Wednesday, 8 May 2013

Cohesion-tension theory: hug a tree

Hug a tree.

Try to pick a trunk that you can only just get your arms completely around, with your hands just touching. Remember this tree. Now go back to your tree at night and try hugging it again. Can your hands still meet? I haven't tried this myself - though I am planning to - but according to cohesion-tension theory, the diameter of a tree trunk is smaller during the day than it is at night. Cohesion-tension theory goes something like this.

The tree absorbs water into its roots, and the water travels up vessels called xylem, which go through the tree trunk. But how can the water travel upwards, against the downward pull of gravity?
a water molecule
Water molecules are polar. This means that one end of the molecule is more electropositive and the other, more electronegative. Opposites attract, so water coheres, or sticks together. That's why if you spill some drips of water, they form little round globules. As well as sticking to each other, water adheres with other things, like when you stack two wet tumblers together and can't pull them apart again.

Now, back to the tree. Water travels up the xylem under tension in a continuous stream. It's under tension because the water molecules are sticking to each other - cohesion - being pulled up by the transpiration pull. (Transpiration is the evaporation of water from the surface of the leaves via pores called stomata.)

The water molecules also stick to the walls of the xylem - adhesion. This pulls the xylem inwards, reducing their diameter. As a consequence, the diameter of the whole trunk reduces too.

The tree trunk is thinner in the day because it's sunny. High light intensity increases the rate of water evaporation but also opens the leaves' stomata, increasing the rate of transpiration. High rate of transpiration makes the transpiration pull stronger, so the water is under greater tension, so adheres to the xylem walls more, pulling them inwards more. That's why tree trunks are thinner during the day!

... Well, I think it's interesting, anyway.

I think the way tree roots and boughs branch out, dividing into smaller and smaller tendrils, is really beautiful. It's an image common in nature - just look at nerves and blood vessels in animals.

an angiogram (blood vessels)


  1. I love the patterns of trees and blood vessels! Some nice fashion finds :D x

  2. You wrote about it! And it was really freaking cool too! I totally understand why trees are thinner in the day now - in my head I'm kind of visualising it like when you stick one end of a drinking straw onto a surface which will stop any air getting in (like the bottom of your cup or your finger) then you suck on the other end and the straw goes all thin and 2D until you release the pressure...yeah..haha. That's crazy that such a small-seeming process in the relatively teeny xylem can have such a large effect on the global shape of the tree.

    Also I really love how you pair your science posts with fashion finds at the end. It works so well :)

    1. I am totally seeing your straw image. Pressure is a cool weird thing. We use the siphon effect to fill the fish bowl:

      So glad you like these science posts. They're so fun to research and write, too.


Thank you ♥

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