The first land plants on Earth probably did not simply move from the sea and set roots into the barren land and start growing. There was little or no soil, which is made up mostly of decayed plant matter, to hold moisture and nutrients.

So when the algae and fungi floating in primordial seas began to colonize the land, a cooperative strategy was needed to survive in the hostile territory, researchers believe. It involved fungi attaching themselves to plants so the fungi could be nourished by the plants and the plants could be given nutrients by the phosphorus- and nitrogen-gathering fungi through a two-way "valve" or arbuscle.

Probably more than 90 percent of the veined plants living now still have such a symbiosis between plant roots and fungi.

But the trouble with the historical tale is that researchers had been unable to find plant fossils that showed a plant root connected to fungi by the arbuscle, which would have been expected if ancient symbiosis were the survival strategy used by the first plants.

But in a recent issue of Science, researchers report the first known occurrence of fossil arbuscles. They were found linked to roots in plant fossils from Antarctica, dated more than 200 million years ago.

Sara Stubblefield and T.N. Taylor of Ohio State University and James Trappe of the Pacific Northwest Forest Experiment Station in Corvallis, Ore., reported the find.

Stubblefield said that it does not prove that symbiosis was a key mechanism used by the first land plants, but provides some evidence that these tiny structures were used early in the history of plants. She said she expects to find still earlier examples as research continues.