Dennis Richmond, age 15, of Columbia, Tenn., for his question:

WHAT IS A DIATOM?

A diatom is a tiny water plant of the kind called algae. Unlike some of the other algae, which include large seeweeds, a diatom consists of only one cell.

There are several thousand species of diatoms, including both salt water and fresh water types. Some appear as brown, blimy coatings on stones and piles in water.

Many of these one celled plants may hang together in chains and in various other arrangements. Still others float free. A diatom can move by giving a jerky, creeping or pendulum like motion.

The free, floating diatoms which grow in mid ocean and in lakes are a very important food for small sea animals. These, in turn, are eaten by fishes. If there were no diatoms, most of the fish of the world would die.

An earthy material composed largely of diatom shells is called diatomite. It is mined in California, Oregon, Washington, Nevada, Arizona and Florida. Diatomite is used as a polishing powder, abrasive, plastic filler, insulator, filter and in manufacturing explosives.