How does water get to the top of a tree?
Photo: mr.rich
You might think that gravity would keep water low in the tree.
But water can rise all the way to the highest leaf because of a phenomenon called “transpiration.”
Transpiration is the loss of water vapor to the atmosphere through leaves and stems – and it works because water has a tendency to diffuse or evaporate. That means water always moves from wetter to drier – even against gravity.
Inside a tree, water is pulled from the roots up to the leaves through a network of microscopic tubes called xylem – made up of dead cells that have holes at the ends that are joined together to make “pipes” that water can flow through. Water is sucked up through the xylem like through a straw.
Plant physiologists calculate that the tension created by transpiration is strong enough to pull water up five hundred feet – higher than any tree is tall. The tallest tree is Australia’s Eucalyptus Regans, which can grow to be over three hundred feet high.
In the leaves, the water evaporates through tiny holes, which open and close to regulate the amount of evaporation. Ultimately, the evaporated water returns to the soil in the form of rain, and the cycle starts again.
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