tree height and atmospheric pressure

[DaveC426913]

I'm sure this has been asked uncountable times before.

How can any tree be more than 32 feet tall?

[dextercioby]

What kind of a question is that???The better question would have been:"Why the heck not??"

Daniel.

[enigma]

Why would 32 feet be a cut-off point?

[russ_watters]

You're not thinking about the height of the water column and the inability to suck it higher than 34 feet due to atmospheric pressure, are you? Trees don't work that way. Its mostly capillary action.

[ramollari]

You're not thinking about the height of the water column and the inability to suck it higher than 34 feet due to atmospheric pressure, are you? Trees don't work that way. Its mostly capillary action.
I believe that capillary action (resulting from surface tension) cannot rise the water column more than ten or twenty cm.

[DaveC426913]

(Guys, the title of the thread clarifies the nature of the question.)

So, capillary action is not limited by atmospheric pressure? Or just less affected?

I guess, since some trees reach 300 feet, it's pretty powerful.

[ramollari]

It should be active diffussion, i.e. osmosis that pushes the water. Ask this question to the biology forum.

[russ_watters]

According to THIS (http://www.madsci.org/posts/archives/apr2000/956789389.Bt.r.html) site, there are 3 force: root pressure (the roots act like pumps), capillary action, and transpiration (osmosis), with transpiration being most of it.

[gerben]

According to THIS (http://www.madsci.org/posts/archives/apr2000/956789389.Bt.r.html) site, there are 3 force: root pressure (the roots act like pumps), capillary action, and transpiration (osmosis), with transpiration being most of it.

yes that site says it all, but just to correct the above: it is osmosis that is causing the pump like action of the roots (via osmosis water is moving into the roots) and evaporation of water from the leaves (transpiration) which provides a pulling/sucking force on the water column. There are many capillaries in a tree trunk on which the leaves are sucking from above while the roots are adding water from below.

[DaveC426913]

That site suggests that transpiration is the dominant force, and the others can only lift a few feet.

But transpiration would be subject to atmospheric pressure. Even a hypothetical tree with 100% transpiration efficiency could not provide more lifting power than vacuum.

[Gokul43201]

Ummm...water gets transported between cells of the xylem through osmotic pressure.

[gerben]

No the xylem tissue does not consist of living cells, it consists of tiny but long tubes. They are really capillaries. If you suck through are very thin tube you can get water very high (because of cohesion and adhesion), that is what the leaves do. Sometimes a water column inside a xylem "tube" breaks, when that happens that xylem tube has lost its function and will never function again.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html

[DaveC426913]

excerpt:
"both root pressure and capillary action can only account for water
rising a few feet above ground. ... The remaining force (transpiration) accounts for the rest of the take up of water."

So, how can transpiration provide the bulk of the lifting power if it should not be able to lift higher than 32 feet?

[DaveC426913]

Here is an article that seems to explain it better:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html

Scroll halfway down to Transpiration-Pull.

"When water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water."


Though, to be honest, I don't see how this gets a the crux of the problem. The phenomena is still subject to atmospheric pressure. Even if the entire column of water were solid, the vacuum created at the top by transpiration will still not be able to lift it.



Although, wow:

"If forced to take water from a sealed container, the vine does so without any decrease in rate, even though the resulting vacuum becomes so great that the remaining water begins to boil spontaneously."

That pull sure is strong...

[ramollari]

It depends on what kind of roots you are talking about. In very big roots, such as those of the giant trees we are focusing on, the root pressure should be more dominant than that.
Before that you have to understand the cause of root pressure. It is active absorption of minerals from the ground, and then water follows by osmosis. The root can spend as much energy for active absorption as it needs.