How do Trees Really lift Water to their Leaves?

Why not? It simply depends where the tension is applied surely.
quote: Originally posted by l_kryptonite
I'm getting way out of my depth here but I don't believe that the same change in tension would cause leaves in one plant to pull one way, and have the reverse effect on another.

I have also thought about the carnivorous plants, and have a venus flytrap in front of me now.
could the insects movement stimulate the release of solutes stored at the leaf to be suddenly released and begin to flow rapidly down the stem, altering the internal pressures in front of the falling solutes to become positive and behind the falling solutes to become negative inducing the leaves to be pulled down around the captured insect by said hydraulic forces?

Andrew

l_kryptonite
Hm, I believe you may be on to something there. I'll look into it further.

Dave
According to:
www.news.harvard.edu/gazette/daily/2005/01/26-flytrap.html
and in more detail
www.ias.ac.in/jbiosci/bobji2652.pdf

The fly trap leaf is designed so when primed it is a bit like those toy rubber hemispheres which you could turn inside out and were just about stable, but would turn back the right way round quite violently (the toys would jump in the air).

The Flytrap leaf is just about stable open but with a small change in the rigidity of certain parts of the trap near the trigger cells (possibly due to some osmosis related mechanism) it will flip into it's preferred closed configuration, trapping the poor fly.

I think Harvard are stating the obvious
You have to admit, it is a bit like saying, when we release the string on a bow, the arrow flies through the air, without adding that we place tension manually on the bow with the addition of a string, then even more tension is added, until the bow string is manually released.

I think what we are trying to establish is how the plant places the charge in its leaves, and the mechanism that causes the leaves to close, which I believe to be a simple hydraulic process, that is easy to demonstrate using very basic and inexpensive lab equipment.

I was hoping that many of the scientists here would rush to defend all the old accepted explanations for fluid transport.

David
The opening of the trap is less interesting as it happens much more slowly, I would have thought it happened by cells changing shape by gaining or loosing water (possibly by pumping ions in and out of the cells and water following them by osmosis) or by the cells actually growing. I believe it is quite an expensive process energetically for the plant as you can kill it by triggering it too many times without feeding it.

What do you mean by "all the old accepted explanations for fluid transport." the stuff in the like I posted earlier seemed to be pretty consistent, what is the exact problem?

There is no paradigm in the accepted literature that comes remotely close to addressing the bulk flow rates observed in plants and trees.
The new Cohesion theory requires non-cavitation to even begin,let alone addressing the fact that it is impossible for leaves to generate the suction required to pull water from the ground and out through those pores, and therefore is a non-starter. Osmosis is utter nonsense when placed against the flow rates, capillary action is laughable and root pressure, well, let's not go there

Dave:
Why is it impossible for leaves to produce the suction required? If you work out the osmotic pressure produced by just the sugar in orange juice (as an example plant fluid it is easy to get figures for) it comes out at about 9.8 atmospheres, enough to suck water up about 100m (about 300 feet), so as long as the water in the xylem doesn't cavitate it should work fine!

A large tree has tens, or hundreds, of thousands of leaves. So each leaf would only have to suck, by osmosis, a few militres a day to make up the 1000 litres a day you quoted earlier. Surely this is a perfectly reasonable rate?

I don't think the cavitation problem is as bad as it sounds - for a start we can observe that there are xylem in a tree that are 100m long and they don't cavitate, and in the link I posted earlier it says that branches have been spun in centrifuges so they are experiencing negative pressures equivalent to 92metres of water. So the question isn't "is it reasonable for a xylem not to cavitate?" but why isn't the xylem cavitating?

I would guess the answer to this is related to nucleation. It is possible to heat water above it's boiling point, without it boiling, if you do so in a very clean container. This is because although it is (free) energetically favourable for all the water to boil, to do so it would have to form a bubble. Creating a bubble is difficult because you have to fight against suface tension, and it turns out that to be stable the bubble has to be more than a critical size. Now if the xylem is smaller than this critical size it would be impossible for a bubble to form until the tension is so large that the critical size of the bubble is smaller than the diameter of the xylem.

I will do some calculations at some point but I guess this will be at considerable negative pressures.

Reply #20 on: 27/04/2005 12:41:00 »
Tubular Water
One way to envision water pulled into and up a capillary tube is to use a suspension bridge model. The column of water is suspended against gravity by its adherence to the walls of the tube. Cohesive force keep all the water molecules together. Capillary movement is greater as tube diameter decreases. Extremely small diameter tubes, pores, or spaces can attract water and move it a relatively long way.
Capillary movement is responsible for within- and between-cell water movement in trees, and small pore space movements in soils. Cell wall spaces are extremely small (interfibral) and can slowly wick-up water. The water conducting tissues of trees (xylem), does not utilize capillary movement for water transport. If xylem were open at its top, a maximum capillary rise of 2-3 feet could be obtained. Xylem transport is by mass movement of water not capillary action.
Capillary movement is a matter of inches, not dragging water to the top of a 300 feet tall tree. Capillary movement components can be seen where liquid water touches the side of a glass. The water does not abruptly stop at the glass interface, but is drawn slightly up the sides of the glass. This raised rim is called a "meniscus." The meniscus is the visible sign of adhesive forces between the glass and water pulled up the side of the glass. The smaller the diameter of the glass, the greater the adhesive forces pulling-up on the water column and the less mass suspended behind.
Tiny Bubbles
Gas bubble formation in water columns is called cavitation. As temperatures rise and tension in the water column increases, more gases will fall out of solution and form small bubbles. These tiny bubbles may gather and coalesce, "snapping" the water column. As temperatures decrease, water can hold more dissolved gasses until it freezes. Freezing allows gases to escape and potentially cavitates water conducting tissue when thawed. Trees do have some limited means to reduce these cavitation faults.
On The Move
Water movement and transportation of materials is essential to tree life. The three major forms of transport are driven by diffusion, mass flow, and osmosis forces.
Diffusion – Diffusion operates over cell distances. Diffusion is the movement of dissolved materials from high concentrations areas to low concentration areas. Diffusion can move a dissolved molecule in water across a cell in a few seconds. Diffusion does not operate biologically over larger distances. It would take decades to diffuse a molecule across a distance of one yard / one meter.
Mass Flow – Most movements we visualize are due to the mass flow of materials caused by pressure differences. Wind, gravity, and transpiration forces initiate and sustain small differences in pressure. These small differences drive water and its dissolved load of materials in many different directions. Because pressure is the driving force in mass flow, (not concentration differences as in diffusion), the size of the conduit is critical to flow rates. If the radius of the conduit is doubled, volume flow increases to the fourth power of the size increase (double conduit radius and flow rate increases by 16 times — 24).
Osmosis – Osmosis is the movement of water across a membrane. Membranes in living tree cells separate and protect different processes and cellular parts. Membranes act as selective filters, preventing materials with large hydration spheres or layers from passing through. Small, uncharged materials may pass freely. The driving force to move materials in osmosis is a combination of pressure and concentration forces called a "water potential gradient."

by Dr. Kim D. Coder
Daniel B. Warnell School of Forest Resources
University of Georgia
6/99

Dave:
Yep that all sounds about right.
As far as I can work out the presently accepted way water gets up a tree is:

The xylem are full of water from the beginning, as each year they grow up from the roots and are full of water from the start.
This column of water is essentially hanging from the top of the column, and is stable (despite being under considerable negative pressure) because the xylem is so small and covered with hydrophillic substances so cavitation is difficult as I described above (this is known as the cohesiveness of water).

Now the tissue around the xylem have more sugar and other salts dissolved in them than are in contents of the xylem so they suck water across the cell membranes surrounding the xylem by osmosis. You are right to point out that osmosis is a slow process, but this is happening over the whole area of the tree so it adds up.

Because the water is cohesive if you pull on the top the whole column moves up like a piece of string so it sucks water in at the bottom.

The water in the cells evaporates concentrating the salts and sugars in them, and allowing them to draw more water in by osmosis. So the energy to power the whole process comes from the sun evaporating water in the leaves.

Where is the problem with this picture