Pulling each other through bad times.

Pulling each other through bad times

Scientists believe that when the early plants first left theoceans and lakes for a home on land, they were greeted by a harsh environment, where dryness, ultraviolet radiation and nutritional problems made life exceedingly difficult. According to a popular theory, the earliest land plants survived these conditions through symbiotic relationships with fungi--a situation quite common today.

In a prevalent form of the modern symbiosis, fungi live in oraround a plant's roots. They aid the plant in the uptake of nitrogen and phosphorus, and receive needed carbohydrates in return. Scientists have yet to find conclusive evidence that this in fact took place in prehistoric times, but a recent find suggests that it did.

In previous support of the theory of early symbiosis, somescientists have pointed to fossils of plants that contain characteristic fungal structures. However, these structures have not given researchers enough information to determine exactly what kind of relationship was occurring, says paleobotanist Sara P. Stubblefield of Ohio State University in Columbus. In an apparent answer to this question, Stubblefield and her colleagues report in the July 3 SCIENCE that they have found fossilized fungal organs, called arbuscles, which Stubblefield says are the most definitive structural evidence for symbiosis. Scientists believe that arbuscles allow the exchange of nutrients between fungus and plant, but fossilized arbuscles had previously been unknown.

Although these fossils, dating back approximately 220 millionyears, do not prove any relationship, says Stubblefield, they strengthen the argument that symbiosis was occurring at that time. This discovery should help scientists in locating even older arbuscles, she says.