BOZEMAN, Mont. — Walking across the campus of Montana State University here, David Sands, a plant pathologist, says the blanket of snow draped over the mountains around town contains a surprise.

The cause of most of it, he said, is a living organism, a bacterium, called pseudomonas syringae.

In the last few years, Dr. Sands and other researchers have accumulated evidence that the well-known group of bacteria, long known to live on agricultural crops, are far more widespread and may be part of a little-studied weather ecosystem. The principle is well accepted, but how widespread the phenomenon is remains a matter of debate.

The accepted precipitation model is that soot, dust and other inert things form the nuclei for raindrops and snowflakes. Scientists have found these bacteria in abundance on the leaves of a wide range of wild and domestic plants, including trees and grasses, everywhere they have looked, including Montana, Morocco, France, the Yukon and in the long buried ice of Antarctica. The bacteria have been found in clouds and in streams and irrigation ditches. In one study of several mountaintops here, 70 percent of the snow crystals examined had formed around a bacterial nucleus.

Some of the bacteria promote freezing as a means of attacking plants. They make proteins that will trigger freezing at higher temperatures than usual and the resulting water ice damages the plant, giving the bacteria access to the nutrients they need.