A redwood leaf 100 m up needs water that started in the soil. How does the water get there? There is no heart and no pump in a tree. The answer, proposed by the Irish scientists Henry Dixon and John Joly in 1894, is the cohesion-tension theory: the water is pulled from the top, not pushed from the bottom.

Here is the chain of events:

• Evaporation. Inside a leaf, water evaporates from the wet walls of cells into tiny air spaces and leaves through the stomata (next lesson). The energy for this evaporation comes from the Sun, not from the tree’s own metabolism.
• Tension. As water leaves, the remaining water in the cell walls forms tiny curved surfaces (menisci) in pores that are only a fraction of a micrometre wide. Surface tension makes these curved surfaces pull the water behind them, so the water is under negative pressure, called tension.
• Cohesion and adhesion. Water molecules stick to each other (cohesion, from hydrogen bonds) and to the cellulose walls of the xylem tubes (adhesion). So the pull is passed down, through continuous unbroken columns of water, all the way to the roots.
• Uptake. Water enters at the roots to replace what was pulled out.