Trees are amazing! They have the ability to pull up and transport large quantities
of water. A big tree can absorb 200 liters of water per day and twice as much on a hot day.
Large amounts of water are absorbed by the roots, transported up the trunk and lost as water vapor
through the leaves in a process called transpiration. Water is used to support cellular
metabolism and photosynthesis in the leaves too, but most of the water exits via transpiration.
How trees can pump water from the roots through the trunk and out to the leaves
has been the subject of studies for over 130 years. After all, if you cut into a tree, you
won t find any pumps just wood. Nevertheless, it is worthwhile reviewing how suction pumps work.
These pumps work by creating low pressure at the top of a column of water. It is the pressure
provided by the atmosphere that overcomes the weight of the water and pushes it up the pipe.
Drinking a beverage through a straw works the same way. Sucking on the straw creates a lower pressure
in the mouth allowing the higher pressure of the atmosphere to push the liquid up the straw.
Similar forces are at play in trees. Transpiration through the microscopic pores in the leaves,
called stomata creates a negative pressure at the outermost reaches of the tree which pulls
the water up the trunk through the xylem, which are tiny pipes running the length of the tree.
But, to get a deeper understanding of how trees transport water, we have to look at
the properties of water itself, namely water cohesion and water adhesion.
Cohesion is the tendency of water molecules to stick to each other.
Cohesion is responsible for surface tension, the tendency for a water droplet to adopt a spherical
shape. Adhesion is the attractive force between water and other surfaces. The ability of water
to stick to surfaces depends a lot on the nature of the surface
Together, water cohesion and adhesion are responsible for capillary action.
Have you seen how water moves up glass tubes? The thinner the tube, the higher the water moves.
Because of adhesion, the water is pulled up the sides of the glass wall and because of cohesion,
the water column is pulled along. Water adhesion and cohesion play important
roles in water transport in trees as well. The theory explaining the transport of water
in vascular plants including trees is called the Cohesion-tension theory. Transpiration powers the
movement of water through the xylem. It creates negative pressure (or water potential) of minus
2 to minus 3 megapascals at the leaf surface and minus point five to minus 1 at the stems and small
branches. The air itself has a water potential of about minus 100 megapascals. At the other end,
in the roots, water is absorbed from the soil through osmosis. Minerals in the interior of the
root cells cause it to be hypertonic (that is, have a higher mineral concentration)
compared to the surrounding water in the soil), thus pulling in water through osmotic pressure.
In the parlance of cohesion-tension theory, the cells in the roots have a lower potential than the
water in the soil, but a higher potential than the leaves, and water always moves down the gradient
from higher potential to lower potential. Cohesion and adhesion also contribute to water transport,
drawing water up through the xylem like water runs up a capillary tube.
At night, transpiration stops, but the water column maintains its integrity because of the
adhesion and cohesion properties of water. In 2020 scientists at Virginia Tech demonstrated
water flow via transpiration in a synthetic tree. They started with a nanoporous ceramic disk
representing a leaf through which water could evaporate. Then they embedded 19 silicone tubes
into the bottom of the disk creating an air-tight seal. The silicone tubes represent a bundle of
xylem. These scientists immersed one end of this capillary tube bundle in a tub of water
and witnessed the upward flow of water against gravity to a height of three meters. The total
mass of the water in the tubes was 458 g (1 lb in water weight). As water evaporated from the disk,
there was a water potential at the leaf, calculated to be about minus 30 kilopascals,
which drew the water up from the reservoir. This is a clear laboratory demonstration of the physics
underlying the cohesion-tension theory. If you d like to see more animated science videos,
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