Tracy Nelson, age 10, of Santa Clara, California, for her question:

Do trees breathe?

Plant respiration involves taking oxygen from the air and returning waste carbon dioxide, which is what we do by breathing. It also involves the use of oxygen as a fuel by all the living cells in a plant. Plant respiration provides the energy to break down the basic sugar manufactured by photosynthesis and to build the vast assortment of substances needed for growth. Oxygen is the fuel for these chemical operations and carbon dioxide is the waste by product.

A tree, of course, has no nose and no lungs to inhale oxygen and exhale carbon dioxide. But numerous small surface pores, mostly on the undersides of the leaves, allow mixed air molecules to seep in and out. When inside, these gas molecules seep among the cells. The cells take the oxygen they need and return carbon dioxide, which eventually seeps back outdoors through the same breathing pores.