Threatened mothers have tougher offspring.

Radish plants and water fleas that soup up their defenses pass the weaponry down to their offspring, even in experiments where the danger has been removed.

"This is the first time anyone has found an induced defense across generations," says Anurag A. Agrawal of the University of California, Davis. He and his colleagues report their find in the Sept. 2 NATURE.

The discovery bears a spooky parallel to the discredited ideas of 19th-century biologist Jean-Baptiste Lamarck, who argued that species inherit acquired characteristics. For example, Agrawal says, "if I'm a bodybuilder, my babies will have big muscles." Lamarck didn't know about genes, but people interpret his idea as implying genetic change.

"This isn't Lamarckian; it doesn't get fixed in the genes," insists coauthor Ralph Tollrian of Ludwig-Maximilians-University in Munich. Still, genes do switch on to produce these defenses, Agrawal points out. Although he wouldn't describe the effects as Lamarckian, he says, "it's on the edge."

Induced defenses turn up in species from one-celled pond ciliates to humans with their fancy immune systems, Tollrian notes. Learning about the inheritance of such defenses might open new agricultural possibilities. He suggests that evoking defenses in a crop could yield seed needing less pest control.

To check for inherited effects, his lab raised one generation of water fleas near predatory phantom midge larvae. The water fleas grew huge helmets, halving mortality rates. When moved to safe water, they produced daughters with big helmets in all three broods that the researchers checked. Granddaughters, too, had somewhat enlarged crests.

Tollrian didn't look for paternal effects because the water fleas reproduced without sex, as they do in the wild for all but about one brood a year. He wouldn't be surprised, he says, to find some influences of dads in other species.

Agrawal did corresponding experiments with wild radish plants. When cabbage butterfly caterpillars chewed the foliage, new leaves unfurled with 10 times the concentrations of mustard oil glycosides and 30 percent more spikes compared with unmolested plants. The defenses work: The fortified plants made 60 percent more seed.

The seedlings of the embattled radishes bristled with defenses, too. Caterpillars gained only 80 percent as much weight when attacking tough seedlings as when eating undefended controls.

What's the mechanism? "That's the million-dollar question," Agrawal sighs.

Ellen L. Simms of the University of California, Berkeley muses that passing down defenses could help explain the puzzling diversity within a plant species' characteristics. "They don't evolve in a really straightforward way," she says.