How come leaves on a tree get water, and this at higher altitudes?

Answer

Dear Ludo,

A very good question indeed.

Water moves through plants or trees from the roots to the leaves through the xylem. This xylem is actually made up of a series of thin “tubes” next to each other. Water in such a xylem vessel actually forms a continuous water column from the root to the leaf.

In a plant, however, a liquid is not pumped upwards, but “pulled” upwards.
The force that pulls the water column upwards is the following:

At the underside of the leaves (in a deciduous tree) are the stomata (small openings). It is through these skin mouths that the water leaves the plant as water vapour. Under the stomata inside the leaf is a cavity (substomatal cavity) in that cavity the air is saturated with water vapor. The air outside the leaf is not saturated. According to the laws of diffusion of physics, water vapor always moves from a place with a higher concentration to a place with a lower concentration of water vapor. So the water vapor is drawn out of the leaf through the stomata by the dry outside air. The substomatal cavity then draws back water from the leaf, pulling the entire water column upwards. Down in the roots, the water is replenished with water from the soil.

Even with large trees (eg 60 m) it is the same floating force that plays. Obviously, greater forces have to be developed here to get the water up.

The driving force behind the upward movement of water through a plant is thus the difference in humidity between the air outside and inside the stomata. So in dry air (warm weather) plants will evaporate more. To limit water loss, plants can close their stomata.

Voila, I hope my answer clears things up.

If you want more explanation, you can always contact me and I can refer you to the necessary literature.

Regards,

dr ir Hans Verbeeck
Laboratory of Plant Ecology
Faculty of Bioscience Engineering
university of Ghent