John Boardman, age 15, of Allentown, Pennsylvania, for his question:

Do trees really use oxygen?

Some weeks ago Andy reported that trees use oxygen and return carbon dioxide to the air. This was challenged by several polite friends    and a couple of people not so friendly. They were sure that Andy told the story backwards because plants use carbon dioxide and give off oxygen. The fact is that both statements are true    and either one tells only half of the story.

Almost everybody knows that green plants absorb carbon dioxide and pour out breathable oxygen. During the miraculous process of photosynthesis, they use the energy of sunlight to manufacture basic plant food from water and dissolved chemicals which they get from the soil    and carbon dioxide which they get from the air. The "waste" gas from this wondrous operation is oxygen. All this is true. But plants also carry on another exchange of gases.

The living cells of both plants and animals use oxygen as a fuel for their chemical activities. These miraculous activities are the very processes of life itself. In animals, this operation is called respiration, or breathing. In plants it is called transpiration. Both respiration and transpiration use gaseous oxygen as a fuel and produce waste carbon dioxide.

We breathe in oxygen and breathe out carbon dioxide day and night. So do trees and other plants. We cannot reuse any of this waste carbon dioxide. But the plants can. Day and night, the plant and animal worlds use oxygen and give off carbon dioxide. During the daylight hours, green plants also use the carbon dioxide and give off oxygen.

The surfaces of green leaves and stems are riddled with small pores, which allow the gaseous air to pass in and out. As this mixture of gases circulates among the living plant cells, they take the oxygen molecules they need and return molecules of carbon dioxide  ¬which eventually seep out through the pores. This goes on day and night.

At daybreak, green chlorophyll cells start up their photosynthesis operation. They absorb molecules of carbon dioxide from the air that seeps in through the pores. Eventually the "waste" gas from the photosynthesis process seeps out through the pores. And it is oxygen  ¬oxygen to supply all the plants, animals and people with gaseous fuel to carry on the processes of life.

We keep plants indoors, thinking that they pour fresh breathable oxygen into the air. So they do, but only during daylight. After dark, photosynthesis which produces oxygen shuts dower. But plant transpiration continues to use oxygen and give out waste carbon dioxide. At night, the plant and animal world compete for the same gases.