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Tree plumbing.
Conifers thrive thanks to special valves that ease upward water flow.

The Grinch Who (Almost) Stole Christmas

Liz is a staff writer for Science.

Those of us who celebrate Christmas tend to take fir and spruce trees for granted around the holiday season. But without a special modification that allows these trees to efficiently transport water, we might be hanging our ornaments on a ficus instead, according to a new study.

In order for photosynthesis to occur, tall trees must supply their uppermost leaves with water, which is pulled up from the roots by evaporation. Angiosperms such as oaks and willows accomplish this using a series of centimeters-long, tube-shaped cellular pipes. Tiny valves made of cellulose membranes connect each "pipe" and help keep air bubbles out. Christmas trees and other conifers have much shorter pipe cells, however, and therefore must use many more valves than angiosperms. This should create more resistance and make it harder for them to transport water. But they don't have any trouble at all, says John Sperry, a plant biologist at the University of Utah in Salt Lake City.

Sperry, Utah graduate student Jarmila Pittermann, and their colleagues discovered why conifers had it so easy by measuring water flow in 18 species, including bald cypress, junipers, and redwoods. They also measured water flow in 29 species of angiosperms--including oaks, maples, and creosote bushes. For all 47 species, the researchers calculated the resistance to flow as water was sucked upwards. It was about the same for pipes of equal diameter, the researchers report 23 December in Science.

The reason, says Pittermann, has to do with key differences in the valves. Angiosperm valves are simple membranes full of miniscule pores. In conifers, the valves consist of a circle of impermeable tissue surrounded by porous tissue. The conifer's pores are 100 times larger than those in angiosperms and allow water to pass through relatively easily. This efficiency more than makes up for the additional valves on the way to the tree top, Pittermann says.

The results "add clarity in how we understand transport of water in wood," says N. Michele Holbrook, a plant physiologist at Harvard. But the work also points to how conifers, which predate angiosperms and are often considered primitive, were able to survive once angiosperms populated Earth. Without these very special cells, says Jean-Christophe Domec, a plant physiologist at Oregon State University in Corvallis, conifers would be less able to compete with angiosperms, and conceivably, "there wouldn't be any conifers anymore."

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