How does water really reach the leaves of trees?
Andrew Kenneth Fletcher
gravitystudy at hotmail.com
Sun Mar 19 06:04:33 EST 2000
Thank you for your post, this is what I am looking for! I agree with most of
The valve placed at the top would act like scissors and cut the water
column. It would then immediately fall on both sides to around 33 feet.
There is no point trying this unless you doubt the 33 ft limit exists in a
single upstanding capped tube.
In one experiment I had a T junction which was air tight. The idea was that
I could inject saline solution via a valve into the loop. It failed because
the water boiled, it was like air was being sucked into the water from the
walls of the tube at the top of the loop. I guess the interruption
The tube I used was 6.5mil strong nylon, the type used in the brewery trade.
I have observed the column breaking during the experiments and noted that
the level in both tubes falls rapidly to the 33 ft mark.
Ray Girvan <ray.girvan at zetnet.co.uk> wrote in message
news:2000031411151571207 at zetnet.co.uk...
> David Allsopp <dallsopp at signal.dera.gov.uk> writes:
> > Andrew Kenneth Fletcher wrote:
> >> By introducing a loop of tubing, instead of a single tube, to
> >> simulate the internal structure of plants and trees, and
> >> suspending it by the centre, the problem of raising water
> >> above the 33 feet limit is solved. The reason a loop of tubing
> >> succeeds where a single tube fails is because the cohesive
> >> bond of water molecules is far stronger than the adhesive
> >> qualities of water observed in Galileo's lift-pump problem.
> >> Using a loop of tubing enables water molecules to bond to
> >> each other in an unbroken chain. It helps to picture the
> >> unbroken loop of water as a cord instead of a liquid,
> >> supported by a pulley in the centre with tension applied to
> >> both ends.
> > Whoa! I can picture chemists across the land falling off
> > their chairs! ... You have some _major_ convincing to do here...
> >> The columns of water held in both sides of the tube exert a
> >> downward force due to the weight of the water contained in
> >> the tube. This force causes the water molecules in the tube
> >> to be stretched, causing the water to behave like an elastic band.
> > What do you mean, 'causing the water to behave like an
> > elastic band' ?
> Actually, I think this is the key to the Brixham Cliff observations.
> I vaguely remembered something from university, and tried a web
> search for "tensile strength of water".
> A constrained column of water does, in fact, possess tensile
> strength; to stretch or 'break' it is doing work against the hydrogen
> bonds between molecules. One of the references mentioned a
> centrifuge experiment showing a water column could take -26 MPa
> pressure without cavitating (1 atmosphere = 0.1 Mpa). This is a
> known phenomenon, and I think Andrew has hit upon an intuitive
> description of this, and an experimental setup that demonstrates it.
> The likely explanation, then, is that the 78 foot column is
> partly supported from below by atmospheric pressure, and partly by
> the tensile strength of the water column itself.
If atmospheric pressure is responsible, why does the water level go up the
tubes when the tube ends are pulled from the vessels? Surely the level would
fall resulting in water flowing from the tubes. This is simply not the case
in the Brixham experiment.
> However, it's nothing to do with the loop setup: I'd predict that a
> single closed column of water would do just as well. Andrew could
> test this by putting a valve or clip at the top of the loop, and
> seeing what (if anything) happens if it's closed after the tube is filled.
It would inevitably fail Ray.
If you would like to come to Paignton and see the video and bench
experiments, and hear what I have to say about how this theory fits with
everything, just give me a call on 01803 524117.
Kettle is always on and my wife Judy is an excellent cook. I will give you
some tube so that you can repeat my experiment and test your own idea.
I would like to share your post with the other groups to see if it will
stimulate some more responses, if this is OK with you?
> ray.girvan at zetnet.co.uk +++ Technical Author +++ Topsham, Devon, UK
> http://www.users.zetnet.co.uk/rgirvan/ +++ The Apothecary's Drawer
Andrew Kenneth Fletcher <gravitystudy at hotmail.com> wrote in message
news:89tdsh$to$1 at newsg4.svr.pol.co.uk...
> Extracts from ENCYCLODAEDIA BRITANNICA:
> : PROCESS OF XYLEM TRANSPORT
> : Normally the proportion of xylem to leaves supplied by that xylem is
> : greater in plants growing in dry habitats than in plants found in wet
> : ones and may be as much as 700 times greater in certain desert plants
> : than in aquatic plants and herbs of relatively humid forest floors.
> : The velocity of sap movement in trees varies throughout a 24-hour
> : period. ... Peak velocities correlate with vessel size; the rate of
> : sap flow in trees with small vessels is about 2 metres (7 feet) per
> : hour; that in trees with large vessels, about 50 metres (160 feet) per
> : hour. The energy required to lift water in both cases is comparable;
> : in trees with large pores, water simply moves faster through fewer and
> : larger vessels.
> : It was demonstrated about 1900 that living cells of the stem are not
> : responsible for water movement.
> That living cells are not responsible for the water movement might be
> correct in the same sense as living cells are not a necessary condition
> for e.g. DNA replication. Polymerase enzymes are able to carry out this
> function also in vitro. The crucial question however is, whether the
> behaviour of polymerase enzymes is consistent with the predictions of
> statistical physics.
> : It is now generally recognized that water in the xylem moves passively
> : along a gradient of decreasing pressures.
> It is clear that in vertical tubes filled with water, gradients of
> decreasing pressures upwards are unavoidable. But such gradients do
> certainly not lead to upwards forces on the water molecules. On the
> contrary the gradients are the result of downwards forces.
> : Under certain special conditions, water is pushed up the stem by root
> : pressure.
> If water is pushed up the stem, then the molecules which produce the
> root pressure must perform "uphill" movements, i.e. they must move
> against a force and lose the energy which is converted into potential
> energy of the pushed water. Such "uphill" movements must not be taken
> for granted.
> : Most of the time, however, water is pulled into the leaves by
> : transpiration. A gradient of decreasing pressures from the base to
> : the top of a tree can be measured, even though pressures are low.
> Isn't this "transpiration pull" hypothesis dreadfully incredible? The
> kinetic energy of water molecules corresponds to a certain statistical
> distibution. Those surface molecules with the highest energy evaporate.
> Because of momentum conservation the water in the pores of the leaves
> suffers rather a downwards push than an upwards pull from upwards
> evaporating water molecules.
> From a purely quantitative point of view, the explanation seems
> plausible. For a gram of water to evaporate, around 2000 Joules are
> needed. For a vertical transport over 100 m however, only 1 Joule
> is needed for the same quantity of water.
> From the fact that water is transported in huges trees after very dry
> winters before the leaves emerge, we conclude that another mechanism
> of water transport must exist.
> : A vacuum pump cannot pull water to a height of more than 10 metres
> : (about 33 feet). ... The hypothesis that water is pulled upward along
> : a pressure gradient during transpiration has been called the cohesion
> : theory. Two critical requirements of the cohesion mechanism of water
> : ascent are (1) sufficient cohesive strength of water and (2) existence
> : of tensions (i.e., pressures below zero) and tension gradients in
> : stems of transpiring trees.
> : Although the tensile strength of water is very high, an excessive pull
> : exerted on a water column will break it. The tallest trees are about
> : 100 metres (330 feet) high. A nonmoving water column at an atmospheric
> : pressure of 1 atmosphere at the base of the tree is exposed to a
> : pressure of -9 atmospheres (i.e., a tension of 9 atmospheres) at the
> : top. ... If ..., the pressure at the top drops to -25 atmospheres.
> Negative pressures in the context of water seems a rather strange and
> questionable concept. Isn't normally an atmospheric pressure of (almost)
> zero enough to separate all water molecules from each other?
> : It has been demonstrated that water columns in the xylem can withstand
> : this tension, or pull, without breaking.
> Maybe it is the actual mechanism of the xylem transport system which is
> responsible for the fact that water columns do not break, and not this
> strange "cohesion hypothesis".
> : Negative pressures and gradients of negative pressures have been
> : shown to exist in trees with an ingeniously simple device called the
> : pressure bomb. A small twig is inserted in a container (the pressure
> : bomb), its cut stump emerging from a tightly sealed hole. As pressure
> : is applied to the container and gradually increased, water from the
> : xylem emerges from the cut end as soon as the pressure being applied
> : is equal to the xylem tension that existed when the twig was cut.
> If I understand correctly, then "pressure bomb" reasoning is based on
> a rather dubious premise: it is assumed that the resistance against
> pushing water through the twig in leaves-root-direction results from a
> one-directional xylem tension. I suppose there is also a resistance
> in the opposite direction (when trying to increase the natural flow of
> water in the twig).
> So the question "how does water really reach the the tops of trees"
> is still open.
> Wolfgang Gottfried G.
> On Brownian motion, diffusion and molecular transport:
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