What's happening?

What makes water climb trees is still somewhat of a mystery. But scientists think it all depends on the special properties of water, and on the fact that the tubes are porous and very narrow.

As the tubes spread out into the leaves, heat from the sun evaporates the water molecules at the top. Because water tends to climb a short way up the walls of certain substances (like drinking glasses, for instance), the next molecules in line move up after those that evaporate.

Water molecules always hold tightly together, and when they're squished into very narrow tubes, they grip even more tightly, with enough strength to pull all the following water molecules along behind them. So as the molecules at the top move up, the whole chain moves up the tree. This only works, however, if the tubes are full of liquid to begin with, so trees and other plants have liquid-filled tubes from their earliest days as seedlings.

• Return to the experiment.

Excerpt from The Jumbo Book of Science by the Ontario Science Centre, used by permission of Kids Can Press Ltd., Toronto.