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| Broadback 2008-02-26, 9:25 am |
| 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"?
| |
| Nick Maclaren 2008-02-26, 9:25 am |
|
In article <62i4vjF23jaobU1@mid.individual.net>,
Broadback <wen@towill.plus.com> writes:
|>
|> 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"?
'Tain't so. Sounds plausible, until you think about it. If that
were so, how would the sap reach the top of a 50' tree? Even a
vacuum will raise water only 30'.
That explanation was proposed and debunked by early in the 19th
century - but, like all myths, cannot be killed by mere disproof.
Regards,
Nick Maclaren.
| |
|
| "Nick Maclaren" <nmm1@cus.cam.ac.uk> wrote in message
news:fq0o9g$gbl$1@gemini.csx.cam.ac.uk...
>
> In article <62i4vjF23jaobU1@mid.individual.net>,
> Broadback <wen@towill.plus.com> writes:
> |>
> |> Just curious. As I understand it the evaporation of moisture is what
> |> draws the sap from the toots up the tree. Does this mean that the sap
> |> does not rise until the leaves unfurl? If so where do the leaves obtain
> |> the energy to be "kick started"?
>
> 'Tain't so. Sounds plausible, until you think about it. If that
> were so, how would the sap reach the top of a 50' tree? Even a
> vacuum will raise water only 30'.
>
> That explanation was proposed and debunked by early in the 19th
> century - but, like all myths, cannot be killed by mere disproof.
>
>
> Regards,
> Nick Maclaren.
Interesting post on this at another forum:
http://www.city-data.com/forum/gard...sap-rising.html
pk
| |
| Broadback 2008-02-26, 1:25 pm |
| PK wrote:
> "Nick Maclaren" <nmm1@cus.cam.ac.uk> wrote in message
> news:fq0o9g$gbl$1@gemini.csx.cam.ac.uk...
>
>
>
> Interesting post on this at another forum:
> http://www.city-data.com/forum/gard...sap-rising.html
>
> pk
>
Very interesting PK, does not answer my question though. OK Nick if that
theory is wrong what is the actual answer as to how sap gets up the tree?
| |
| Nick Maclaren 2008-02-26, 5:25 pm |
|
In article <62is6aF22nd6dU1@mid.individual.net>,
Broadback <wen@towill.plus.com> writes:
|>
|> >> |> 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"?
|> >>
|> >> 'Tain't so. Sounds plausible, until you think about it. If that
|> >> were so, how would the sap reach the top of a 50' tree? Even a
|> >> vacuum will raise water only 30'.
|> >>
|> >> That explanation was proposed and debunked by early in the 19th
|> >> century - but, like all myths, cannot be killed by mere disproof.
|> >
|> Very interesting PK, does not answer my question though. OK Nick if that
|> theory is wrong what is the actual answer as to how sap gets up the tree?
Well, it appears that I was wrong! Mostly.
It seems that the plant physiologists do regard evaporation of moisture
as being the driving force, but the books I looked at gloss over the
physics so badly as to make the explanation physical nonsense. And they
were undergraduate plant physiology references, too! My daughter
confirms that corresponds with what they were taught.
Reading between the lines, it seems that the mechanism is driven as
you might expect, but with surface tension of the hydrophilic cell walls
being a critical aspect. However, the way that solar energy is translated
into potential energy is still baffling the plant physiologists - after
over a century of studying the problem!
And neither book had even a hint of an answer to your question.
Regards,
Nick Maclaren.
| |
| Charlie Pridham 2008-02-27, 3:25 am |
| In article <fq1qgb$g9s$1@gemini.csx.cam.ac.uk>, nmm1@cus.cam.ac.uk
says...
>
> In article <62is6aF22nd6dU1@mid.individual.net>,
> Broadback <wen@towill.plus.com> writes:
> |>
> |> >> |> 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"?
> |> >>
> |> >> 'Tain't so. Sounds plausible, until you think about it. If that
> |> >> were so, how would the sap reach the top of a 50' tree? Even a
> |> >> vacuum will raise water only 30'.
> |> >>
> |> >> That explanation was proposed and debunked by early in the 19th
> |> >> century - but, like all myths, cannot be killed by mere disproof.
> |> >
> |> Very interesting PK, does not answer my question though. OK Nick if that
> |> theory is wrong what is the actual answer as to how sap gets up the tree?
>
> Well, it appears that I was wrong! Mostly.
>
> It seems that the plant physiologists do regard evaporation of moisture
> as being the driving force, but the books I looked at gloss over the
> physics so badly as to make the explanation physical nonsense. And they
> were undergraduate plant physiology references, too! My daughter
> confirms that corresponds with what they were taught.
>
> Reading between the lines, it seems that the mechanism is driven as
> you might expect, but with surface tension of the hydrophilic cell walls
> being a critical aspect. However, the way that solar energy is translated
> into potential energy is still baffling the plant physiologists - after
> over a century of studying the problem!
>
> And neither book had even a hint of an answer to your question.
>
>
> Regards,
> Nick Maclaren.
>
Trees do all sorts of clever stuff, I remember reading something about
Oak trees pumping water with their deep tap roots and pushing this out
into the soil near the surface where their feeder root system is in
prolonged dry weather. So they obviously have quite a degree of control
in the process are are not just at the mercy of the elements. I will try
and find the artical as it may shed some light on the way the system
works (which I always thought was mainly capillary, but then they must
be able to turn that off in winter?)
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea
| |
| Broadback 2008-02-27, 9:25 am |
| Charlie Pridham wrote:
> In article <fq1qgb$g9s$1@gemini.csx.cam.ac.uk>, nmm1@cus.cam.ac.uk
> says...
> Trees do all sorts of clever stuff, I remember reading something about
> Oak trees pumping water with their deep tap roots and pushing this out
> into the soil near the surface where their feeder root system is in
> prolonged dry weather. So they obviously have quite a degree of control
> in the process are are not just at the mercy of the elements. I will try
> and find the artical as it may shed some light on the way the system
> works (which I always thought was mainly capillary, but then they must
> be able to turn that off in winter?)
Thanks peeps, even though it does not answer my question. Makes me
wonder if they do resolve it they may have it on a solution to harbour
the sun's energy, then perhaps we can get rid of those ugly wind farms!
| |
| Mary Fisher 2008-02-27, 9:25 am |
|
"Broadback" <wen@towill.plus.com> wrote in message
....
> Thanks peeps, even though it does not answer my question. Makes me wonder
> if they do resolve it they may have it on a solution to harbour the sun's
> energy, then perhaps we can get rid of those ugly wind farms!
Not everyone thinks that they're ugly.
Mary
| |
| Amethyst Deceiver 2008-02-27, 9:25 am |
| In article <47c5488e$0$770$4c56ba96@master.news.zetnet.net>,
mary.fisher@zetnet.co.uk says...
>
> "Broadback" <wen@towill.plus.com> wrote in message
> ...
>
>
> Not everyone thinks that they're ugly.
Indeed. We were quite delighted to spot a new windfarm on the way to
work recently.
--
Linz
Wet Yorks
| |
| Broadback 2008-02-27, 1:25 pm |
| Amethyst Deceiver wrote:
> In article <47c5488e$0$770$4c56ba96@master.news.zetnet.net>,
> mary.fisher@zetnet.co.uk says...
>
> Indeed. We were quite delighted to spot a new windfarm on the way to
> work recently.
No accounting for taste, I do know that I would not like them near me,
their noise pollution is tremendous. My AD they must have been erected
quickly! ;-)
| |
| ®óñ© © ²°¹°-°² 2008-02-27, 1:25 pm |
| On 26 Feb 2008 10:05:36 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote
and included this (or some of this):
>In article <62i4vjF23jaobU1@mid.individual.net>,
>Broadback <wen@towill.plus.com> writes:
>|>
>|> 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"?
>
>'Tain't so. Sounds plausible, until you think about it. If that
>were so, how would the sap reach the top of a 50' tree? Even a
>vacuum will raise water only 30'.
Then try Osmosis. It's very powerful, and not limited to 30'
http://en.wikipedia.org/wiki/Osmosis
--
®óñ© © ²°¹°-°²
| |
| David in Normandy 2008-02-27, 1:25 pm |
| 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.
--
David in Normandy. DavidinNormandy@yahoo.fr
To e-mail you must include the password FROG on the
subject line, or it will be automatically deleted.
| |
| Nick Maclaren 2008-02-27, 1:25 pm |
|
In article <v54bs35g8b22vmlq601inueon3rp7sh5pb@4ax.com>,
®óñ© © ²°¹°-°² <ron@spamall.com> writes:
|>
|> >'Tain't so. Sounds plausible, until you think about it. If that
|> >were so, how would the sap reach the top of a 50' tree? Even a
|> >vacuum will raise water only 30'.
|>
|> 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.
Regards,
Nick Maclaren.
| |
| ®óñ© © ²°¹°-°² 2008-02-27, 1:25 pm |
| On 27 Feb 2008 17:06:46 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote
and included this (or some of this):
>
>In article <v54bs35g8b22vmlq601inueon3rp7sh5pb@4ax.com>,
>®óñ© © ²°¹°-°² <ron@spamall.com> writes:
>|>
>|> >'Tain't so. Sounds plausible, until you think about it. If that
>|> >were so, how would the sap reach the top of a 50' tree? Even a
>|> >vacuum will raise water only 30'.
>|>
>|> 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.
--
®óñ© © ²°¹°-°²
| |
| Charlie Pridham 2008-02-27, 5:25 pm |
| In article <478bs3pbfj9lq878n8gsuj49q50ei27u2f@4ax.com>, ron@spamall.com=20
says...
> On 27 Feb 2008 17:06:46 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote
> and included this (or some of this):
>=20
>=20
> Osmosis is not perpetual motion. The attractant is fuelled by our
> beneficent sun.
>=20
>=20
>=20
Can't be that alone, as if you prune something off it bleeds all night=20
long.
--=20
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk=20
Holders of national collections of Clematis viticella cultivars and=20
Lapageria rosea
| |
| Nick Maclaren 2008-02-27, 5:25 pm |
|
In article <478bs3pbfj9lq878n8gsuj49q50ei27u2f@4ax.com>,
®óñ© © ²°¹°-°² <ron@spamall.com> 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.
| |
| Charlie Pridham 2008-02-27, 5:25 pm |
| In article <MPG.222fad7444c91b459897fc@news.wanadoo.fr>,
DavidinNormandy@yahoo.fr says...
> Broadback says...
>
> 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
| |
| ®óñ© © ²°¹°-°² 2008-02-28, 3:25 am |
| On 27 Feb 2008 19:32:06 GMT, nmm1@cus.cam.ac.uk (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/butte...anspiration.asp
Regards
--
®óñ© © ²°¹°-°²
| |
| Nick Maclaren 2008-02-28, 3:25 am |
|
In article <hsrcs315019s3fppp3vj6a9bsmqjh13o6m@4ax.com>,
®óñ© © ²°¹°-°² <ron@spamall.com> 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/butte...anspiration.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.
| |
| Stewart Robert Hinsley 2008-02-28, 9:25 am |
| In message <fq5rbt$bmj$1@gemini.csx.cam.ac.uk>, Nick Maclaren
<nmm1@cus.cam.ac.uk> writes
>
>In article <hsrcs315019s3fppp3vj6a9bsmqjh13o6m@4ax.com>,
>®óñ© © ²°¹°-°² <ron@spamall.com> 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/butte...anspiration.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
| |
| Chris Hogg 2008-02-28, 9:25 am |
| On 27 Feb 2008 19:32:06 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote:
>
>In article <478bs3pbfj9lq878n8gsuj49q50ei27u2f@4ax.com>,
>®óñ© © ²°¹°-°² <ron@spamall.com> 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.
I recall seeing in the telly some years ago, an experiment in which a
fully grown forest tree was encased in scaffolding to support it. The
trunk was then cut through at the base and a section removed and
replaced by a tank of nutrients that was replenished as required. The
tree thrived, apparently. If the osmosis argument is being advanced,
then presumably the tree roots are assumed to be acting as the
semi-permeable membrane. But the tree in this experiment thrived
despite being severed from its roots, which suggests that osmosis
isn't the key to getting sap up into the canopy.
Two slightly rhetorical questions: do tall-growing trees have higher
sugar concentrations in their sap than shorter growing trees? Is the
ultimate height of a tree species controlled by its sap composition?
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?
It does surprise me to hear that the botanists don't yet have the
definitive answer to all this.
--
Chris
Gardening in West Cornwall overlooking the sea.
Mild, but very exposed to salt gales
E-mail: christopher[dot]hogg[at]virgin[dot]net
| |
| Charlie Pridham 2008-02-28, 9:25 am |
| In article <i9ads314hdj8rpgfuda7hacpod1997f41u@4ax.com>, me@privacy.net=20
says...
> On 27 Feb 2008 19:32:06 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote:
>=20
>=20
> 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).
>=20
> 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.
>=20
> 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=20
to prevent anything but water being transpired and can control the amount=
=20
of chemical in solution, that they use osmosis and capillary action and=20
are a lot smarter and more proactive than most of use give them credit=20
for!
NB Sugar content can be quite high, think sugar maple!
--=20
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk=20
Holders of national collections of Clematis viticella cultivars and=20
Lapageria rosea
| |
| Nick Maclaren 2008-02-28, 9:25 am |
|
In article <i9ads314hdj8rpgfuda7hacpod1997f41u@4ax.com>,
Chris Hogg <me@privacy.net> 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.
| |
| Cat(h) 2008-02-28, 9:25 am |
| On Feb 27, 7:50=A0pm, Charlie Pridham <char...@roselandhouse.co.uk>
wrote:
> In article <MPG.222fad7444c91b45989...@news.wanadoo.fr>,
> DavidinNorma...@yahoo.fr says...
>
>
>
n[color=darkred]
>
>
>
> 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)
| |
| Chris Hogg 2008-02-28, 5:26 pm |
| On 28 Feb 2008 13:43:46 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote:
>
>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!
--
Chris
Gardening in West Cornwall overlooking the sea.
Mild, but very exposed to salt gales
E-mail: christopher[dot]hogg[at]virgin[dot]net
| |
| Nick Maclaren 2008-02-28, 5:26 pm |
|
In article <098es312g8v5eg4c1j99bdbn2mjprti3a4@4ax.com>,
Chris Hogg <me@privacy.net> 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.
| |
| Chris Hogg 2008-02-28, 5:26 pm |
| On Thu, 28 Feb 2008 21:03:34 +0000, Chris Hogg <me@privacy.net> wrote:
>On 28 Feb 2008 13:43:46 GMT, nmm1@cus.cam.ac.uk (Nick Maclaren) wrote:
>
>Not sure I understand you here, but no matter. If the botanists
>haven't sorted it out by now, I doubt that we will!
PS: Just found this on wikipedia, for what it's worth.
http://en.wikipedia.org/wiki/Cohesion-tension_theory
--
Chris
Gardening in West Cornwall overlooking the sea.
Mild, but very exposed to salt gales
E-mail: christopher[dot]hogg[at]virgin[dot]net
| |
| Nick Maclaren 2008-02-28, 5:26 pm |
|
In article <8t8es397lkfjhchsjdanl6igt4qeg5jmqf@4ax.com>,
Chris Hogg <me@privacy.net> 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.
| |
| Charlie Pridham 2008-02-29, 3:25 am |
| In article <fq78ql$elr$1@gemini.csx.cam.ac.uk>, nmm1@cus.cam.ac.uk
says...
>
> In article <8t8es397lkfjhchsjdanl6igt4qeg5jmqf@4ax.com>,
> Chris Hogg <me@privacy.net> 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
| |
| Broadback 2008-02-29, 1:25 pm |
| 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.
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