Matt Phillips, age 15, of Watertown, N.Y., for his question:

WHAT IS A DIATOM?

A diatom is a tiny water plant of the kind called algae. But unlike some of the other algae, which include large seaweeds, a diatom consists of only one golden brown cell.

There are several thousand species of diatoms, including both saltwater and freshwater varieties. Some appear as brown, slimy coatings on stones and piles in water. Many of these one celled plants may hang together in chains, and in various other arrangements while others float free.

Diatoms can move by themselves through the water with jerky, creeping or pendulum like motions. The cell walls of the individual diatom are made up of two nearly equal halves, called valves. They are joined together somewhat as the two halves of a pillbox.

Floating diatoms 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 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.