Kathy Johnston, age 11, of Richmond, Virginia, for her question:

Are diatoms plants or animals?

A microscope reveals that a tiny diatom lives inside a tiny glass house made from a pair of shells. Many animals, of course, build nests and burrows and many others have shells for houses. But most people would pooh pooh the idea of a shell building plant. Neverth less, a diatom is a plant.

Diatoms are related to the floating seaweeds and other members of the alga plant family. They are single celled plants, and the naked eye sees them as tiny dots. The microscope reveals them to be shaped like exquisite little jewel boxes. At least 10,000 different varieties have been classified and choosing the most beautiful would be impossible. Diatoms, like seaweeds, are water plants. They teem in the oceans and rivers, in streams and creeks, in stagnant ponds and pools. Medium sized diatoms measure 1,000 to an inch and there is no end to the variety of their shapes and delicate designs.

Each little jewel box is a pair of shells made mostly from silica. This durable mineral is dissolved from rocks by running streams, and invisible fragments are dumped into the sea. The single celled diatom plant extracts silica from water, fragment by fragment, and uses it to build two almost identical shells around itself. The shells are like two tiny trays, joined together by a slender seam to form a box. This is the basic plan. But every variety of diatom adds to it with delicate artwork.

Being a plant, the diatom contains green chlorophyll, which it uses in the presence of sunlight to manufacture its basic food from carbon dioxide and water. As a rule, the diatom also contains a yellowish pigment called diatomin. This may mask its chlorophyll green with amber. A tinge of green or brownish yellow shows through the glassy shell, and teeming diatom populations sometimes color the surface of seas and rivers.

You are not surprised to learn that the diatom shells are glassy because we use sand to make glass, and sand, remember, is made mostly of silica. We can make dozens of different shapes of glass and have countless designs to add to its natural beauty. But in the field of glassy artwork, the little diatoms can outdo the best of us. Among them we find squares and triangles, ovals and oblongs, globes and dewdrops, sticks and sickles, full moons and half moons, discs and daintily pointed doilies. Each of these basic shapes is further adorned with its own delicate design with buttons and bumps, pits and packets grooves and ridges. And some are etched with lacy nets of the finest lines.

But nature, it seems, does little or nothing to cherish this miniature artwork. The diatom's main purpose in life is to provide food for the fish and other water dwelling creatures. Diatoms by the ton are a vital link in the food chain that sustains life in the hungry ocean. They are rich in plankton, the food of the balleen whale. So you see, the world's biggest animal depends on some of the world's tiniest plants.

Diatoms need light to make their food and hence they must live near the sunlit surface of the sea. They also need carbon dioxide, and the ocean surface teems with fish that add their waste carbon dioxide to the water. The diatoms use it and return fresh oxygen for the fish. This parallels the give and take of these gases between plant and animal life on land. Every season, zillions of diatoms perish and their durable shells sink to the bottom. Some of these diatom deposits are mined and used to make cleaning powders and dozens of other useful items.