Jacqueline Strayer, age 12, of Fairfield, Connecticut, for her question:

Are viruses plants or animals?

Last week a question about slime molds took us down to creatures that populate a borderline world between the animal and plant kingdoms. The viruses take us deeper into this strange region. Their invisible world verges between the realm of life and nonliving chemical molecules.

Viruses are parasites that attack the living tissues of both palms and animals. No doubt there are more varieties, but the 50 or so known to science are all bandit microbes. A generation ago, some scientists were sure that the infinitesimal objects were not really living things at all. In dormant stages they may endure for decades under temperatures and other hardships that would destroy any plant or animal cell permanently. What's more, when they enter a suitable living cell they are aroused to activities that are very different from the normal processes of life.

In the past, it seemed logical to suspect that viruses may be non living chemicals that cause destructive reactions in living tissues. Many poisons, of course, are destructive, inorganic or nonliving chemicals.

And some experts suspected the viruses were toxic chemicals of this sort. These scientists refused to classify the viruses as living things, either plants or animals.

Even modern scientists have not settled this problem definitely. Chemists who study.' the medical aspects of viruses, tend to think of them as nonliving chemical substances. , Biologists who study the life processes of the infinitesimal parasites regard and treat viruses as living things with basic qualities that make them unique in the realm of life. But they do not know enough about them to classify them as plants or animals.

It takes at least a million of the miniature microbes to measure an inch. Aft electron microscope is needed to study their shadowy shapes. Their activates are studied by intricate equipment that appears to be borrowed from some weird science lab of the far future. The infinitesimal microbes may resemble bats or balls, sausages or commas. In suitable surroundings, they move and multiply and, like living cells, the teeming multitudes can mutate, or alter their structures.

Each virus is a wad of simple chemicals, the very same chemicals in the tiny nucleus at the heart of every living cell. All plants and animals, of course, are built from living cells and each cell is organized and run by its tiny nucleus. Its key chemicals are nucleic acids known as DNA and RNA. And a miniscule virus is a wad of these complex organic chemicals. It seems to be identical with the vital core of a living cell. This is why microbiologists treat it as a form of life, though the evidence is much too scanty to assign it to the plant or animal kingdom.

In the nucleus of a healthy cell, the DNA molecules. carry the blueprint for the busy .life processes. They issue orders that are obeyed and handed on by RNA molecules. A virus is small enough to filter through the tissues of a suitable plant or animal. It reaches the. nucleus and takes over the duties of the DNA. It forces these blueprint molecules to issue different orders of its own.. The cell must neglect its normal duties to produce copies of the virus. In some virus infections, a p ant or animal can fight back and. bring the disease under control. But sometimes the living organism is completely destroyed by the swarming invasion of a virus parasite.