How giant redwoods hoist water up 30 stories
 Q:
How is a giant redwood able to hoist water all the way from the roots to the top
of the tree?
[National Park Service] Sequoias--the largest living
organisms.
A: It's certainly a long way up--as much as 100 meters, as
tall as a 30-story building. A tree releases as much as 160 gallons of water in
a day through its leaves and, by necessity, moves that water up from its roots.
How does the redwood manage?
Redwoods have a system of interconnected wood cells for carrying water. The
hollow, short, thin cells are stacked intricately to form an incredibly tall
column, extending from the roots through the branches and stems to the leaves.
The cells are dead wood that function as pipes with pitted openings along the
sides of the pipe for water to pass between adjacent cells. The small cells
rarely exceed a quarter of an inch in length (5 millimeters) and are only about
30 microns in diameter (about three times the diameter of a red blood cell).
Redwoods must form a continuous column of water within this pipe in order to
move water through the pipe. We believe redwoods form the column when the tree
is a newly germinated seedling. The tree maintains the water column intact
throughout its life time. Consider the wind and how it tosses trees around. Only
the millions of small compartments that enclose the water keep the water column
whole. A single pipe would not work.
Two forces move the water: a push and a pull.
Roots do most of the pushing but capillary action (the tendency of water to rise
in a thin tube by flowing up the walls of the tube) kicks in a small pressure.
The push can support a column of water about three yards high (two to three
meters). The pull takes over from there.
The driving engine behind the pull is the evaporation of water and the
attraction between water molecules. The molecules have both a positive and a
negatively charged part and, therefore, stick together with a strong
force--experimentally determined to be25 to 30 atmospheres. That's enough to
crush a World War II submarine at 1000 feet below the surface. A redwood tree
routinely maintains a negative pressure of 14 atmospheres at the top of the
water column (with one atmosphere at the base) and the column withstands this
pressure without breaking. This is how it happens:
Water evaporating from the leaves starts the suction pull. A water molecule
evaporates from a leaf and pulls on the molecules around it as it departs. This
creates a small suction in the water column and pulls water from adjacent
water-conducting leaf cells. These molecules, in turn, attract those around
them. The chain continues to the ground and moves water from the roots to the
tree top just as a pump brings water to the surface from a well.
The Sun's energy, which evaporates the water from the leaf surface in the
first place, is the pump engine.
(Answered by April Holladay, science correspondent, April 18, 2001)
Further Surfing:
Scientific American: how trees get water to leaves
Comment
Readers' Comments:
- If water leaving the leaves generates the power to pull water up in a
tree, why bother. obviously the tree is filled with water or the tube system
wouldn't work. Steve, Corona, Canada
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