Kitty Azar, age 11, of Montgomery, Alabama for her question:
How does moss get started on a tree trunk?
Most of the shy little mosses grow on the ground. It is easy to imagine how they can spread from one patch of shady moist soil to the next one. But some of them start life above the ground, perhaps high on a tree trunk. How they get there seems most mysterious. Actually it is possible because the mosses have an unusual life style which is somewhat complicated.
Most plants grow from seeds that produce plants like their parents. A dandelion, for instance, grows a feathery round ball. When the breezes scatter the fluff, they also spread around the attached seeds.
The same breezes can help a new moss plant to get started on the trunk of a tree. However, in the world of mosses, seeding is more compli¬cated than it is among oak trees and dandelions. It seems that parent mosses go to a lot more trouble to launch their offspring.
The mosses are big enough to be seen as small patches of velvety greenery. With patience and know how we could count hundreds of differ¬ent species. But reproduction occurs on a very small scale. We need a hand lens or a magnifying glass to observe the details. When the time comes, a parent moss sprouts a forest of tiny green stems. Each stem is topped with a mini package of mini mini egg cells. Meantime, another parent moss sprouts a forest of stems, each topped with a package of tiny male sperm cells.
The first reproductive stage depends on the merging of one male cell and one female egg cell. Such meetings are possible if the ripened sperm cells pop open when the velvety moss is slippery with dewy mois¬ture.
The sperm cells have hairy little tails to swim around through the fine film of moisture. Now and then, a lucky one manages to reach an egg capsule—and merge with an egg cell. In most plants, a merging of this sort would produce a fertilized seed, all set to produce a new a plant. The mosses go through an extra stage.
Actually the tiny fertilized moss cell is a true embryo seed. It settles on its soft velvety parent and sprouts a small stem. At the top is a tiny capsule, sealed with a neat lid. The capsule is stuffed with powdery seedlets called spores, finer than fine dust.
When the spores are ripe, the capsule lid pops open. The dusty spores blow away on the breezes. Most of them are lost. A few land on suitable moist patches of soil. Once in a while a moss spore is wafted aloft and lands in a moist crevice of bark, high on a tree trunk.
The lofty little spore does not need chemical rich soil, as most plants do. For it has no true roots. All it needs is a pocket of dependable moisture and something solid in which to hook its fingers. It is not a parasite plant, which takes nourishment from a host plant. The tree growing moss will manufacture its own supplies, using the energy of sunlight to make its food from air and water.