Doreen Hom, age 11, of San Diego, California, for her question:

Why must deciduous trees shed their leaves?

Some people plant a maple or some other deciduous tree outside a big window or beside a porch. In winter, its bare branches let in the sunshine and reveal the frosty scene. In summer its leafy branches provide cool shade. However, nature did not design deciduous trees to provide us with this nice seasonal arrangement. It so happens that their delicate leaves cannot survive the winter. So they are shed in the fall and a new crop grows in the spring.

Deciduous leaves are papery thin. Their cell walls are thinner¬ than thin and the cells are filled with liquid sap. Plant sap is mostly water and we know what happens to water when the temperature drops below 32 degrees F. It freezes to solid ice. If deciduous trees kept their leaves through the winter, their cell walls would be ripped and torn by tiny daggers of ice. The shattered tissues would wither and turn black. Instead, deciduous trees usually shed their leaves before the first frost.