Mary Nines, age 10, of Winston Salem, Forth Carolina, for her question:

Why does moss grow on the north sides of trees?

Some people expect the mosses to guide them when lost in the woods. This would be logical if these pretty little plants grew only on the north sides of the trees. But if we search the woods, especially in the dim, shady corners, we find mosses growing on all sides of the tree trunks. If we trust them to guide us, we certainly will never get home in tire for dinner.

The soft velvety mosses covered some of the earth's first plants. They arrived hundreds of millions of years ago and thousands of different varieties thrive in the modern  world, All of them are alike in many ways. But each variety has a few features that make it special.  It is true that most mosses like moist shady places. And most of those that grow on tree trunks and prefer the shady north side, where the sunbeams never fall directly upon them, However, a few types enjoy a few hours of sunshine and some even thrive right cut 11.m the full light of day. These mosses are at home on the east, west or even the south side of a tree.

There are naturally reasons why most of these velvety little plants prefer moist and shady places. They have no true roots and no true stems or leaves, no true flowers or seeds. Their outlets soak up water with dissolved minerals. But the little plants need a supply of dew drops and misty moisture nestled among their dense greenery. When the sun dries; them out, their delicate foliage withers and dies. For this reason, all but a few hardy types thrive in shady places and those that grow on trees north of the equator do better on the north side.

You need a magnifying glass and plenty of time to behold the wondrous beauty of the little mosses. Some look like fairy bushes, others like doll house vines. Some stand like midget trees in a crowded pygmy forest. Other types are topped with tassels or ferry fronds, with ruffles or leafy frills. Others look like fat pin¬cushions of vivid green. These solid bumps are the sturdy ones most likely to grow on the sunny side of a tree.

Each type strives hard to select suitable spots for the .next generation. All mosses produce millions of tiny spores that drift in the breezes like dusty clouds. Some sail out to sea, others waft up snowy mountains and many come to rest on sunbaked  

Why does moss grow   for Thursday, June 3, 1971 plains and hillsides. None of these spores survive. But a few lucky ones land in suitable, moist shady places. They anchor themselves with threads called rhizoids and start to absorb moisture with dissolved minerals. They use their nourishment to grow new cells and soon they become new patches of moss.

Many plants grow flowers and seeds to reproduce themselves. But the parent moss plants sprout skinny little green threads, topped with capsules of either sperm cells or egg cells. On a dewy morning, when the moss is wet and slippery, the sperm capsules break open and the cells swim over to fertilize the egg cells. Each fertilized cell grows rootlets down into the moss. Then it sprouts a little stalk, topped with a capsule of spores. At last this tiny box pops its tiny lid and the ripe spores float off on the breezes.