Dina Meredith, age 11, of Wichita, Kan., for her question:

ARE BACTERIA PLANTS OR ANIMALS?

There are perhaps a couple of million different living things on the earth. The science of creature classification is merely a few generations old. Actually the patient experts have done wonders to sort out all the plants and animals in such a short time. So let's be extra polite when a few items are still on the undecided list.

If we could line up a row of 25,000 average size bacteria, shoulder to shoulder, they would measure about one inch. Hundreds could perch on the head of a pin and, no doubt, thousands reside in various crevices of your skin, in your mouth and on your hair. Others perform useful duties down there in your digestive system.

Naturally such bitsy beings could not be studied until the microscope was invented. They were spotted first by Anthony van Leewenhoek, who lived in the Dutch town of Delft.  While examining tooth scrapings under his microscope, he saw some tiny strangers. Since they seemed to be wrigglers, he called them animalcula, meaning little animals. We now call them tooth decay bacteria.

For a long time bacteria were regarded as mini members of the animal kingdom. With microscopes and lots of patient research, scientists learned more about the tiny organisms and their way of life. In some ways they seemed more like plants than animals. And until a few decades ago, most experts regarded bacteria as mini members of the plant kingdom.

Now we come to modern times, when researchers use very high power microscopes and very precise methods of analysis. Since bacteria have qualities of both plants and animals, most experts refuse to classify them in either of the two major kingdoms. Hence it seemed logical to place them in a separate group of their own.

The new classification was called Protista. It was coined from an older word meaning first  and suggests the original, most simple forms of life. Protista may be regarded as a kingdom, a third kingdom of tiny organisms that, strictly speaking, are neither plants nor animals.

Some scientists prefer to use two kingdoms to classify the realm of mini organisms. Those with more animal like features, such as the protozoa, are classed in the kingdom Protista. Bacteria, blue green algae and others with more plant¬like features are classed in the kingdom Monera. In any case, though, bacteria have both plant and animal features and they belong in a separate group of their own.