we have with the inverted U tube is water molecules cohesively joined to water molecules in an unbroken column of water from one vessel to another.

If we raise our water filled tubes open ends from the two vessels while the tube is suspended vertically the water in both sides appears to be elasticised and rises up both sides equally.

Now the water could flow out one side or the other as one would expect based on current thinking about liquids. Yet it does not! It remains suspended because the water has been stretched down both sides held only by the link to other water molecules.

If we then blow up one side to increase the pressure you would expect the water to flow out the other side immediately. It does not! The water level on the side you increase the pressure rises but not enough to tip the balance and cause water to empty from the other side.

These experiments have been repeated many times, even at greater heights than the 24 metres at Brixham.

If you would like me to bring a demonstration to your school I would be delighted to do so. Or you could phone Mr Smith (head of science at Paignton Community College and ask him what he thought about water flowing up to the top floor of the college. Mr Smith added; “I have no problem whatsoever with this experiment. This is exactly how trees lift water, but what can I do about it? I still have to teach the curriculum!”

To understand the experiment one needs to abandon pre conceptions about how water behaves under tension and think of it as stretching and contracting.

The video link about the ocean circulation was readily dismissed by yourself. That video shows clearly how a sinking denser ocean water can cause a dragging effect on water from the equator pulling up warm less dense water thousands of miles.

This is highly significant in understanding the U tube experiment. The same downward flowing salt in the one side causes the dilute fluid to flow up the other side. Nothing to do with pressure either. We can eliminate air / atmospheric pressure in the closed loop experiment.
 

I wish you could tell me the difference between the top of a U tube and the top of  a single tube. Particularly if the tube had a 'domed' top.  Assume that the vertical height is greater than the 10m , conventional, limit.
The molecules need to stick to the top surface whether it's a U or just the top of the tube.
On the attached diagram, the region in the upper section of both the single vertical and the U tube are under exactly the same conditions. How is the water supposed to stick to the top of one yet not to the other? How do the molecules 'know' that they are in different bits of apparatus so that they can behave differently? A loop of string stuck to the top of either curve would pull away from the upper surface just as easily.  Whatever the tension may have been, the liquid would not 'stick' to the top any easier for either case.
Can't you see my problem?

[BenV]

I still don't see how your U tube experiment relates to how water is pulled up a tree - a tree is not a closed system - we know that water transpires from the leaves.  Fair enough, this would increase the solutes in the leaves, which would then flow back down, but as it's not closed, it can't be the mechanism you suggest.  Would your tube experiment still work if you put holes in the top of the tube, out of which some water could evaporate?

[Andrew K Fletcher]

Ben
This flow and return system does not need any tubes to operate. The experiment was to show how strong the tension is and how efficient the density flow system is, not to represent a perfect mechanical tree.

If my experiment was inside a sleeve filled with water to represent an outer bark, then yes we could have pores in the top of the tube and yes it would cause water to exude at the top as the downard flow would drive the dilute fluid higher as shown in the video exp on youtube.

In fact as I said before Strasburger's experiment used an actual tree that had been killed by picric acid and showed circulation continuing for weeks after the tree had died, eliminating any living processes involved with transport. Killing the tree inevitably liberated solutes and this trickle down of salts from top to bottom maintained the circulation, generating the suction at the base of the trunk submerged in the tub of acid required to draw in the fluid. acid exuded from the dead branches for several weeks and this can also be addressed by using the U spirit level demonstration again on Youtube.

[lyner]

The problem with this thread is that it involves three or more  topics which have been drawn together in order to 'prove' something. In reality, they should be discussed separately  as they do not necessarily support each other.
Much of the previous post (the reply to my post, that is) is fine BUT. You still do not say how the molecules (I assume you subscribe to molecular theory) behave.
Pressure in a fluid acts equally in all directions  (I think you will agree). There are forces between the water molecules and the inner surface of the tube. If there were not, then there would be nothing to keep them in contact when there was low / negative pressure in the tube.  Take a piece of dough, say, and stretch it. The sides move in as the length increases. That is the effect of tension. If you wanted to oppose these forces, then you would have to stick the sides to the inside of a tube - with some other forces. If your column of water is to stay at the same width (i.e. fitting the inside of the tube) there must be forces keeping the surface of the water in contact with the tube wall. These can either be due to pressure (pushing)  or adhesion (pulling) but they have to exist. You have already said that there must be negative pressure because it is at a height greater than 10m so the forces must be adhesive. These forces must exist whether the tube is single or a U.

The temporary tensile strength of an unbroken volume of water is one issue and it has been shown and measured. The conditions for it to work are very critical and it is not a common phenomenon to observe because, once the saturated vapor pressure is reached, evaporation can occur and cause bubbles. That's fine.
BUT you need to do much better than you have done if you want to explain the totality of your experiments in the terms you have done so far.
You surely agree that we need any explanations to be as consistent as possible over all situations and that the behaviour of a substance depends upon local molecular conditions.  Liquids can exist as liquids even when their temperature is above the normal boiling point - well known. That situation breaks down as soon as there is a nucleus around which a bubble can form. Well known.
The situation of a molecule next to the wall of a tube of any shape, under the same pressure conditions must be the same. How can you dispute that? If you cannot tell me how the situations are different then how can your explanation be satisfactory?  I take it that you did not try the experiment with a single tube? You 'assumed' the outcome of that experiment?

The video link about the ocean circulation was readily dismissed by yourself. That video shows clearly how a sinking denser ocean water can cause a dragging effect on water from the equator pulling up warm less dense water thousands of miles.

Your argument would also imply that it is the hot air 'pulling' cold air in underneath it that causes convection - i.e that even gases can exhibit tension. The more sustainable explanation is that  the more dense, cooler air displaces the warmer, less dense air due to pressure.
Why do you need tension to explain the ocean circulation when all you need is to realise that there is a bit of excess pressure (the more dense stuff sinks and PUSHES the less dense stuff upwards)? You seem to be confusing cause and effect, here.

Benv's comments make good sense. The structure of the equivalent 'tube' in a tree is very different. Not only is there some venting at the top but the Xylem  'tubes' are much thinner than your plastic tubing. Furthermore, they are very irregular and they have connections with the plant all the way up via  perforations.  It is surely the extremely small size of the tubes which gives a clue as to how they work. Adhesive forces between water and many substances are stronger than cohesive forces between water molecules. Narrow tubes (capillary) in the Xylem make use of this difference.

[lyner]

For anyone who doesn't know exactly 'how trees work' or has their own ideas about it, I suggest you take a look at this link. It is a Google Book review and does not show all the pages but there is plenty of evidence which you can read of a well thought out bit of Science which takes the magic out of the mechanisms used by plants to raise water.
The main point about it, as far as I am concerned, is that it depends upon the Xylem tubes being extremely thin. Cavitation is always a problem and can stop the process.

The mechanism does not rely on 'flow' or inverted U tubes. It is described and explained in terms which make sense and do not go against any established ideas.

[Andrew K Fletcher]

A nice sweeping blanket statement as per usual. For a start, you do not know exactly how trees work!

Cavitation is not a problem for trees and does not stop the process, because every single xylem will cavitate and repair the cavitation!

It is not discribed in terms which make sense. Why do you think there is as much debate on the acent of sap in tall trees as there has ever been? Just because it is in the curriculum does not make it correct!

BTW: Didn't assume anything. Tried a T junction at the top of the U tube and found it failed at 10 metres as the junction simply caused the water to fall rapidly to the 10 metres and vacuum above it as expected. Thought I could have got away with it. However, even with the valve closed the join caused a seed point for the water to boil.

It is possible that there is a delay between a sink and a return flow that could be measured to prove whether it's tension or pressure changes that causes the Atlantic conveyor to function the way it does.

Gas could be tested in the same inverted U tube to determine if it also has a tension added to it. Something I have thought about doing with Co2 due to it’s weight. If the gas flows out of the U tube then you are correct. What do you think should happen if we test Co2 in this way? BTW, I am not sure what will happen with gas either so would rather wait to see what happens. Though the atmospheric pressure will equalise at the same height as the water so we would need a much higher U tube to compensate for this. Interesting.

Andrew K Fletcher

For anyone who doesn't know exactly 'how trees work' or has their own ideas about it, I suggest you take a look at this link. It is a Google Book review and does not show all the pages but there is plenty of evidence which you can read of a well thought out bit of Science which takes the magic out of the mechanisms used by plants to raise water.
The main point about it, as far as I am concerned, is that it depends upon the Xylem tubes being extremely thin. Cavitation is always a problem and can stop the process.

The mechanism does not rely on 'flow' or inverted U tubes. It is described and explained in terms which make sense and do not go against any established ideas.

[lyner]

I am, basically, a reductionist. If there is a theory which explains a phenomenon and it doesn't involve needlessly new complications then I tend to find it acceptable. Science, in general, looks to explain the World with a minimum of 'laws'.
The book in that link manages to give explanations for the phenomena involved with tree sap movement which don't need to introduce any new 'fanciful' ideas. Actual numerical values are quoted and that always reassures me that someone knows what they are talking about. The effect of adhesion and cohesion, taken together is considered and there is a very reasoned discussion of the actual forces involved and the requirement for tubes of the sort of size that Xylem uses. No magic and nothing actually new - just an intelligent approach which uses values drawn from elsewhere in Science.

I mention cavitation because that is something which couldn't be dealt with if the cavities were large.

I did not suggest a T piece should be added; it would obviously be a nucleus for local boiling. I suggested a smooth termination to the top of the tube. Your use of an unbroken length of u tube gave you the best chance of a smooth internal surface in the higher sections. Your explanation that you need tension all round the loop is not the only one which explains the phenomenon.
Yet again, I ask you to tell me the difference in the situation for my two molecules at the top of the two tubes. If you can't either assure me that you did the experiment I suggested or come up with some adequate theory about my two molecules then your explanation (being the 'new' one) is not proven.
As far as I am concerned, there is no need to use your 'cohesion - not- adhesion' idea until you prove it. Conventional arguments about tree sap  use adhesion and cohesion and make good sense.

Measuring the delay in the Atlantic Conveyor could be difficult; the time involved is over 1000 years. Why do you want to find tension everywhere? Pressure differences explain all these phenomena - even the Brixham experiments (the indoor and the outdoor ones) where the two ends are exposed to the atmosphere and one column just happens to be heavier than the other. If you adjusted the heights of the two reservoirs, you could stop the flow or even reverse it. The pressure at the bottom of a column of fluid is ρgh (h is height and ρ is density) If ρ is greater then h will be smaller for the same pressure.

It is not surprising that there is still some 'debate' about the tree sap thing; it is not straightforward, there is a sort of 'magic' about it because it is counter intuitive, in many ways and it is the sort of topic beloved of fringe Science.

There is also a lot of aimless debate about Income Tax, personal health and the Moon Landing 'conspiracy'. It doesn't meant that all views are equally valid.

Have you heard of the Van der Vaal forces? They apply in gases and in liquids. They modify the gas laws, particularly at high pressure and are caused by the asymmetry of charges around electrically neutral molecules. What sort of experiment did you have in mind which could reveal gas tension? Gases are usually only too happy to expand as much as you let them. They have already boiled.

[Andrew K Fletcher]

I am, basically, a reductionist. If there is a theory which explains a phenomenon and it doesn't involve needlessly new complications then I tend to find it acceptable. Science, in general, looks to explain the World with a minimum of 'laws'.
The book in that link manages to give explanations for the phenomena involved with tree sap movement which don't need to introduce any new 'fanciful' ideas. Actual numerical values are quoted and that always reassures me that someone knows what they are talking about. The effect of adhesion and cohesion, taken together is considered and there is a very reasoned discussion of the actual forces involved and the requirement for tubes of the sort of size that Xylem uses. No magic and nothing actually new - just an intelligent approach which uses values drawn from elsewhere in Science.

1st Paragraph beginning with the word Basically adds nothing intelligible.

I mention cavitation because that is something which couldn't be dealt with if the cavities were large.

2nd paragraph on the other hand admits there is one big problem for the cohesion tension theory as it stands today. Cavitations. Yes, these tiny bubbles have a habit of forming bigger ones and when they form in the fine xylem tubes, which incidentally are much larger than capillary tubes used experimentally to show capillary action, fail to function because of the same 10 metre limit shown in the barometer and Water pump puzzle: Galileo, Toricelli.

This completely destroys the tension theory as it stands. Another problem is and I have said it time and time again, there are 40 metre trees here in Devon, Larch and some Ash-growing in close proximity to each other that have few branches on the top and very few leaves, yet obviously are capable of surviving for many years. The argument for the cohesion tension theory is that the collective loss of moisture from the leaves causes more water to be pulled up and out of the tree. How pathetic is that?

Capillary action. Well is the tree towering a hundred metres able to cause water to be soaked up through the massive trunk of a Californian Redwood at 5 thousand litres a day? I don’t think so. If this were the case then rising damp in walls would do the same and it does not.

Root pressure: Don’t even get me started on this one.

And then we have Strasburger’s experiments which you failed to address in my previous post. Just in case you missed it. Here it goes again. Take one large tree. Suspend it over a bath of picric acid after it has been recently removed from the soil. While submerged in the toxic soup, saw off the roots, or indeed leave them intact. The acid rises and kills all the living processes in the tree. Now for some 3 weeks or more the tree continues to draw acid up and transpiration continues, even though for all intensive purposes the tree is a skeleton.

Now somewhere I think you mentioned a straw analogy. Take one straw, say 10 metres or more tall. Suck as much as your lungs can bare to suck and se if water rises and flows out the end. But this is not quite like the leaves and branches on the tree is it? As you pointed out in other posts my experiments are too simple to represent a tree. (which incidentally was never my intention) But let us make our straw we are frantically sucking on more like a tree. The leaf for example has pores in it that allow gas to escape. So we stab a few holes in it. Some are closed and some are open, so like a flute we place a few fingers on some of the holes and suck. Do we suck air in or do we suck water up when we couldn’t even suck water up without the holes in it?

Now let us look at the barometer. Here we have a glass tube as you suggested we should try. Mercury has been added instead of water so that the it can be scaled down, after all mercury is a liquid much denser than water and it was good enough for Toricelli and the barometer, which incidentally was a happy accident while trying to sort out the age old 10 metre limit problem, has indeed already got the rounded end you suggested should be tried with water. Let us not forget also that Galileo used a longer tube with water. You fail to accept that what this brilliant man concluded about his own experiments and your suggestion was indeed correct in a single suspended vertical capped tube. The barometer does not show mercury adhering to the top of the tube but shows vacuum above it. Toricelli was trying to recreate the pump problem using this set up and failed, but found the reason why it fails and there was born the barometer.

Now for your analogy of lowering one vessel to initiate a siphon.

Our suspended U tube filled with water, ignore the salt for a minute.

We lower one vessel. Remember the observed elastic properties of water I mentioned in a previous post, again ignored by yourself? Well all that is going to happen is that the water will stretch until it breaks. Note you will not pull the water up the other side. Yes I have tested it.

And no there is no “sort of magic” about how trees lift water just a lot of stupid people that can’t see the wood for the trees.

Andrew K Fletcher

[Andrew K Fletcher]

----- Original Message -----
  From: Andrew K Fletcher
  Sent: Friday, June 02, 2006 2:15 PM
  Subject: [IAWA Forum] More questions on Circulation within a tree.



  A while ago, the question of density changes in residual leaf and
branch fluids as a direct result of the efficient transpiration from the
leaves of a tree was put to the group. Judging from the responses which
were also posted at the request of one of the groups members, it would
be fair to deduce that there is a general acceptance that density
changes would be an inevitable consequence of the evaporation of 98% of
the water from the leaves.

  several members also began to question what would happen to the sap
once the density had increased and indeed it was suggested that it would
be acted upon by gravity and that the sap would be moved as a result of
this interaction with gravity.

  This brings me to the next part of this important question for the
group.

  Explaining the results of Eduard Strasburger's experiment
  Andrew K Fletcher
  Evaporation from the leaves alters the density of the sap at the leaf,
and gravity pulls the denser sap down. This generates a positive
pressure in front of the falling sap, and a tension / negative pressure
behind the falling sap, which initiates a simple flow and return, much
the same as found in a simple flow and return domestic central heating
system, where the heat from the boiler alters the density of the water
causing the heated water to rise, where it is cooled inside the hot
water tank via a coiled copper tube, returning the cooled water back to
the boiler.
  The German botanist Eduard Strasburger's famous experiment - where he
killed all of the cells in a tree by cutting off the roots, while
submerged in a bath of picric acid - demonstrated that transpiration and
circulation was maintained for three weeks, after the death of the tree.

  I put it to the group that either the picric acid or the copper
sulphate solution used by Strasburger, caused the minerals and sugars
held within the dying leaves and branches to be released over the 2
weeks and that this was all that would be required for a simple flow and
return system to maintain the circulation and transpiration.
Furthermore, the experiment does suggest that no living process need be
involved in the bulk flow of a tree.

  This would result in a downward flow caused by the liberated solutes
and this would in turn generated suction at the base of the tree
sufficient to draw in more dilute solution from the bath, and that this
flow would continue until the liberated salts and sugars had either all
reached the picric acid / copper sulphate bath, or that the liberated
salts and sugars had changed the density of the fluid within the tub to
counterbalance any falling solutes.

  Andrew K Fletcher, UK

http://4e.plantphys.net/article.php?ch=4&id=98

Strasburger himself was an adherent of the school of physics and provided some strikingly efficient demonstrations of water being lifted to considerable heights without any involvement of living cells (Figure 1). He showed that woody stems with their lower end immersed in concentrated solutions of copper sulfate or picric acid and severed by a cut made below the surface of the liquid, will readily suck the solution up. Immediately upon contact, the poisonous fluid kills all living cells in its way, but the copper or the acid arrive in the transpiring leaves and kill them as well. The uptake of the solution and the loss of water from the dead leaves may continue for several weeks, and new solutions of a different color may be lifted in a dead stem.