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There are several "ultimately important" biological phenomena that together have done most to change the face of the earth. Transpiration is one of them.* Indeed, so importantly that were transpiration not to exist, life as we know it would not exist. Vascular plants are THE efficient photosynthesizers making up the bulk of plantlife, upon which members of the other kingdoms feed. Interestingly, while the overall phenomenon of water movement from the roots up to the leaves is well known, how it happens has been a matter of conjecture for nearly two centuries.
Several proposals have been offered and, with the passage of decades, have become entrenched as "facts." These and their weaknesses are discussed in a section at the bottom of this page.
For now, let's pick up a generally overlooked study that opens the doors for supporting a newly discovered combination of biochemistry and biophysics. This work shows that transpiration is ATP-driven and is heavily based on osmotic principles. Then we shall make an osmotic device that will "transpire" just like a tree. So now let's split out osmosis and study that in some detail.
This osmosis component of transpiration will be studied in two aspects:
These Do NOT Explain Transpiration (reference)
Capillary activity. The height to which a capillary can lift water is a function of the diameter of the tube: the narrower, the higher the water will go. However, there is a limit. The limit is the functional diameter of a bunch of hydrogen-bonded water molecules. If the tube is narrower than that, the water will not rise. Thus the highest that a capillary can life water is somewhere in the range of one meter - high enough for most greenhouse plants, but not for trees. What is more is that the narrower the capillary, the less water will be able to flow up that tube. Large trees have trunks that transport 150 liters per day. That's a lot of water! Especially since it is in the form of viscous sap. So let us mark our score card for one meter of lift due to capillarity.
Leaf draw. Another way of saying this is "suction." Unfortunately, any life-form depending on suction to draw water upwards has a limit: atmospheric pressure. A hydrostatic head of about 10 meters equals atmospheric pressure. One cannot draw water upwards more than that. Anything more will result in pulling a vacuum. Envision a mercury barometer. There is the column of mercury that is pushed up the tube by atmospheric pressure. Above the mercury is a vacuum. Were water to replace the mercury, the column would exceed 10 meters - but not much more. Our scorecard now has 10 meters added to the capillary's 1 meter for a total of 11 meters.
Root pressure. This idea has a lot of truth in it, but alone it cannot be the whole story. For example, when was the last time you cut off a plant near its base at ground level, and saw the sap squirt up as the cut was made?
Take a look at for another experiment.
Wright, Karen. Sept. 2002. "Antigravity Plumbing" in Discover Magazine 23: 20-21.
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