One old biology book had a line saying that for biology, it is still a mystery how exactly trees and plants work the ascent of sap all the way to the top of very large trees, or to other plants for that matter. Capillarity? Suction? Cell circulation? There were no convincing theories. Another doper in a thread on mysteries of science did also mention, recently, that this elusiveness is still the current state of affairs!
Is it really a mystery?
I thought the water that evaporated from the leaves of trees pulled more water in through the roots, and this is what moves molecules up a tree. Evidently you can hear these chains of water snapping on hot days.
I was under the same impression… capillary action and atmosphereic pressure differences weren’t enough to pull water all the way up through tall trees, but the evaporation of water is also a driving force and combined with the tensile strength (not sure if that’s the right word… force it takes to pull apart a sealed column of water) of the water in the capillaries was what kept the water moving up. It was explained to me in a physics class, and it was apparently no mystery; I just didn’t care enough to remember the exact explanation and reasoning.
My biology book has an entire chapter devoted to transport in plants. An overview of the chapter can be found here. In essence, the bulk flow of sap is solar-powered.
As I recall, the action of pulling water, nutirents, etc through evaporation in the leaves is called transpiration.
It’s still open for debate, Discovery Magazine had an article within the past three months that explained some theories and studies.
Here are great links:
http://www.massmaple.org/flow.html
And, Discover’s newest issue revisits the issue, discussing “negative pressure”.
Thanks for all those links guys!
Looks like more research is needed, I can see there are leading theories but it looks that plants are doing it with a combination of them; I guess the mystery is how do plants juggle those forces in a way that even the limit of how high suction can go (about 33 feet) is broken by taller trees.
Explaining the results of Eduard Strasburger’s experiment
Andrew K Fletcher
How do trees lift water to great heights? Evaporation from the leaves alters the density of the sap at the leaf, and gravity pulls the denser sap down. This generates a positive pressure in front of the falling sap, and a tension behind the falling sap, which causes a simple flow and return. This is a non-living physical force, that must flow, wherever evaporation takes place from a liquid that contains solutes.
The German botanist Eduard Strasburger’s famous experiment - where he killed all of the cells in a tree by cutting off the roots, while submerged in a bath of picric acid - demonstrated that transpiration and circulation was maintained for three weeks, after the death of the tree. This new paradigm explains how the bulk flow continued in the dead tree.
Andrew K Fletcher, UK