• Question: How does a tree pass water round itself without the use of a pump? :)

    Asked by to Amelia, Izzy on 20 Mar 2014. This question was also asked by .
    • Photo: anon

      anon answered on 20 Mar 2014:

      Think of the stem of a plant, or even the trunk of a tree like a very very long straw.

      When you suck on a straw you create a pressure difference, because water sticks to itself (like in water droplets) the water moves up the straw. This process is called mass flow

      In the many leaves of a plant, processes take place which remove water and create this pressure difference, and this is enough of a differnce to pull the water a very long distance indeed!

    • Photo: Isabel Webb

      Isabel Webb answered on 20 Mar 2014:

      This all works because of something called water tension. Water tension is forces that occur between water molecules, holding them together. These forces are very strong.

      Water is taken up in the roots, and then evaporates out of the leaves, with vessels linking the two. These vessels contain long streams of water with no air gaps – like a straw. As water evaporates, it pulls on the water behind it in the vessels, dragging it upwards. This drags all the water upwards, all the way from the roots. The plant then takes up more water from the roots. Plant scientists are fascinated by the ability of huge trees to do this – but they can! The forces inside these tall trees are massive.

      If air bubbles or other things get into the water vessels (these vessels are called xylem) they can break the chain of water and stop water moving up through the plant, causing it to wilt or even die.

      If you want, there is a good experiment to see this movement of water. Find some white flowers and instead of putting them in just water, put them in water mixed with ink. After a few days you will see the leaves and petals of the flowers turning the same colour as the ink, as it gets drawn up the stem!

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