In article ,
®óñ© © ²°¹°-°² writes:
|
| | Then try Osmosis. It's very powerful, and not limited to 30'
| |
| | http://en.wikipedia.org/wiki/Osmosis
|
| An old and good physical rule is that, if your explanation allows
| you to construct a perpetual motion machine, your explanation is
| wrong.
|
| Osmosis still requires energy to drive it.
|
| Osmosis is not perpetual motion. The attractant is fuelled by our
| beneficent sun.

That is an evasion. Osmosis requires a concentration gradient or
similar, and causes a pressure differential. A reduction cannot
raise water by more than 30' and all of the books indicate that the
maximum actual excess pressure is nothing like the 1 MPa per 30m
needed. Good try, but no banana.

It would be easy if plants had anything like venous return valves,
to act as a pump, but they don't.

The question remains is HOW does the sun's radiation get transferred
into the movement of the water? And I believe that is unknown.


Regards,
Nick Maclaren.

In article ,
says...
Broadback says...
Just curious. As I understand it the evaporation of moisture is what
draws the sap from the toots up the tree. Does this mean that the sap
does not rise until the leaves unfurl? If so where do the leaves obtain
the energy to be "kick started"?


I think that at least part of the mechanism involves the
roots pumping the water upwards. I seem to remember from my
student days that this was sometimes with considerable
force which can actually made the leaves "bleed" - a
process called "gutation" if I remember correctly.

There are also other mechanisms such as capillary action to
draw water along very fine tubules.

Found the artical I was looking for The words belong to John Tulett of
Edinburgh, the spelling mistakes are mine!

Plant roots take up inorganic ions from the soil and transfer them to the
xylem from which they can not leak back. Water is drawn in by Osmosis
which creates a positive pressure in the xylem . Because of this pressure
xylem sap leaks from pores. Guttation happens at night where the normal
stomata (pores) close and the water is then forced out through the
hydathodes, we see this as dew drops. (some plants do this more than
others ie grass)
Some useful ions are probebly recovered by the hydathodes and some of the
ions in the xylem may have been recirculated.
It is a similar process that brings calcium to developing fruits and when
this is interupted as when the atmosphere in a greenhouse at night is too
dry tomato fruits can suffer blossom end rot and drop off.

It would appear that although the above was written as an answer to a
question on why does dew form it at least starts to explain what
pressures the system
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea

On 27 Feb 2008 19:32:06 GMT, (Nick Maclaren) wrote
and included this (or some of this):

| Osmosis is not perpetual motion. The attractant is fuelled by our
| beneficent sun.

That is an evasion. Osmosis requires a concentration gradient or
similar, and causes a pressure differential. A reduction cannot
raise water by more than 30' and all of the books indicate that the
maximum actual excess pressure is nothing like the 1 MPa per 30m
needed. Good try, but no banana.

It would be easy if plants had anything like venous return valves,
to act as a pump, but they don't.

The question remains is HOW does the sun's radiation get transferred
into the movement of the water? And I believe that is unknown.

Does this help?

http://www.earthsbirthday.org/butter...nspiration.asp

Regards

--
®óñ© © ²°¹°-°²

In article ,
®óñ© © ²°¹°-°² writes:
|
| | Osmosis is not perpetual motion. The attractant is fuelled by our
| | beneficent sun.
|
| That is an evasion. Osmosis requires a concentration gradient or
| similar, and causes a pressure differential. A reduction cannot
| raise water by more than 30' and all of the books indicate that the
| maximum actual excess pressure is nothing like the 1 MPa per 30m
| needed. Good try, but no banana.
|
| It would be easy if plants had anything like venous return valves,
| to act as a pump, but they don't.
|
| The question remains is HOW does the sun's radiation get transferred
| into the movement of the water? And I believe that is unknown.
|
| Does this help?
|
| http://www.earthsbirthday.org/butter...nspiration.asp

No. Anyway, plant physiologists currently believe that osmosis is
NOT the primary mechanism. As I said, I looked it up in two reference
books (one British, one American) intended to teach undergraduates
the basics of plant physiology. They are likely to be more reliable
than random Web pages written by and for laymen.


Regards,
Nick Maclaren.

In message , Nick Maclaren
writes

In article ,
®óñ© © ²°¹°-°² writes:
|
| | Osmosis is not perpetual motion. The attractant is fuelled by our
| | beneficent sun.
|
| That is an evasion. Osmosis requires a concentration gradient or
| similar, and causes a pressure differential. A reduction cannot
| raise water by more than 30' and all of the books indicate that the
| maximum actual excess pressure is nothing like the 1 MPa per 30m
| needed. Good try, but no banana.
|
| It would be easy if plants had anything like venous return valves,
| to act as a pump, but they don't.
|
| The question remains is HOW does the sun's radiation get transferred
| into the movement of the water? And I believe that is unknown.
|
| Does this help?
|
| http://www.earthsbirthday.org/butter...nspiration.asp

No. Anyway, plant physiologists currently believe that osmosis is
NOT the primary mechanism. As I said, I looked it up in two reference
books (one British, one American) intended to teach undergraduates
the basics of plant physiology. They are likely to be more reliable
than random Web pages written by and for laymen.

Especially one that gets the definition of osmosis wrong in the first
paragraph - "Osmosis is the push that water gives as it seeks to make
plant roots as full of water as the damp soil around them". Osmosis is
the process which equalises the concentrations of solutes on either side
of a solvent (water in this case) permeable membrane. (Everything else
being equal.)

(It's aimed at ten year olds, so perhaps that's what they think a ten
year can understand. But, I expect that if a U-tube with a suitable
membrane could be provided, ten year olds could understand the concept.)


Regards,
Nick Maclaren.

--
Stewart Robert Hinsley

In article ,
says...
On 27 Feb 2008 19:32:06 GMT, (Nick Maclaren) wrote:


In article ,
®óñ© © ²°¹°-°² writes:
|
| | Then try Osmosis. It's very powerful, and not limited to 30'
| |
| | http://en.wikipedia.org/wiki/Osmosis
|
| An old and good physical rule is that, if your explanation allows
| you to construct a perpetual motion machine, your explanation is
| wrong.
|
| Osmosis still requires energy to drive it.
|
| Osmosis is not perpetual motion. The attractant is fuelled by our
| beneficent sun.

That is an evasion. Osmosis requires a concentration gradient or
similar, and causes a pressure differential. A reduction cannot
raise water by more than 30' and all of the books indicate that the
maximum actual excess pressure is nothing like the 1 MPa per 30m
needed. Good try, but no banana.

It would be easy if plants had anything like venous return valves,
to act as a pump, but they don't.

The question remains is HOW does the sun's radiation get transferred
into the movement of the water? And I believe that is unknown.


Regards,
Nick Maclaren.

I am wholly ignorant as to how sap gets from the roots to the canopy
of tall trees, but a few points may be worth making (at the risk of
stating what you already know perfectly well).

The 'driving force' for osmosis comes from the decrease in the free
energy of the dilute solution (usually water) as it migrates into the
concentrated solution.

The osmotic pressure generated by a 0.4 molal aqueous solution of
sucrose (136.8 g of table sugar in a litre of water or approximately a
13.7% solution) is about 10 atmospheres (~1 MPa). Leaves generate
sugars by photosynthesis, mostly glucose IIRC, and sunshine causes
leaves to transpire and lose water, raising the concentration of those
sugars in the sap. I have no idea what that concentration might be,
although I suspect 13.7% is way too high.
Almalgamating various snippets of info it looks like the trees are able
to prevent anything but water being transpired and can control the amount
of chemical in solution, that they use osmosis and capillary action and
are a lot smarter and more proactive than most of use give them credit
for!
NB Sugar content can be quite high, think sugar maple!
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea

In article ,
Chris Hogg writes:
|
| The osmotic pressure generated by a 0.4 molal aqueous solution of
| sucrose (136.8 g of table sugar in a litre of water or approximately a
| 13.7% solution) is about 10 atmospheres (~1 MPa). Leaves generate
| sugars by photosynthesis, mostly glucose IIRC, and sunshine causes
| leaves to transpire and lose water, raising the concentration of those
| sugars in the sap. I have no idea what that concentration might be,
| although I suspect 13.7% is way too high.

Yup, 13.7% is definitely too high - maple, birch etc. sap needs a LOT
of boiling down!

1 MPa would be enough for a 100' tree. 3 MPa is needed for the largest.

| My last point relates to capilliary suction. As you say, atmospheric
| pressure (i.e. 'suction') will only support a column of water roughly
| 10 metres high in a wide bore tube. But the situation is rather
| different in a fine capilliary, where surface tension alone will cause
| the liquid to rise. I don't know the size of the finest capillaries in
| trees, but if my fag-packet calculation is correct, a capilliary with
| radius 1 µm (10^-6 metres) will lift a column of water nearly 15
| metres. The figure varies inverse linearly with the capilliary radius,
| so a 0.1 µm radius gives 150 metres. In the same vein as the questions
| above, do tall-growing trees have finer capillaries than shorter
| growing trees? Is the ultimate height of a tree species controlled by
| capilliary size?

Dunno, but it doesn't help. While that is true, it won't cause any
liquid to FLOW once the whole tube is wet- conservation of energy.
That was my point about perpetual motion machines.

Even if it were pure water and being evaporated by the sun at the
top (i.e. with an adequate source of energy), it would still get to
only 30' without positive pressure, as the surface tension would
pull the column down as readily as it would pull it up. And all
measurements seem to indicate only a small positive pressure (say,
0.1 MPa).

| It does surprise me to hear that the botanists don't yet have the
| definitive answer to all this.

Charlie Pridham is right that things are a lot cleverer than they
appear!


Regards,
Nick Maclaren.

On Feb 27, 7:50*pm, Charlie Pridham
wrote:
In article ,
says...



Broadback says...
Just curious. As I understand it the evaporation of moisture is what
draws the sap from the toots up the tree. Does this mean that the sap
does not rise until the leaves unfurl? If so where do the leaves obtain
the energy to be "kick started"?

I think that at least part of the mechanism involves the
roots pumping the water upwards. I seem to remember from my
student days that this was sometimes with considerable
force which can actually made the leaves "bleed" - a
process called "gutation" if I remember correctly.

There are also other mechanisms such as capillary action to
draw water along very fine tubules.

Found the artical I was looking for The words belong to John Tulett of
Edinburgh, the spelling mistakes are mine!

Plant roots take up inorganic ions from the soil and transfer them to the
xylem from which they can not leak back. Water is drawn in by Osmosis
which creates a positive pressure in the xylem . Because of this pressure
xylem sap leaks from pores. Guttation happens at night where the normal
stomata (pores) close and the water is then forced out through the
hydathodes, we see this as dew drops. (some plants do this more than
others ie grass)
Some useful ions are probebly recovered by the hydathodes and some of the
ions in the xylem may have been recirculated.
It is a similar process that brings calcium to developing fruits and when
this is interupted as when the atmosphere in a greenhouse at night is too
dry tomato fruits can suffer blossom end rot and drop off.

It would appear that although the above was written as an answer to a
question on why does dew form it at least starts to explain what
pressures the system
--
Charlie Pridham, Gardening in Cornwallwww.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea- Hide quoted text -

- Show quoted text -

Thank you all for a fascinating thread! It's when I read something
like this that I think there's life in the usenet beast yet :-)

Cat(h) (and that I am reminded just how brilliant Nature is)

In article ,
Chris Hogg writes:
|
| Even if it were pure water and being evaporated by the sun at the
| top (i.e. with an adequate source of energy), it would still get to
| only 30' without positive pressure, as the surface tension would
| pull the column down as readily as it would pull it up. And all
| measurements seem to indicate only a small positive pressure (say,
| 0.1 MPa).
|
| Not sure I understand you here, but no matter. If the botanists
| haven't sorted it out by now, I doubt that we will!

I am just using physics, in the above. But I agree.


Regards,
Nick Maclaren.

In article ,
Chris Hogg writes:
|
| PS: Just found this on wikipedia, for what it's worth.
| http://en.wikipedia.org/wiki/Cohesion-tension_theory

That's what the books had, and they waved their metaphorical hands
equally vigorously while spouting comparable platitudes. Doubtless
someone has put some mathematics around that, though it may be
pretty dubious, but I have not seen it.


Regards,
Nick Maclaren.

In article ,
says...

In article ,
Chris Hogg writes:
|
| PS: Just found this on wikipedia, for what it's worth.
| http://en.wikipedia.org/wiki/Cohesion-tension_theory

That's what the books had, and they waved their metaphorical hands
equally vigorously while spouting comparable platitudes. Doubtless
someone has put some mathematics around that, though it may be
pretty dubious, but I have not seen it.


Regards,
Nick Maclaren.

Having read that I still can not see how it explains how a tree can raise
its sap before it has any leaves if the main force was evaporation there
must be some pretty powerful back up forces at work too.

--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea

Cat(h) wrote:
Snip
Cat(h) (and that I am reminded just how brilliant Nature is)

As I wrote in the starter for five it was just a curious question,
especially as to what "kick starts" the leaves. If the answer confounds
the intelligentsia here, never mind the plant physiologists, just
imagine how my head is spinning. Thanks for all the contributions.