SSS 2008-06-12（在线收听）

This is Scientific American's 60-Second Science, I'm Karen Hopkin. This'll just take a minute.

With temperatures approaching 100 degrees in the eastern U.S. this week, it’s amazing that the leaves on the trees don’t simply burst into flames. Maybe one reason they don’t is that no matter how hot or cold it gets, leaves tend to maintain themselves at a nice, climate-controlled 70 degrees. That’s according to a report in the online issue of the journal Nature. Scientists had always assumed that the temperature of a leaf was the same as the surrounding air. And why wouldn’t they? It’s not like trees are warm-blooded. But University of Pennsylvania researchers examined leaves from 39 tree species in 25 different locations, ranging from inside the Arctic circle to the island of Puerto Rico. And they found that no matter where the leaf comes from, its average temperature is the same. The explanation: leaves from the tropics have evolved mechanisms for keeping cool, say by angling themselves away from the sun. And trees in polar regions have come up with ways to heat their leaves, by putting out more leaves per branch, for example, so they can essentially huddle for warmth. After all, what choice does a tree have in bad weather? It can’t just pack up its trunk and leaf.

Thanks for the minute for Scientific American's 60-Second Science, I'm Karen Hopkin.