This is one of those questions we all think we know the answer to :

Q : How does moisture from the soil reach the top of tall trees?
A : Capillary action, in which the surface tension of water drives moisture up the many narrow tubular vessels within the tree.

Were it true, the world’s tallest trees would be the size of a pencil

However there is a problem with this explanation : were it true, the world’s tallest trees would be the size of a pencil. Putting in the numbers, capillary action is likely to raise water in trees by just a few inches at best. Whatever the means by which the 350ft-plus redwood trees of northern California get their moisture, capillarity action is not the answer we’re looking for.

The real answer is transpiration, in which the water inside trees is pulled upwards by the effect of the sun’s heat. The myriad columns of water deliver nutrients and moisture throughout the tree until they reach the leaves, where the water evaporates into the surrounding air. The clever little tree can also regulate the rate of transpiration by the degree of stomatal opening (Stomas are openings or pores on the outside of a plant’s stem).

The real answer is transpiration

So powerful is the effect that it allows water to defy both gravity and the frictional drag of the plant tissues. Inevitably, however, there does come a point where the process simply can’t hoist the water any higher – not least because the column of water is torn apart under the competing forces. Even so, a study published in 2004 in the journal Nature showed that transpiration could still allow even the world’s tallest trees to grow another 50 ft, to a giddying height of 425 ft.

Transpiration and Water Movement in a Plant : A Laymans Guide


Water on the surface of the cells inside a leaf evaporates and then diffuses out of the leaf. This is transpiration. More water is drawn out of the xylem cells inside the leaf to replace what’s lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to up to the leaves.