Trees suck up water through their roots and disperse it through the rest of the tree. How does, say, a giant redwood, generate enough pull to get the water up that far?
Capilliary action mostly, I think - the water pulls itself up there because the vessels are so narrow.
But surely the weight of water is too much for capillary action to pull up?
That is largely the case IIRC; transpiration on the leaf surface allows water to evaporate, to be replaced by new water from the body of the plant;
this water is partly contained by thin xylem vessels, which form a continuous network from the leaves to the roots.
Water lost at the top of the tree pulls new water up the xylem vessels, against gravity and atmospheric pressure;
(transpirational pull)
if it were not for capillary action the chain of water suction could not get any higher than about 10 metres before the weight of water balanced atmospheric pressure and the water would stop rising.
As we know, trees are often taller than 10 metres, so the additional height is achieved by capillary action, which gains energy from surface tension.
Oh yes, there is a bit less than a metre of push from the roots via osmosis, as well.
It isn’t so much of a case of the water being ‘pulled’ or ‘sucked’ up in one continuous column (or there would be a problem with the 302 foot limit) - it is more a case of the water ‘soaking’ up into a porous medium.
Ooops (32 foot limit)
According to this, it’s a combination of capillary action, osmosis (water equalising across a membrane), water tension (surface tension/attraction between water molecules), and transpiration (evaporation from the top creating a vacuum).
Transpiration pulls water up as the evaporation of water creates low pressure at the top, but this will only work to a certain height, about 10 metres (the same limit as for supporting a volume of water by atmospheric pressure). As to the limits on the other forces, I don’t know.
According to this article in Discover, there are competing theories and it is not a settled matter.
Also, the ~32 foot limit is not hard and fast, since a water column boils and falls apart if “sucked” down to zero pressure, yet water can only start boiling if nucleation centers are present. If the water lacks the tiny cavities which get the boiling-bubbles started, then it can be put under considerable tension or “negative pressure” yet the column of water doesn’t break apart. This is similar to the phenomenon of superheating. If something stops the formation of bubbles, then the top of the “solid” water column can support more than 32ft of water below. The pressure is allowed to go negative.
Hmmm. I wonder if trees are limited in maximum height because of vacuum-boiling… but the height limit is simply much greater than 32ft?
You are imagining a discrete single tube reaching hundreds of feet into the air. That ain’t it.
Every few millimeters this water is passing horizontally through membranes, into side vessels, and cells. The water is then held up by the physical walls of the cells, and passes upwards by osmosis, capillary action, and transpiration to another level, where it collects again inside discrete cellular walls. The fact is that it takes a long time for water to go all the way from the roots to the crown of a mature tree. But time is one thing trees got lots of.
Tris