How do Trees Really lift Water to their Leaves?

Stefan:
Are you saying that you know better than the rest of us what gravity is and how it does and does not work?

The plates on the following paper show clearly the size of xylem and number of xylem in comparison to phloem. Tiny amount of return flow in the phloem = solvent dragging on all of the more dilute sap in the xylem, evaporation at the leaves reduces the volume of sap but increases the density of the sap and the denser sap moves down the tiny by comparison phloem vessels.

Experiments with different diameter tubes has shown this to be worth considering.

Using an inverted U tube with 3 tubes one of which contained a small amount of coloured salt solution added at the upper part where the tubes were joined with a T junction, the salt solution moved down as expected and the salt free water in the other two tubes moved up, caused by solvent dragging on all of the water molecules.

In the case of the tree, the narrowing of the xylem vessels in the canopy compared to the xylem vessels in the trunk and branches affords a method of extruding the large volume of water shedding off the majority to evaporation and returning the solutes down the phloem vessels.

The paper also relates to the problem with addressing constant cavitations known to take place and also known to refill and repair the cavitations. The cohesion tension theory relies on root pressure for this, yet the rattan does not exhibit root pressure.

www.amjbot.org/cgi/reprint/89/2/196.pdf
American Journal of Botany 89(2): 196–202. 2002.
XYLEM OF RATTANS: VESSEL DIMENSIONS IN
CLIMBING PALMS1
JACK B. FISHER,2,3,5 HUGH T. W. TAN,4 AND LESLIE P. L. TOH4
environmental factors. During periods of limited rainfall, rattans
and other lianas can experience severe water stress. At
such times, both stomatal closure and stem water storage
would aid survival. In other lianas, water-storing tubers or succulence
of stems and leaves are common (Fisher and Ewers,
1991). Rattans lack tuberous roots and their narrow stems have
a small proportion of parenchyma that could function in water
storage. However, their long stems with a relatively large volume
of water in wide vessels represent a significant water reservoir
that would become available if cavitation of vessels
occurred during periods of extreme water stress (Holbrook,
1995). If cavitation of wide vessels does play a role in water
supply during draught periods, then the question of vessel refilling
must be addressed. Further studies should also focus on
the water capacity of rattan stems compared to nearby nonclimbing
palms, as well as their relative degrees of stomatal
control.
At present, we have no information on production of embolisms
in rattan xylem. Yet the low percentage of nonfunctional
vascular bundles in old stems suggests either a lack of
vessel cavitation or a mechanism for refilling vessels (and tracheids).
Other lianas have root pressure that is sufficient to
refill air-filled xylem, as in Vitis (Sperry et al., 1987), or to
decrease xylem tension and thus assist in removal of embolisms
(Fisher et al., 1997). In a nonclimbing palm, Sperry
(1986) found that embolisms were dissolved when xylem pressure
potential approached that of the atmosphere during periods
of rain. When stem bases of cultivated species of Calamus,
Daemonorops, and Desmoncus (a climbing nonrattan palm)
were cut at dawn during rainy periods, no exudation appeared,
thus indicating no root pressure (Fisher et al., 1997); however,
there was an indication of root pressure in one species of Calamus
cultivated in a mountainous rainforest. We suggest that
future measurements for possible root pressure are needed to
better understand water conduction for rattans growing in natural
environments.


environmental factors. During periods of limited rainfall, rattans
and other lianas can experience severe water stress. At
such times, both stomatal closure and stem water storage
would aid survival. In other lianas, water-storing tubers or succulence
of stems and leaves are common (Fisher and Ewers,
1991). Rattans lack tuberous roots and their narrow stems have
a small proportion of parenchyma that could function in water
storage. However, their long stems with a relatively large volume
of water in wide vessels represent a significant water reservoir
that would become available if cavitation of vessels
occurred during periods of extreme water stress (Holbrook,
1995). If cavitation of wide vessels does play a role in water
supply during draught periods, then the question of vessel refilling
must be addressed. Further studies should also focus on
the water capacity of rattan stems compared to nearby nonclimbing
palms, as well as their relative degrees of stomatal
control.
At present, we have no information on production of embolisms
in rattan xylem. Yet the low percentage of nonfunctional
vascular bundles in old stems suggests either a lack of
vessel cavitation or a mechanism for refilling vessels (and tracheids).
Other lianas have root pressure that is sufficient to
refill air-filled xylem, as in Vitis (Sperry et al., 1987), or to
decrease xylem tension and thus assist in removal of embolisms
(Fisher et al., 1997). In a nonclimbing palm, Sperry
(1986) found that embolisms were dissolved when xylem pressure
potential approached that of the atmosphere during periods
of rain. When stem bases of cultivated species of Calamus,
Daemonorops, and Desmoncus (a climbing nonrattan palm)
were cut at dawn during rainy periods, no exudation appeared,
thus indicating no root pressure (Fisher et al., 1997); however,
there was an indication of root pressure in one species of Calamus
cultivated in a mountainous rainforest. We suggest that
future measurements for possible root pressure are needed to
better understand water conduction for rattans growing in natural
environments.

Bored Chemist: aka BC

Quote from: Andrew K Fletcher on 15/11/2009 09:54:11

And all of you know what gravity is?

Please let me know at your earliest convenience as this problem has eluded the greatest minds and to this day still does!


No, but I do know some of it's properties.
For exaple;
It is the force that holds me down on the planet.
It isn't responsible for the heat from the sun
It doesn't push water up trees.
It doesn't drive all life on earth.

The world's great scientists mighht not understand it fully but, unlike you, they know what not to atribute to it.
Why dd you post that rubbish?

Gravity is what drives the sun! therefore gravity is responsible for the heat from the sun. No gravity = no sun No gravity = no BC and no earth to stand on! No gravity = nothing

Madidus_Scientia:
Gravity is a force, not energy.

Rosy:
Hehe.

Ok, now I'm laughing. We're in real making-stuff-up territory now.

Sure the sun wouldn't be able to exist without gravity, because the matter wouldn't be concentrated in one place... but to say that gravity drives the sun is just ludicrous. What "drives" the sun (in the sense of providing the actual energy) is nuclear fusion. Nuclear fusion is not, repeat not, driven by gravity.

Where does the immense pressure come from that forces the atoms together to create fusion?

BC:
Who cares?
If it were not for fusion all you would get was squashed gas.

Saying the sun is driven by gravity is like saying my central heating runs on a match.
OK, without a match to light the pilot light, the heating wouldn't work.
But it's still gas that heats my house.

It's still time for you to learn some physics and maths.
Why did you post that rubbish?
Why don't you listen?

Titre du document / Document title
Osmosis and solute-solvent drag : Fluid transport and fluid exchange in animals and plants
Auteur(s) / Author(s)
HAMMEL H. T. ; SCHLEGEL Whitney M. ;
Résumé / Abstract
In 1903, George Hulett explained how solute alters water in an aqueous solution to lower the vapor pressure of its water. Hulett also explained how the same altered water causes osmosis and osmotic pressure when the solution is separated from liquid water by a membrane permeable to the water only. Hulett recognized that the solute molecules diffuse toward all boundaries of the solution containing the solute. Solute diffusion is stopped at all boundaries, at an open-unopposed surface of the solution, at a semipermeable membrane, at a container wall, or at the boundary of a solid or gaseous inclusion surrounded by solution but not dissolved in it. At each boundary of the solution, the solute molecules are reflected, they change momentum, and the change of momentum of all reflected molecules is a pressure, a solute pressure (i.e., a force on a unit area of reflecting boundary). When a boundary of the solution is open and unopposed, the solute pressure alters the internal tension in the force bonding the water in its liquid phase, namely, the hydrogen bond. All altered properties of the water in the solution are explained by the altered internal tension of the water in the solution. We acclaim Hulett's explanation of osmosis, osmotic pressure, and lowering of the vapor pressure of water in an aqueous solution. His explanation is self-evident. It is the necessary, sufficient, and inescapable explanation of all altered properties of the water in the solution relative to the same property of pure liquid water at the same externally applied pressure and the same temperature. We extend Hulett's explanation of osmosis to include the osmotic effects of solute diffusing through solvent and dragging on the solvent through which it diffuses. Therein lies the explanations of (1) the extravasation from and return of interstitial fluid to capillaries, (2) the return of luminal fluid in the proximal and distal convoluted tubules of a kidney nephron to their peritubular capillaries, (3) the return of interstitial fluid to the vasa recta, (4) return of aqueous humor to the episcleral veins, and (5) flow of phloem from source to sink in higher plants and many more examples of fluid transport and fluid exchange in animal and plant physiology. When a membrane is permeable to water only and when it separates differing aqueous solutions, the flow of water is from the solution with the lower osmotic pressure to the solution with the higher osmotic pressure. On the contrary, when no diffusion barrier separates differing parts of an aqueous solution, fluid may flow from the part with the higher osmotic pressure to the part with the lower osmotic pressure because the solute molecules diffuse toward their lower concentration and they drag on the water through which they diffuse. This latter osmotic effect (diffusing solute dragging on solvent or counterosmosis) between differing parts of a solution has long been neglected and ignored when explaining fluid fluxes in plant and animal physiology. For two solutions separated by a semipermeable membrane, osmosis is the flow of its solvent from the solution with the lower solute concentration into the solution with the higher solute concentration. For two contiguous solutions not separated by a semipermeable membrane, counterosmosis is the flow of solution with the higher solute concentration toward the solution with the lower solute concentration.
Revue / Journal Title
Cell biochemistry and biophysics ISSN 1085-9195 CODEN CBBIFV

Experiment showing solvent drag and solute density applied tension to water filled vertically suspended silicone tubing.