Gases such as CFCs―originally developed for use in refrigeration units and air conditioners―are most often blamed for the depletion of the ozone layer, but many of the gases eating away at the ozone are actually not man-made. Until recently, the origin of these substances had eluded scientists. Preliminary tests, however, suggest that a common type of forest fungus may be responsible for at least some of the ozone-depleting gases in the earth's atmosphere. The fungus grows around the roots of trees, where it gathers nitrogen and phosphorus from the soil and shares these vital nutrients with the trees. In return, the fungus receives carbohydrates from its tree hosts.    This helps to maintain the health of temperate rainforests where the fungus is found. The same may not be true for the ozone layer. While each gram of the fungus gives out only a few millionths of a gram of gas per day it makes up as much as 15 % of organic matter in the soil in which it grows in forests throughout the world. Among the gases the fungus gives out are methyl bromide, which is believed to cause about 10 % of ozone destruction, and methyl chloride, which is blamed for another 15 % of the destruction. Scientists have long known that some of the methyl bromide in the atmosphere comes from farming and other human activities, but they have not been able to identify the source of at least a quarter of the atmosphere's methyl bromide. The search has now begun.   Further tests are being conducted to see whether the fungus really is responsible for a significant amount of ozone-depleting gases. In addition, scientists want to know how the world's changing climate may cause changes in fungus activity. In any case, the seemingly benign forest fungus is being looked at in a completely new light.