Talk:Transpirational pull

From Wikipedia, the free encyclopedia

Something important is missing here. According to the article -"Water movement within the xylem conduits is driven by a pressure gradient, not by capillary action." A pressure gradient can only account for ~34 feet of suction lift. For anything higher then that the water column has to support tensile stress. Alternatively there could be a large positive gauge pressure in the roots.

The article also seems to imply that the water in the xylem can be under negative absolute pressure. Negative absolute pressure is a bogus concept - seems like there's been a confusion of stress and pressure. Toiyabe 20:25, 4 December 2005 (UTC)

This is almost correct. It seems that many people think that capillary forces are a misconception. They fail to realize that there needs to be some very very strong force to hold the water coloumn against gravity. What the author wrote about pressure gradient is correct, and it is responsible for the movement of water, but it is certainly NOT responsible for holding the column. The ~34 feet you are reffering to is assuming the only pressure in town is atmospheric pressure in which case,

rho g h = 1atm  therefore h = 1atm/ rho g = 1atm / 1 gm/cm^2 10 m/s^2 = 10 m ~ 34 feet

But, there are other forces present in water which are due to surface tension. By the way, when people say negative pressure they really mean tension. If you take a rubber band and pull on it, it streches it and you are effectively providing negative pressure (yes, this is a stupid way of saying, there is tension in the rubber).

[edit] A small correction

I am not a specialist on this area but I have been trying to make sense by reading this page and the linked pages. I realy had trouble understanding the principle of operation of transpirational pull because of two wrong words used in the following paragraph. I capitalized these words and I believe they must be replaced by the parenthesised words. This way, what this article and the related ones claim makes sense.

"A common misconception is that water moves in PLANTS(XYLEM) by capillary action, the movement of water along a small-diameter conduit (such as a capillary) as a result of surface tension in the meniscus at the leading surface of the moving water. Surface tension does play a critical role in water movement in PLANTS(XYLEM), as described above, but the relevant surface occurs at the site of evaporation within leaves, not within the xylem conduits, and that surface does not typically proceed along any conduit. Water movement within the xylem conduits is driven by a pressure gradient, not by capillary action."