Wednesday, 5 October 2016

How does water get to the top of a large tree? An alternative theory

Alan Warwick’s insight 

Although this blue eucalyptus on Hawaii barely achieves fifty metres another eucalyptus, Eucalyptus regnans can achieve a hundred metres
It is quite remarkable that water can rise to the top of a large tree. There would seem to be a limit to how high it can go. The world’s tallest trees is a coastal redwood Sequoia sempervirens which has achieved 116 metres. Some scientists have suggested that 120 metres is the theoretical limit to the height of a tree.

Imagine a hundred meter sports track, turn it vertical and add some more
David Attenborough did a TV programme from up in the tree tops. Apparently he illustrated nature’s achievement by pumping up hundreds of gallons of water. A huge red fire engine was parked at the bottom. Large trees raise water this high for themselves every day. 

The tallest sequoia in the UK - in Wales - is a mere forty metres. Introduced in about 1850 we do know not high it will go! I took this picture in Oxford 
Alan Warwick was a mature student at Askham Bryan College thirty years ago. As a retired ICI executive and PhD chemist he was a joy and a challenge. It was the one year certificate craft course in horticulture. Many great gardeners and horticulturists started their careers this way. Alan now tells me that he then knew nothing about gardening but had a vision of retiring to a garden and small holding. He now spends all and every day in his horticulturally fascinating acre. That’s when he is not sunning himself for two months every midwinter at his son’s in Australia! Now at 87 it is about time to tell his story.

Philip Orton was giving his botany lecture about water movement in plants. He explained the still current theory that water is pulled upwards in plants driven by the forces of leaf transpiration as they suck up water that evaporates from the leaf surface. The key element of this theory is that a thin tube of water molecules adhere together with considerable force. Whole columns of water are dragged up from the ground.
Alan went quietly apoplectic and had a mild  and discrete tantrum (give me a little poetic licence). At the end of the lecture he took Philip to one side. Absolutely impossible! Only botanists could believe such nonsense. Any self respecting physicist would debunk it.
As an industrial chemist Alan had experience of designing industrial solutions to carrying great quantities of liquid to very large heights. He immediately offered what he thought to be a much better explanation - the Pholé lift phenomenon. He has maintained this belief for the last thirty years.
Pohlé is a patented lifting system for fluids
I met Alan again recently at a social event. I later stopped by at his garden. It’s absolutely fantastic. I offered to use my blog as a public platform for his theory. I don’t expect my readers to judge an unorthodox theory any more than I understand it myself. Never-the-less the power of the internet is huge. Who knows who might read this from a Google search! 
Alan says he does not expect to live long enough for the botanists to be shown to be wrong!
You can read the modern synthesis of tree water uptake in this series of articles in the Scientific American

Alan Warwick

Alan now spends his time not digging and winning prizes for his vegetables and flowers
He was a model Askham Bryan student. I remember on one occasion when I mentioned chalk in my lecture about lime and a student asked me whether chalk was toxic. I flamboyantly raised a great cloud as I dusted the board and declared that it did me no harm (some hope of that). Later Alan took me on one side. No, my blackboard chalk is Calcium sulphate not Calcium carbonate, horticultural lime.
Alan actually listened to my lectures! In a throwaway line I once said that a greenhouse receives sufficient water in a year that if you could collect it all it would be enough to irrigate the plants within throughout the same year. He later put this into practice ! He purchased huge used dye vats from ICI for 'half a crown', transferred them home on the roof of his car. They are still part buried alongside the length of his very large greenhouse and they work very well.                                                                                                                                                                                                                                                                                     

Plenty of water here to irrigate his tomatoes

His cacti need rather less water and he has plenty to spare
He has been applying farmyard manure to the top of his heavy clay soil for the last thirty years with no cultivation. It is in superb condition and he wins all the prizes at local vegetable  shows.

Pohlé lift theory

Alan explained to me it is the same force that when chemistry school boys using pipettes get an air bubble in the tube and get a mouth full of chemical - in the good old days! As a former victim that represents a measure my ability in this area and my competence to judge Alan’s theory. 
A further example is when you open a glass of fizzy tonic and water overflows everywhere. When you pour your morning champagne it is what makes the glass spill over.
It is the force that is released when a dissolved gas is released from pressure.
Note that although I mention bubbles this does not mean that they occur in a plant stem! There are no bubbles in a bottle of champagne until it is opened or shaken.

More about the source of upward force
A plant exerts root pressure as a result of osmosis. It is a not inconsiderable two bars strength - don’t ask. According to Alan’s adaptation of pholé theory the sap as it rises is released from pressure and dissolved air is released and propels upward movement.
Although root water contains dissolved and hugely significant oxygen Alan considers there will be greater amounts of dissolved and more-soluble nitrogen. Nitrogen release is the same when an ascending diver gets the bends.
Alan’s personal experience of pohlé is when at ICI they designed equipment to elevate large quantities of fluid in an industrial process. Twenty gallons per minute were continuously lifted about eighty feet high. The elevating force was achieved by introducing compressed air at the base of the ‘tube’.
Alan says that he could use pohlé lift to force fluid in a capillary tube to the height of St Paul’s Cathedral!

Alan explains that in a plant there will be relatively gentle upward pressures from root pressure, capillary rise and pull from replacement of evaporating liquid. As the rising sap column proceeds the pressure will reduce and increasing amounts of gas released will speed upward movement taking water a very long way. Alan claims that if anyone measured internal pressure at intervals up the height of a very tall tree the pressure would gradually reduce and provide evidence for his theory.

Alan’s efforts to promote his ideas
If I might dare say so he was a little naive. He first wrote to David Attenborough who was very interested and in a hand written letter which Alan used to cherish suggested he approach the RHS! The RHS have an excellent science department that provides excellent advice to amateurs. It is not in their brief or inclination to promote new botanical theory. I do absolve their scientists of the dodgy science frequently purveyed in the RHS journal. I think Alan submitted to the Open University too. Again the wrong place.
Rejected, Alan took his bat home. But the flame has continued to burn


My own competence only enables me to throw a few straws into the wind.
Several forces are involved and work together to power the flow of water in plants. Root pressure is derived from the osmotic pressure generated in the root and is driven largely as a result of the pull across root membranes from dissolved sugars. Capillary rise and surface tension move water into small spaces. The fabled capillary rise is unable to pull water beyond thirty foot high. And even that could only be produced from a laboratory vacuum and forces that would collapse plant cells. In actual practical botanical terms capillary rise is piddling.
And of course in the standard theory the energy comes from the power of sunshine and the pull from the leaves as water evaporates. This Alan disputes and does not accept that water molecules hold sufficiently together

The well recognised phenomenon of root pressure is actually very significant and tends to be underrated in some text books. Any gardener knows sap’s exuberant rise in a tree trunk and branches in Spring. Wounded branches spill sap from up to thirty foot high. On deciduous trees there are not yet leaves to generate the botanists’ proposed pull. I wonder if any arboricultural readers can tell me whether such bleeding goes higher. Last year I watched my pruned leafless vine bleed profusely at ten foot high for more than a day.

I wonder how the alleged pull from the leaves starts up in Spring on very tall deciduous trees?

The physical effect of gas released from pressure is a very real phenomenon. One wonders when one considers a plant’s plumbing how it could not have an effect in a stem!
I am a great believer in evolution’s ability to exploit any available solution to a ‘problem’. If the pholé lift is real and it is possible to harness it nature would have found a way.

As usual when I have a scientific problem I turn to botanist Peter Williams. He is something of an expert on water movement in plants and is fascinated by it. He ponders about the sideways movement of water in a tree trunk. He is quite comfortable with current theory - but then as Alan would say botanists are.
Peter marvels at the surging rise of sap up a tree’s xylem and reminded me that xylem is a tube of dead tissue. (He of course accepts this as a simplification). He discussed the problems of living cells extracting water from the flow. His analogy was dipping into a surging river to extract water.

In a later exquisite explanation of the air and water pressures involved - I barely understood - Peter happened to mention that sometimes an air bubble breaks the continuous flow in an ascending xylem column and the flow interrupts with an audible plop. He discerned my face falling. This took the wind out of my attempt to bring together both theories. In conciliation he has sent me these beautiful pictures of guttation!

Guttation takes place through specialist cells

It is the way that certain plants reduce internal pressures when it is too humid for speedy evaporation
Many gardeners think they have a problem.They have not although the sap can be a bit messy
Perhaps I should take Peter to see Alan’s wonderful garden and see the sparks fly.

The worlds tallest trees are all evergreens and are almost all conifers. Huge(evergreen) eucalyptus is a principle exception. As you climb higher the leaves on extremely tall trees get very much smaller.

Alan’s parting shot. If the cohesion theory is correct then the pressure at the base of a  hundred metre tree would be about ten bars. You would need very thick stainless steel containers to contain such pressure
And his theory is not about bubbles in plants! (In this respect I misinformed Peter!)

My parting shot. The world’s tallest tree Sequoia sempervirens is thought to be an ancient hybrid between metasequoia and sequoia. A wonderful example of hybrid vigour

Final statements
As ever when I have finished writing further information is forthcoming. I present it as final statements for the two sides of the discussion

Peter Williams
Water simply moves from the soil to the air, via the plant because of a gradient of water potential and the existence of a continuous body of liquid water from the surface of soil particles to the cells lining the sub-stomatal cavities in the leaves.    
Bubbles are not needed to provide a ‘helping upward push’ because the suction generated by evaporation in the leaf  is quite enough on its own.  The air has enormous drying potential as we all know from the way clothes dry on the washing line, and water evaporation from puddles, or from the sea to form clouds in weather systems.  The air is usually so dry that it ‘sucks’ water out of the leaves of even the tallest trees.  To quantify this a little, we need to consider the units of water potential. The units are usually quoted in megapascals (MPa) and 1 MPa is equivalent to 10 bars (or 10 atmospheres in obsolete terminology). 

When air is totally saturated with water vapour, its drying potential is zero. There is no suction and water cannot flow. 
When the air is 99% saturated however- very humid indeed - it has a drying potential equivalent to 1.3 MPa, and on a typical sunny day when the % saturation might be 40%, the drying potential may be well over 100 MPa or 1000 bars!
Water will only leave the soil and enter a plant if the water potential of the plant is lower than the soil.  When soils are very dry after periods of drought, their water potential may be as low as the air on a sunny day and so water does not enter the plant and it experiences severe water stress and wilts.   Usually however, the water potential of the air is much lower than the soil and water flows through the plant.  This is possible because within the plant there is a continuous body of water throughout the   conducting systems.  This continuous body has to extend to upward of 100 metres in the tallest trees and this is possible because water molecules ‘stick’ together and to solid surfaces by hydrogen bonding.   Now, bodies of water in trees are subject to gravity and the even more powerful ‘suction forces’ of dry air.  The cohesion of water molecules is very strong and has been shown experimentally to be of the order of 3 MPa or 30 bars and has been calculated theoretically to be at least 10 times this value.  The observed value is strong enough to allow the continuous body water to extend upwards of 300m metres - far in excess of the tallest trees.
When plants are under severe water stress, or physically damaged by pests or diseases, the continuous water may be challenged and may partially fail.   This damage is localized because of the modular structure of the xylem – any bubble that forms is isolated within a single cell or group of cells, because bubbles cannot cross cell walls.  This is very important because the break in the water body would prevent water from being sucked up.   Bubbles would appear to hinder water movement rather than promote it!  Plants can cope with isolated components being bubble filled because the water can flow in the cell wall and thus bypass the bubble region.
In plants, and especially the tallest trees that are generally coniferous, there are no continuous columns of water.  The conducting elements are individual cells and do not form long vessels.  Water passes through tiny pores in adjacent, ascending cell walls that are too small to allow bubble movement.  

I feel there are numerous problems with the Pohlé lift mechanism operating in plants. 
(Peter wrote more…)

Alan Warwick
Thanks for your latest email containing Peter’s comments. You are not a nuisance. I have both the time and the inclination. Perhaps I have not made it clear enough. My theory is not a true Pohlé lift in that pohlé has no suction and no dissolved gases and relies on the air bubbles injected to lift the liquid in effect reducing the average specific gravity in the tube that my theory proposes.
If cohesion theory was correct, which it cannot be, the pressure at the base of a hundred meter tree would be about 10 bar and the root pressure would have to be more than this which it cannot be as the osmotic pressure is related to the difference in concentration of solute on either side of the membrane. 
The bubbles are formed by a reducing pressure from a saturated solution of gas in your article’s analogy of tonic or champagne. Has my mention of the tonic water confused?

There must be an unbiased Professor of Botany somewhere, with the resources and the courage to discuss with fellow physicists and engineers my theory.
Do you think it worth a mention that deciduous leaves are fragile and would collapse under the vacuum proposed by the cohesion theory?
With regard to Peter’s comments
he cohesion of water molecules is very strong as I explained in an earlier email. Water in the form of H2 0 only exists in steam and although without the cohesion of liquid water life would not exist it has no relevance to the cohesion lift theory. 
How difficult is it to make water droplets in your sprayer? In my day Peter’s cohesion was and probably still is called Hydrogen Bonding.
How many zillion molecules are there in a single string 300 foot high and why does a full vacuum only lift 30 foot?     
In my theory gas passes through the stomata as water vapour. As I understand it the cohesion theory says water vapour is formed by evaporation from the stomata. In the atmosphere the two theories have the same effect.
PS It was David Attenborough himself who referred my theory to The RHS who were his programme advisors. Soon afterwards I got the brush off. 
I was told “it’s cohesion and a continuous stream, no gas, and it was then I gave up.
Regards Alan

I wanted to end by asking whether I should in enquire whether it was ‘push or pull’ but Peter and Alan told me it was inappropriate!

I would rather have questions that can’t be answered  than answers that can’t be questioned
Richard Feynman

Appendix added February 2017
Over Christmas I have read forester Peter Wohlleben’s bestseller book ‘The Secret Life of Trees’ and have posted about it here
He does not believe the standard explanation of water uptake to the top of tall trees.
A group of scientists in his forest in Germany listened at night to the trunk of large trees. They concluded the fizzing they could hear was made by thousands of carbon dioxide bubbles as they are released from root pressure. (Alan Warwick suggests nitrogen).
Wohlleben takes this to negate the cohesive column theory. 

He is of course unaware of the Pohlé lift phenomenon and offers no alternative mechanism.
He does write from his own lifetime’s  experience that each Spring he observes the huge root pressures built up at the base of a tree trunk - in his words ‘sufficient to swell it’. He emphasises that this is before deciduous trees’ leaves grow and there is no transpiration.

Food for thought?

Metasequoia can growth fifty metres high in the wild. As a deciduous plant where are the leaves to drive upward flow in Spring?


  1. I will try again.I would love to see the calculations that peter williams talks about.For example 1000 bar is 30,000 ft of water and my guess is that there arnt more than a dozen vessels capable of withstanding this pressure.Its clear that botanists do not appreciate the power of pressure.My guess is that the 10 Bar of the cohesion theory would require a vessel with 4 inch stainless steel walls and that atree trunk would burst.Your car tyres work at about 2Bar,how many people have been killed as a result of a burst tyre?

    1. I suspect that there will be more discussion about this Alan. I only hope that I have reported correctly what both you and Peter have been telling me.


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