Ruth Weber, age 11, of St. Louis, Missouri, for her question:
How can moss grow on bare rocks?
The mini mosses need moisture, lots of moisture, and most of them grow in damp shady woods. A few types manage to survive on bare rocks, sometimes in dry desert regions. Or so it seems. However, more often than not, what looks like a moss plant growing on a bare rock turns out to be something else. Chances are, it is a little lichen.
The modest little mosses have no true roots and no internal systems for transporting moisture from cell to cell. Like all plants, they must have moisture, but because of their limitations they must absorb it directly from the air and soil. Most of them live among the shady foliage of larger plants, where silvery raindrops from yesterday's shower are stored in secret crevices and moisture from the morning dew lingers in the air.
When we use a hand lens or some other lens that magnifies up to ten times, the hundreds of different mosses remind us of rich velvet carpets, clusters of delicate ferns and mini pine forests. A few do adjust to drier surroundings. For example, the wire rock moss thrives on a surface of bare rock. The rock orchard moss often grows on slopes where stony outcroppings poke through the soil.
If you examine these surroundings with care, you see that these mosses are anchored in rocky cracks and crevices where moisture tends to collect and linger. If what seems to be a moss plant is growing on a bare dry rock right out in the blazing noonday sun, chances are it is a lichen.
A lichen has its own remarkable system to cope with life on the surface of a bare rock. The system is called symbiosis, which means living by sharing. Actually, each amazing little lichen is two simple plants living together as one. One of the partners is a green or a blue green alga of the waterweed. The pasty fungus has no green chlorophyll to manufacture its own food from air, water and sunlight. But the spongy pockets among its tangled threads make it an excellent storer of moisture. The colored alga, of course, has chlorophyll to manufacture plant food. To perform this miracle it shares moisture from the spongy fungus. And the partners have a very complex system for sharing the alga's plant food.
From time to time, showers and dew drops drench even the desert rocks. The lichen partnership is built to store this moisture for days and even weeks. By swapping and sharing food and moisture, it can survive and thrive on the surface of a bald bare rock.
In most cases, the various lichens are flatter than the mosses. A few are green or greenish grey but many are tinted with pastel pinks and blue and some are golden yellow. Some of the lichen partners look like little buttons, some are delicate rosettes and many look like dainty designs painted on the rocks by artistic pixies.