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Each spring enormous amounts of water rise up in trees and other vegetation. What causes this stream upwards?

Edit: I was under the impression that capillary action is a key factor: the original question therefore was: what are the fundamental forces involved in capillary action?

Can perhaps anybody make an estimation of the amount of gravity energy that is involved in this process earth-wide each season?

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What is gravity energy ? –  Cedric H. Nov 5 '10 at 11:23
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With gravity energy I mean the potential energy in a gravitational field. –  Gerard Nov 6 '10 at 0:23
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up vote 11 down vote accepted

Water is transported through xylem tissue, which reassemble just a passive bundle of pipes. They are narrow enough to provide quite a huge capillary effect, but this is not a process of transport because it converges fast to equilibrium water levels and stops.
The flow is powered by two other processes; first and most important is evaporation of water from leafs (transpiration pull) and the second is a osmotic pressure in roots (root pressure). The first one is a passive process, powered by the difference of water potentials in air and soil; the second is active, plant consumes its own energy to produce ion gradient causing osmotic pressure.

I won't give you the accurate numbers in the second part; to give some imagination, about 90% of water absorbed from soil is transpired into atmosphere, so it is near transpiration rates. A single tree growing in humid continental climate zone (most of Europe and USA) transpires yearly something like 100 tons of water.

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What is important is that the active pump acts in cycles and there's never a continous body of water all the way top to bottom, or it would create enourmous hydrostatic pressures at the bottom, being above zero on top. –  Pavel Radzivilovsky Nov 30 '10 at 22:06
    
@Pavel I don't think so -- any citation? –  mbq Dec 1 '10 at 0:08
    
Citation not found - and maybe because the biologists are not into hydrostatics paradoxes? But, you must agree that otherwise it makes very little sense. There exist trees 150m high, and no biological membrane can hold 15bar pressure difference between the sides. –  Pavel Radzivilovsky Dec 1 '10 at 6:53
    
@mbq there is no "transpiration pull". The evaporation in the pores of the leaves comes from a tiny hole filled with water. This water surface will exert some capillary suction "downwards". But capillary forces are very limited, as sigoldsberg wrote. In general, no "suction" does more lift than about 7 meters. –  Georg Jul 21 '11 at 5:18
    
@Georg I wrote that; transpiration pull is due to evaporation. –  mbq Jul 21 '11 at 6:12
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As pointed out above, it is actually other forces, not capillary action that gets water/sap to the top of tall trees.

Notwithstanding that misunderstanding, in capillary action the fundamental force is surface tension and wetting forces on the side of the tube. It can raise water in plants only about 1 meter high. See http://en.wikipedia.org/wiki/Capillary_action for an explanation and a formula.

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