Uffe Kousgaard wrote:
> I P1 har der netop været et indslag, hvor en herre (skovfoged tror jeg han
> var) udtalte sig om, at store bøgetræer kan suge vand op i trækronen med et
> tryk på 150 atm. Hvordan kan man suge med et tryk >1 atm? Måske han mener
> 1/150 atm ?
>
> Se f.eks.
http://da.wikipedia.org/wiki/Vakuum
>
> hilsen
> Uffe
Hej Uffe
Der er 2 "transportbånd" i træer - xylem (opadgående) og phloem
(nedadgående):
Opadgående "vandtransportbånd" i træer:
http://en.wikipedia.org/wiki/Xylem
Citat: "...
The xylem transports sap from the root up the plant: xylem sap consists
mainly of water and inorganic ions, although it can contain a number of
organic chemicals as well.
...
By far the most important cause of xylem sap flow is transpirational
pull. The reverse of root pressure, this is caused by the transpiration
of water. In larger plants such as trees, the root pressure and
transpirational pull work together as a pump that pulls sap from the
soil up to where it is transpired.
..."
Transpirational pull:
http://en.wikipedia.org/wiki/Transpirational_pull
Citat: "...
Transpirational pull results ultimately from the evaporation of water
from the surfaces of cells in the interior of the leaves. This
evaporation causes the surface of the water to pull back into the pores
of the cell wall. Inside the pores, the water forms a concave meniscus.
The high surface tension of water pulls the concavity outwards,
generating enough force to lift water as high as a *hundred meters* from
ground level to a tree's highest branches. This only works because the
vessels transporting the water are very small in diameter, otherwise
cavitation will break the water column.
...
Water movement within the xylem conduits is driven by a pressure
gradient, not by capillary action.
..."
Bemærk: 10 meter vandsøjle svarer til ca. 1 atm. trykændring. 100 meter
svarer derfor til ca. 10 atm. trykændring.
Nedadgående "sukker/vand-transportbånd" i træer:
(
http://da.wikipedia.org/wiki/Phloem )
http://en.wikipedia.org/wiki/Phloem
Citat: "...the phloem is composed of still-living cells that transport
sap...Cells in a sugar source "load" a sieve-tube element by actively
transporting solute molecules into it. This causes water to move into
the sieve-tube element by osmosis, creating pressure that pushes the sap
down the tube. In sugar sinks, cells actively transport solutes out of
the sieve-tube elements, producing the exactly opposite effect..."
-
Nogle træer laver "vandbanker" i op til 13 meters jorddybde:
2006-01-13, Sciencedaily: Deep-rooted Plants Have Much Greater Impact On
Climate Than Experts Thought:
http://www.sciencedaily.com/releases/2006/01/060112035906.htm
Citat: "...The tap roots transfer rainwater from the surface to
reservoirs deep underground and redistribute water...increases
photosynthesis and the evaporation of water...by 40 percent in the dry
season...During the wet season, these plants can store as much as 10
percent of the annual precipitation as deep as 13 meters (43 feet)
underground, to be tapped during the dry months...tree roots acting like
pipes to allow water to shift around much faster than it could otherwise
percolate through the soil..."
mvh/Glenn