Kay Keisler, age 12, of Columbia, S: .for her question:

What is the smallest plant?

The smallest plants are too small for our eyes to see. We call them microscopic because we need a microscope to study them. They are one‑celled plants called protophyta, meaning the first plant. Protophyta are the most simple and perhaps the first plants to exist on earth. ‑They include the algae, the yeasts, molds, mildews and. the dainty diatoms. Trillions of microscopic red algae color the Red Sea. Trillions of green algae form a green pond. scum. Millions of yeast cells make our daily bread and. there are 20 million diatom shells in every thimbleful of cleansing powder.

Maybe a very large alga is just big enough for our eyes to see as a tiny dot, nut most of these protophyta measure from 10,000 to 25,000 to an inch. They are truly microscopic but there are still smaller plants which make these midgets look like giants. These little one‑celled plants are the bacteria, so far as we know the smallest plants on earth.

Maybe you shudder at the word bacteria, thinking of germs and microbes. But this is hardly fair, since only a few of the countless varieties are our enemies. Most bacteria ignore us, some are our friends and some are absolutely necessary to life as we know it. Many bacteria measure only half a micron, or 50,000 to an inch.

There are bacteria in the soil, maybe 200 million of them in a shovelful. There are bacteria in the air, light enough to float on the breezes. There are bacteria in all outdoor waters ‑ rivers, lakes and strepma. There are bacteria in the polar snows and in the desert sands. In fact, there are but two places where bacteria cannot live. They cannot live. inside a fire and they cannot live inside healthy body cells.

Some bacteria can endure almost boiling water. They thrive in the: steamy waters of geysers and hot springs. Some have been tested and proved able to endure cold to within a few degrees of absolute zero: Strangest of all, there are bacteria which can live without oxygen.

Bacteria has no green chlorophyll, therefore cannot make its own food by photosynthesis. Its food must be liquid. Juices ooze through the cell wall to partly digest the food which is then absorbed inside the cell for complete digestion. Some bacteria live on decaying plant and animal material, some live as parasites in the body of a host. Some live on simple minerals such as salts and sulphur. When conditions are right, a bacterium can divide into two bacteria every 20 minutes.