How Trees Bend the Laws of Physics


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How Trees Bend the Laws of Physics



Hope this was worth the wait! So many people helped with this video: Prof John Sperry, Hank Green, Henry Reich, CGP Grey, Prof Poliakoff, my mum filmed for me in beautiful Stanley Park and Jen S helped with the fourth version of the script.

Prof John Sperry http://biologylabs.utah.edu/sperry/john.html
Hank Green (SciShow) http://www.youtube.com/user/scishow
Henry Reich (minutephysics) http://www.youtube.com/user/minutephysics
CGP Grey http://www.youtube.com/user/cgpgrey
Prof Poliakoff (Periodic Videos) http://www.youtube.com/user/periodicvideos

Also thanks to the Palais de la Decouverte – they helped me with the whole vacuum pump setup in Paris. No, I could not actually suck water up 10m – I did about 4m, but the vacuum pump was easily able to do it and I saw spontaneous boiling on all of our various trials. Footage from this may end up on 2Veritasium.

Trees create immense negative pressures of 10’s of atmospheres by evaporating water from nanoscale pores, sucking water up 100m in a state where it should be boiling but can’t because the perfect xylem tubes contain no air bubbles, just so that most of it can evaporate in the process of absorbing a couple molecules of carbon dioxide. Now I didn’t mention the cohesion of water (that it sticks to itself well) but this is implicit in the description of negative pressure, strong surface tension etc.

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44 Comments

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  1. if a tree cant have air bubbles within it otherwise they would act as nucleation sites, and your justification is that the xylem is air tight from the start then how do tree clippings that you plant avoid this?

  2. He saying an average 20 years old trees consume more water than a 20 years old teenager in one single year, there are even 40 years old teenage infact on top of a thousand year old tree in which the two of them are not likely to meet up for a drink of water

  3. I also heard another additional explanation. The water contains dissolved salts and sugars, when then the water evaporates, these materials are left behind and so makes the remaining water heavier, so when it sinks down, it drags the lighter water with it in a circulation system.

    Also, in cold winters, the strings of water may break. Does that mean the water may start to boil in the spring?

  4. i don't see why there is a big differentiation between gas and liquid pressure. I've been taught that gas behavior is just like a less-dense liquid, that we inhabit the bottom of an ocean of gases. Negative pressure is less of a far-fetched idea to me than the concept that liquid pressure behaves inherently different from gas pressure.

  5. Great video. I knew that you can not suck water higher than 10m and I knew that capillary could not bring the water higher than several dm. But I never ever thought about how the water could go up in trees.