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Why do Christmas trees survive?

A new report shows conifers circumvent their circulation handicap using highly efficient valves

Graciela Flores(graciela_flores@nasw.org)

The secret to the success of conifers—which include the planet's tallest and oldest trees--lies on their highly efficient valves that let water flow through simple conduits as easily as in the more complex angiosperm conduit system, researchers report in this week's Science.

In conifers, water flows upward through short, single-celled tracheids. In angiosperms, in contrast, the pipes consist of longer, multicellular conduits called vessels. Both systems have connecting pits—or valves—concentrated at the end-walls of the conduits. In conifers, water has to cross many more valves, suggesting that they may have higher flow resistance than angiosperms. However, the researchers found exactly the opposite. They showed that conifers apparently compensate for their structural handicaps because they have evolved pits that have much lower resistance than the average angiosperm—59 times lower, in fact.

"When you compare a conifer tracheid with an angiosperm vessel," explained study author John S. Sperry of...

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