Jane Karr, age 12, of Clendenin, W. Va., for her question:

How does frost kill a  plant?

A plant is made of little boxy cells, all fitted together. The cells are fed by veins which carry sap to and fro, much as our blood vessels carry the nourishing blood through the cells of our bodies. In evergreens and sturdy tree trunks, this watery sap is protected from the air by thick cell walls. But the fragile leaves of certain smaller plants and the delicate leaves of the elm and oak have no such protection.

A frosty morning freezes the sap in the foliage of these plants. The watery sap swells and turns to ice which rips and tears the veins. Even when the ice melts, the torn veins can no longer carry sap throughout the plant. The once frozen leaves shrivel and turn black from want of nourishment.