Diana Henderson, age 11, and Bruce Henderson, age 8, of Fredericton, New Brunswick, Canada, for their question:

How do trees breathe after they lose their leaves?

In the plant world the process of breathing is called respiration. It is an exchange of gases between internal living cells and the air outside. The system is scaled down to a microscopic level to allow single molecules of gases to enter and exit. Almost all the so called breathing pores are located in the leaves that many trees shed in the fall. It is natural to wonder how these trees survive the winter months without the respiration necessary to keep their cells alive.

Deciduous trees that shed their leaves slow down all their cellular operations during the winter season. Their bare brown twigs stop lengthening and their secret buds form slowly, very slowly. A few meager layers of small woody cells form thin rings around their woody trunks, under the bark. During the summer, the leafy boughs provide plenty of oxygen to make possible the manufacture of sugary plant food, provide fuel for growth and a multitude of busy cellular activities.

The key to all this is the tiny breathing pores called stomata. Through their portals there is a constant exchange of oxygen and carbon dioxide gases in the air. Most of them crowd in the leaves, but they are also on the surfaces of stems and tender young twigs. In later life, the stomata on twigs and branches usually are partly overgrown by porous cells of bark. However, a certain number of them remain open and these may let in enough oxygen molecules to supply some of the minimum operations that go on during the leafless winters.

But the trees do not have to depend on them, for their miraculous living cells have another system to maintain their chemical activities. Its fuel supply is locked in molecules of sugar manufactured by the summery leaves and now stored in the cells. Without the help of oxygen, the sugar is broken down to yield carbon dioxide, alcohol and chemical energy to carry on cellular activities. This operation is called anaerobic respiration    meaning respiration without air. It may proceed during the leafless winter and a certain amount of anaerobic respiration also goes on during the summer in deeply buried cells that get less than their share of oxygen from outdoors.

Anaerobic activity may produce the chemical energy for a limited amount of growth and cellular activity. But remember, its energy is released from stored sugar molecules. To create these minuscule powerhouses, the plant needed a continuous exchange of gases with the air outdoors. And for this operation, it needed those stomata breathing pores on the surfaces of its leaves.

Stomata vary in different plants. As a rule, a pinprick is wide enough to hold 20,000 of their tiny, open portals. There may be from 25,000 to 250,000 in one square inch of leaf surface. Each stoma pore is set in cells of the leaf epidermis. It has two guard cells that open and close the tiny pores to adjust the exchange of gases to temperature, humidity, brilliant sunshine and the amount of water inside the plant.