Re: Tree sap question


In article <i9ads314hdj8rpgfuda7hacpod1997f41u@xxxxxxx>,
Chris Hogg <me@xxxxxxxxxxx> 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 ***-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.
.

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