Dean Waldschmidt, age 11, of Henry, Illinois, for his question:

Do trees use carbon dioxide in winter?

This problem baffles many children and grown ups too. Perhaps they expect to solve it in a neat row, like one plus one equals two. However, the world of nature tends to ignore such simple arithmetic. For example, the gaseous exchanges of the trees operate continuously in endless cycles. What's more, there are wheels within wheels as the cycles change with the seasons.

Everything alive exchanges gases with the atmosphere. Day and night, we trade our waste carbon dioxide for fresh oxygen. So do the animals and, day and night, so do the plants. If this were the whole story, in a short while the atmosphere would be overloaded with carbon dioxide and every living plant, animal and person would be gasping for oxygen.

Obviously this exchange of gases is merely one phase of a far more complicated global cycle. Somehow the atmosphere must manage to acquire continuous supplies of fresh oxygen    and also dispose of its surplus carbon dioxide. These vital problems concern the survival of all life on earth. And in matters of this sort, we can count on nature to come up with a tricky scheme that practically operates itself.

The plant world can create its own basic food, simply by using sunlight to build sugar from carbon dioxide and water. This solves the problem of surplus carbon dioxide in the atmosphere. The plants use it to make their basic .food. It so happens that the waste gas from this photosynthesis operation is oxygen, which pours into the air. This solves the problem of renewing the atmosphere's oxygen supplies.

This double cycle is simple enough to understand. However, to do their part, the plants need a miraculous green substance called chlorophyll. Evergreens keep their chlorophyll all through the year. Whenever the sun shines, summer and winter, they take in carbon dioxide and return oxygen as they carry on their photosynthesis. But the deciduous trees, those that lose their leaves in the fall, do this only during the time they carry leaves. In winter, they do not take carbon dioxide from the air.

However, let's not forget the other phase of the gaseous exchange. During the process of respiration, plants absorb oxygen and return carbon dioxide to the air. They do this as we do, day and night through all seasons. In winter, the leafless trees use more oxygen than they give back.

You might expect that the atmosphere has less oxygen during the winter. Not at all. This problem is averted by a tricky switch of seasons. Summers and winters alternate north and south of the equator. And the planetary winds distribute the atmosphere's two vital exchangeable gases around the globe. North and south, day and night, summer and winter these complicated cycles weave around for the good of one and all.