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| ▲ | cj 2 hours ago | parent | next [-] | | Not that it really matters, but the article also refers to it as “drawing water to the top”. That seems more representative of reality than “pumping water from the bottom”. | | |
| ▲ | chowells 2 hours ago | parent | next [-] | | If you think of it that way, you have a real problem. It only takes about 10 meters for the weight of a column of water to create enough downward force that it starts vaporizing, at which point no pumping action works. This is why any deep well has a submerged pump. You simply can't pull water upward further than that with negative pressure in the Earth's atmosphere. It must be pushed with positive pressure instead. This is why the question is interesting. You can't just suck water to the top of a 60 meter tree. There must be some kind of positive-pressure pumping involved. | | |
| ▲ | pulvinar an hour ago | parent | next [-] | | The trick for trees is capillaries, which change the equation. The 10 meter limit only applies to larger columns. With capillaries there's a high negative tension that allows evaporation from leaves to pull the xylem sap up 100 meters or more. There's no free lunch here. The Sun drives the evaporation, and if the tree were in a closed system with no solar input, the humidity would eventually get high enough to stop it. | | |
| ▲ | hinkley an hour ago | parent | next [-] | | One of the things Susan Simard proved was that deep rooted trees that had found subterranean water continue pulling that water at full speed at night when transpiration is low, and that water finds its way into the fungal networks in the soil and into nearby plants. Simard attributes intention to this, but osmosis is “fair”. It seeks to move water to where sugars are and sugars to where water is. So a plant giving up sugars will receive water, and one low on water will give up sugars in the process of equalization. Do fungi contain pumps to maintain disequilibrium in this work? I could not say. But even when they first learned the trick of tapping roots the basic premise would have worked in a rudimentary fashion woth no further optimization. | |
| ▲ | tenuousemphasis an hour ago | parent | prev [-] | | >if the tree were in a closed system with no solar input ... that would be the least of the tree's problems. | | |
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| ▲ | an hour ago | parent | prev [-] | | [deleted] |
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| ▲ | margalabargala 2 hours ago | parent | prev [-] | | Yeah it's the difference between creating low vs high pressure. | | |
| ▲ | card_zero 2 hours ago | parent [-] | | The low pressure is up there already, for free. Or the high pressure is down here, whichever way you want to look at it. |
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| ▲ | rolph 2 hours ago | parent | prev | next [-] | | more like capillary action. https://en.wikipedia.org/wiki/Xylem#Cohesion-tension_theory | | |
| ▲ | rolph an hour ago | parent | next [-] | | the research is relevant to the issue of transpiration column hieght as a postulated limitation to overall hieght of any tree. a column of water is pulled by hydrogen bonding between molecules in a tug of war fashion, the top of the column is where water is dissociated from the column at such a rate as to maintain low pressure with respect to the column[xylem] in summary water moves from bottom to top in a transpiration stream, that ultimately ejects water vapour from the leaves, resulting in a low efficiency mechanism, that loses a lot of the water but occurs at such a rate that the low efficiency is "good enough" for whats needed. | |
| ▲ | card_zero 2 hours ago | parent | prev [-] | | Oh, so we don't really know how it works. Fun. |
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| ▲ | gitaarik 2 hours ago | parent | prev [-] | | “Trees contain lots of thin, hollow vessels and they suck water upwards by creating low pressure at the top,” So sucking / pulling? | | |
| ▲ | IsTom 2 hours ago | parent [-] | | So a suction pump? | | |
| ▲ | card_zero 2 hours ago | parent | next [-] | | Same principle as chimneys. But I also noticed this line: > leaves which have adapted to withstand greater water stress before wilting. That must be one of the "adjustments to water transport" mentioned. So I suggest that they do, in fact, have trouble pumping water to top branches. | | |
| ▲ | gitaarik 2 hours ago | parent | next [-] | | Maybe it's not more trouble pumping, eh, sucking water up. But that the top branches are the last ones to get water in periods of draught, and have therefore more resilience? | |
| ▲ | DANmode 2 hours ago | parent | prev [-] | | Or, it’s simply a rate to variably adjust to, so the tree is neither flooding nor parching the leaf. |
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| ▲ | hinkley an hour ago | parent | prev [-] | | My recollection is that capillary action is a
little from column a and a little from column b. |
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