The electron microscope images show bird's-eye-views of
the valves that allow water to move upward through conifer
trees (upper left) and angiosperms or flowering trees
(upper right), while the graphics show cross-sections
of water flow through both kinds of valves. Conifers --
including Christmas trees -- have valves with a central
"torus" through which water cannot pass, surrounded
by a porous "margo" through which water flows
easily. Flowering trees have homogenous porous valves
through which water flows less efficiently.
download high-resolution click here:
Credit: Springer Science and Business Media, G.L. Comstock,
W.A. Côté and E. Wheeler.
Conifer trees such these in Utah's Wasatch Range dominate
many of Earth's temperate forests despite an internal plumbing
problem: very short "pipes" that carry water up
from the roots. A University of Utah study found that conifer
trees have highly efficient valves that make up for that
handicap and let water flow easily, allowing conifers to
compete well with flowering trees.
download high-resolution click here:
Credit: Uwe Hacke, University of Utah.
Dec. 22, 2005 – Conifers such as Christmas trees suffer
a severe plumbing problem. The “pipes” that carry
water through firs, pines and other conifers are 10 times shorter
than those in flowering trees. But a University of Utah study
suggests why conifers not only survive but thrive: efficient microscopic
valves let water flow through conifers about as easily as it flows
through other trees.
“When you are sitting around and admiring your Christmas
tree, consider that it owes its existence in part to this clever
microscopic valve,” says John Sperry, a University of Utah
biology professor who led the research team. “Without these
valves, conifers could be much less common than they are, and
conceivably their survival might be marginal.”
The journal Science is publishing the study Dec. 23,
two days before Christmas.
Sperry says that if conifers had not evolved easy-flow valves
to make up for the short length of their water pipes or conduits,
“it is doubtful they could hold their own with angiosperms
[flowering trees] in today’s forests. It’s doubtful
they would dominate whole regions of North America.”
While scientists cannot really know if conifers might have gone
extinct without their efficient type of water valve, “what
this study shows is that without this valve, it would be 38 times
harder for conifers to take up water, which would put them at
a serious disadvantage in competition with flowering trees in
temperate forests,” says Sperry.
The study was part of a University of Utah doctoral thesis by
Jarmila Pitterman, now a postdoctoral fellow at the University
of California, Berkeley. She and Sperry conducted the study with
other University of Utah biologists: Uwe Hacke, a research assistant
professor; lab technician James Wheeler, who has since left for
graduate school at Harvard University; and Elzard Sikkema, an
The Plumbing System of Trees
The numerous parallel “pipes” that carry water upward
through the woody trunks of evergreen coniferous trees are single-celled
conduits called “tracheids” and are only a few millimeters
long (about one eighth of an inch). In flowering trees, the pipes
are multicellular conduits called vessels and are 10 times longer,
or a few centimeters long (more than one inch).
As a result, water moving up through an evergreen must pass through
10 times as many valves (known technically as “pits”)
as water moving up through the trunk of other trees. Sperry said
that should be a severe handicap for conifers in competing against
flowering trees for water.
Yet conifers thrive, and they dominate forests in many regions
of Earth. Hacke says the planet’s tallest trees are conifers:
redwoods and sequoias. So are the oldest trees, bristlecone pines.
So how did conifers overcome the handicap of short pipes?
Scientists already knew that the valves between water pipes or
conduits are far different in conifers than in angiosperms, or
flowering trees, but they did not know how that difference affected
water flow. In the new study, the biologists measured water flow
through twigs from 18 species of conifers and 29 species of angiosperms.
Conifers studied included Douglas fir, subalpine fir, white fir,
lodgepole pine, various spruces, Utah juniper, Rocky Mountain
juniper, redwoods, bald cypress and conifers known as podocarps
and araucarias from New Zealand and New Caledonia.
Angiosperms included oaks, willows, ash, various maples, hickory,
mulberry, creosote bushes, manzanita, serviceberry, mountain mahogany,
grapevine and others.
The researchers connected both ends of each twig to plastic tubing,
used an elevated reservoir’s gravity to force water into
one end of each twig, and then used an electronic balance to weigh
water dripping out the other end. Then, based on the number of
conduits and valves in twigs and their known dimensions, the biologists
calculated the resistance to water flow of both the conduits and
The scientists found that for conduits of the same diameter, resistance
to water flow in conifers was only 1.2 times greater than in flowering
trees – essentially the same. Sperry calls that “remarkable.”
And water flow actually was better in conifers than in flowering
trees in terms of resistance to flow per unit area of wood.
The pits or valves that connect the water conduits in trees not
only carry water up trees from the roots, but also prevent air
from entering the conduits and killing trees.
The Structure of Water Valves in Trees
Sperry says the range of conduit diameters vary but overlap for
conifers and flowering trees. The conduits or tracheids in conifers
range from 10 to 50 microns (millionths of a meter) in diameter,
while the conduits or vessels in flowering trees range from 15
to 110 microns.
The valves are in “end walls” at both ends of water
conduits. In conifers, there are about 25 to 50 valves at each
end of a conduit; in flowering trees there are many more.
These valves are disk-shaped membranes. In flowering trees, the
membranes are homogenous, with water seeping through microscopic
pores. But in conifers, the valve membranes have what is known
as “torus-margo” structure that resembles a bird’s-eye-view
of a circular trampoline.
“It’s like a trampoline in that the torus is the mat,
and the margo represents the supporting springs with holes between
them,” Sperry says. “The margo holds the torus in
place just like springs hold the trampoline in place.”
Water cannot pass through the central torus, but easily flows
through the margo pores, which are about 100 times larger than
the pore in flowering tree valves – on the scale of one
10-millionth of a meter versus one billionth of a meter.
The bottom line is that conifers have shorter conduits and fewer
valves, both of which would increase resistance to water flow,
“but they compensate for that because each individual valve
is so much more efficient,” Sperry says. “The flow
resistance through a valve of a given size is 59 times lower in
a Christmas tree than in an oak tree.”
Flowering trees have longer, more efficient conduits, but less
Evolution Produces Two Ways to Water a Tree
Sperry says conifers and flowering trees evolved with “two
solutions to the same problem.”
Conifers, which arose more than 280 million years ago, have primitive
conduits that are short and inefficient and evolved in some of
the oldest plants some 400 million years ago. The highly efficient,
torus-margo valves evolved in conifers and their relatives no
later than 220 million years ago, Sperry says.
Flowering plants evolved at least 146 million years ago and retained
inefficient valves that first appeared some 400 million years
ago in ferns, cycads and other primitive plants. But flowering
plants evolved longer conduits to get around the problem.
“The evolution of the specialized valve and the specialized
conduit are both ways of achieving more efficient water transport
within a tree,” for conifers and flowering trees, respectively,
He says that as angiosperms evolved and competed with conifers
for water, “it is quite possible that if conifers hadn’t
evolved this efficient valve, they wouldn’t have been as
conspicuous an element of today’s forests. Being at such
a tremendous disadvantage in the competition for water, it is
unlikely they would be such a dominant element in modern forests.”