How do Trees Really lift Water to their Leaves?

Reducing the pressure at the leaf would undoubtedly assist gas exchange. tension = reduced pressure = efficient gas exchange Andrew

Lyner:
What, still peddling this 'tension' idea? Where does it come from? Why doesn't it apply all the time?

Pay attention to the varicose vein study and learn how solutes change pressures and apply tension to liquids. Cohesion Tension Theory mean anything to you?

That's surface tension - not internal tension. It's a feature of the asymmetry of forces at an interface - as you know, it is a very small force.
If it exists, it is amazing that it hasn't been used in some useful machine.
We've been here before but all of those examples work under positive pressure and their operation is totally explicable in terms of pressure differences.
I would love it if you could explain to us how a single molecule, which knows nothing about its surroundings apart from its nearest neighbours, would know whether it was in a loop of tube or just an inverted tube? Why should it behave differently?

Andrew:
If they work under positive pressure then how the hell does a tube get pulled in? Stop thinking of water as water, start thinking of it as a solid. When the solutes flow down one side they apply tension to all of the molecules in the upward flowing side and a compressive force to the water molecules in front of the falling solutes. The tension generated causes the return flow tube to neck indicating that the molecules are pulling on the inside of the tubular wall while at the same time they in turn are being dragged upwards.

If the water was in the ocean rather than in a closed loop of tubing then the water molecules would still pull on all of the other molecules and initiate circulation and as there are no walls to pull on we do not see the same effects, yet the same affects apply and can be shown in tidal changes dragging water from one side of the planet to the other as the moon interacts with the earth's gravity. Same applies when pouring fuel into a car from a funnelled can. First the fluid flows slowly then it speeds up as the fuel flowing into the tank drags more fuel out of the can.

www.pubmedcentral.nih.gov/articlerender.fcgi?artid=59218

The current controversy about the “cohesion-tension” of water ascent in plants arises from the recent cryo-scanning electron microscopy (cryo-SEM) observations of xylem vessels content by Canny and coworkers (1995). On the basis of these observations it has been claimed that vessels were emptying and refilling during active transpiration in direct contradiction to the previous theory. In this study we compared the cryo-SEM data with the standard hydraulic approach on walnut (Juglans regia) petioles. The results of the two techniques were in clear conflict and could not both be right. Cryo-SEM observations of walnut petioles frozen intact on the tree in a bath of liquid nitrogen (LN2) suggested that vessel cavitation was occurring and reversing itself on a diurnal basis. Up to 30% of the vessels were embolized at midday. In contrast, the percentage of loss of hydraulic conductance (PLC) of excised petiole segments remained close to 0% throughout the day. To find out which technique was erroneous we first analyzed the possibility that PLC values were rapidly returned to zero when the xylem pressures were released. We used the centrifugal force to measure the xylem conductance of petiole segments exposed to very negative pressures and established the relevance of this technique. We then analyzed the possibility that vessels were becoming partially air-filled when exposed to LN2. Cryo-SEM observations of petiole segments frozen shortly after their xylem pressure was returned to atmospheric values agreed entirely with the PLC values. We confirmed, with water-filled capillary tubes exposed to a large centrifugal force, that it was not possible to freeze intact their content with LN2. We concluded that partially air-filled conduits were artifacts of the cryo-SEM technique in our study. We believe that the cryo-SEM observations published recently should probably be reconsidered in the light of our results before they may be used as arguments against the cohesion-tension theory.

Concretions found walking along the river dart Estuary at low tide. The dead tree trunk and roots were above the high tide water level and surrounded by living trees that did not exhibit these mineral deposits. I suspect the high level of salt and minerals deposited on the roots eventually killed the tree.

Very similar to lime scale deposits found on urinals probably having more in common with solutes excreted from urine. In an estuary the salinity of water varies from sea water to brackish water depending upon the flow of the river water mixing with either incoming salt water from the sea or outgoing less salty water as the tide turns. The trees ability to draw this water into it’s system presents the tree with a toxic overload. In the case of the kidneys, filtration from the blood increases the density of minerals in the urine, which builds up over time on urinals that are inadequately cleaned forming very similar concretions to those pictured on this tree.
A question that arises from this observation is why are the living trees not affected by the same concretions as the one that died. Clearly this cannot be a sedimentary deposit coating the tree from the outside but must be a concretion that is excreted from the root and trunk, otherwise other trees on the same bank around the same age would also exhibit the same characteristics as those pictured.





This is a reply to Blaine's post about www.thenakedscientists.com/forum/index.php?topic=14146.0

Andrew K Fletcher

B.C.
Just a thought re the trees in the post abvove: If only the dead trees roots are covered in mineral deposits could it be that the causation runs the other way. It's not the minerals that killed the tree, but the deadness of the tree that caused the mineralisation. For example, could bacteria feeding on the dead tree's roots have excreted something that precipitated the minerals?
For example, CO2 might be a potential reason for the mineralisation.

Good point BC there is a possibility that the death of the tree caused the concretions. The remaining trunk and roots were coated in the same mineral deposits. I suspect not as a bacterial process, although this also could be worth investigating, but as a result of gravity and decay causing the migration of solutes back towards the roots. We discussed this possibility in another thread relating to fossilized tree trunks found in deserts and volcanic locations where the trunk has turned to stone. Polished these reveal the ring structure of the tree and are used for ornamental and scientific interests.

This is what drew my attention to this particular trunk while walking along the estuary in Dartmouth. It may also have been a combination of the trees equivalent blocked arteries and the resulting decay that caused the solutes to exude from the trunk and form the concretions.

Either way, Gravity plays an important roll in both the trees survival and it’s inevitable death.

A 4 metre loop of soft walled latex or silicon tubing filled with clean water, add a little salt solution and close the loop using a union. Invert the tube and the salt flows down one side causing it to bulge visibly, while the opposite side of the vertically suspended loop is pulled in. As for a useful machine using it, it may interest you to know that our own organic machine we call the body uses it to great effect. So does another organic machine called a tree. And then there’s the massive machine called the Atlantic Conveyor system that drives the ocean currents, regulates the temperatures and powers the worlds weather, and is believed to have caused the last ice age and many others.
Quote from: sophiecentaur on 19/07/2008 23:34:46

That's surface tension - not internal tension. It's a feature of the asymmetry of forces at an interface - as you know, it is a very small force.
If it exists, it is amazing that it hasn't been used in some useful machine.

On the siphon thread. I provided you with an introduction to the spinning Z tube, designed to test the cohesion in water. There were measurements there. This is what Hammel referred to in his letter to me. You dispute the necessity for a U tube adding that a single capped tube would enable the water to remain in the tube over the 10 meter mark. I provided you with a very early attempt to do this. This was rejected because you said the pipe used was lead. Presumably then we need to alter Galileo’s Limit to using lead pipes only?

Now, why do you suppose that spinning a Z tube was preferred? Why not spin a capped ended tube? Why not spin a straight length of tube?
Both elbows in the Z tube provide the same stress on water as my experiment.

Your meniscus does not represent the strength of adhesion any more than it represents the strength of cohesion, it tells us nothing except that gravity is pulling the water down and some of the water sticks to the inside of a vessel.

Adhesion is very important for your single capped / domed ended vertical water filled pipe to work, and yes I understand why you would require adhesion to be stronger in this case. But the fact remains that no one to date has observed this. If we look around in nature for examples, do we see upright capped tubes or do we see circulation involving a flow and return system? Circulation being circular. In the body we see arterial flow and a return venous flow. In the tree we see phloem and xylem flow. In the ocean we see the conveyor system etc etc. Chop a carrot in half and we see circular patterns. We do not see a tree open at the top and bottom that is able to pull water from the soil like a giant straw and here lies the problem with science having a go at explaining circulation in a mono-directional paper, when in reality the fluids in the tree circulate.

I have seen trees 30 metres high, mostly larch with very few leaves and branches at the top, living by drawing water from root to soil. I can photograph these if required because they destroy the cohesion tension theory, yet according to the circulation theory, little in the way of evaporation would be required for it to continue and more to the point the required tension on the sap for the current cohesion tension to work has never been observed. Despite many attempts.


Summary
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).

Received: 16 October 2003 Accepted: 30 January 2004


www3.interscience.wiley.com/journal/1187...ct?CRETRY=1&SRETRY=0

Lyner:
There is no point in discussing the siphon phenomenon any more because your appreciation of it is incomplete and you don't seem to want to advance your knowledge at all in that direction.
Yes, trees are wonderful. Yes, there have been experiments on cohesion. But your explanation is what lacks rigour and depth.
I have no quarrel with your external references because they talk sense and do not neglect the effects of adhesion. I can only conclude that you don't understand all they say.
I can't take you seriously if you can't even discuss the water meniscus in normal terms. Is that piece of theory all wrong as well?

lacking rigour and depth as you put it could also equate to keeping it simple. Investigating how this flow and return system fits with human, animal, tree and plant physiology has been trully fascinating. Observing a recovery from cerebral palsy where a girl gets out of a wheelchair and climbs stairs in school after 12 years of being unable to walk: Priceless