Shut up! A thunderstorm's on the way.

For the first time, biologists have shown that a flower pinches shut during thunderstorms, shielding its reproductive parts from pounding, drowning rain.

Despite their reputation as a stick-in-the-mud, most plant species can move some part of their anatomy, explains William K. Smith of Wake Forest University in Winston-Salem, N.C. However, little research has focused on movements of blooms themselves.

Lab experiments in the 1930s showed that flowers of the narrow-leafed gentian, Gentiana algida, close when temperatures drop. A native of high-mountain and northerly zones in North America and Asia, the gentian's small, tubular flowers typically open in threesomes, sticking upright from tufts of leaves. Smith decided to take a closer look at the plant at the behest of a long-time hiker in the Rocky Mountains who suspected that gentian blooms close during summer thunderstorms.

Working at three spots in the Rockies, Smith and Michael R. Bynum, now of the University of Wyoming in Laramie, found that the flowers indeed constrict about 10 percent per minute as a storm builds. When the squall blows away, the blooms reopen.

The researchers also mimicked storm conditions by blowing ice-chilled air over gentian blooms, which responded by closing up. The blooms reopened when the researchers warmed them.

Smith and Bynum also forced flowers to stay open by tucking waxed-paper cones inside. Blooms that couldn't close for several weeks produced less than a third as many seeds as unfettered flowers did, the researchers report in the June AMERICAN JOURNAL OF BOTANY. The continuously-open blooms also lost up to half their pollen. Smith says that such reproductive disasters could explain what drove the evolution of storm-shutting flowers.