Robert Ogden, age 13, of Longview, Wash,, for his question:

To what class do the ferns belong?

Scientists divide the plant world into groups from the simplest ones to the most complex ones. The scientific name for each group ends with phyte, which is coined from an older word for plant. The simplest group, Thallophyte, includes the algae, the fungi and the lichens. The next group includes the Bryophytes, meaning the moss plants. Group three includes the Pteridophytes. The first part of this fine sounding name is coined from the Greek word for fern.

The club mosses and the horsetails are also classed as pteridophytes, though neither looks at all like ferns. The club mosses look like fairy pine trees, just an inch or two tall and topped with cones. The horsetails are jointed stalks, two or three feet tall and tufted with circles of wiry leaves. The club mosses and the horsetails are classed with the ferns because all of these plants follow the same life cycle. Whatts more, their life story is different from that of any other plant.

In the modern world, the ferns are outclassed by the countless plants that bear seeds and flowers. But this was not always so. Some 300 million years ago, the ferns were among the most advanced of the plants. This was their heyday and they thrived in great varieties. Along with their relatives, the horsetails, they covered vast areas of the world with swampy forests. These were the forests which formed our coal beds. The imprint of many an ancient frond was left between layers of glossy black coal.

We have some 9,000 different ferns. Most of them carpet the damp woods with their feathery fronds. A few attach themselves high on friendly tree trunks and some of them are tropical ferns 40 feet tall.

For every fern, life is a complicated story in two stages. This life story in two parts is called the alternation of generations.

A feathery fern frond is actually a midrib bordered with fringes of dainty little leaves, A new life cycle begins when clusters of brown blisters form under the fronds. Each bump is a spore case, teeming with dusty spores. At the right time: a jointed rib springs apart, tearing the case and scattering the spores. Moso of them will be lost. The lucky spore that falls in a damp and shady spot will grow.

The developing spore becomes a prothallium a green leafy heart about a quarter inch wide. The prothallium puts down little roots to feed itself. It also grows egg cells and sperm cells. The ripe sperm cells wait for a moist morning when the prothallium is covered with a film of water. Then off they swim to join the egg cells. The work of the prothallium, a word which means before the plant, is now complete. Each fertilized egg is ready to carry on stage two of the fern life story. It grows from the prothallium and becomes a new fern plant.