Mosses (bryophytes) are primitive plants with leaves and stems, but without true roots and without a vascular system--the network of channels through which liquid moves, like the blood vessels in mammals. Mosses are low, mat-forming plants usually under 4 inches in height. They often colonize in moist, shady forests with acid soil, but can be found on desert soil, city pavement and rocks in mountain streams.
The Moss Plant
What most people think of as a moss plant is, botanically, a gametophyte --a structure that bears gametes, sperm and eggs. It may have rhizoids, root-like threads that attach it to wood, soil or stone. The stems may be horizontal or vertical, and thin leaves, usually only one cell thick, are attached all along the stems. The minerals and water needed by the plant are absorbed through the leaves.
The moss plant serves to raise the important structures, the reproductive organs, above the ground so that the spores can be dispersed over a wider area. This is much like flowering plants, where the stems, branches or trunks raise the flowers high enough for seed dispersal to be effective.
Male and female sex organs, antheridia and archegonia, grow at the tips of separate male and female plants. Sperm are produced within the club-shaped antheridia, and an egg is created within a vase-shaped archegonium.
When the sperm are released from the antheridia, they are splashed by drops of water onto the archegonium. They travel by means of whip-like flagellae down to the egg, fertilizing it.
The Sporophyte Stage
After fertilization, an embryo begins to grow within the archegonium. A placenta develops at the base to transfer nutrients from the gametophyte to the growing sporophyte, or spore-producing stage.
The sporophyte grows a stalk with a spore-producing capsule at the top. The whole length of the sporophyte may be no more than a quarter or half an inch.
Within the capsule, spores are produced and, when ripe, the capsule dries, the tip opens and the spores are released.
When a one-celled spore lands on a moist spot of the type appropriate to the species, it sprouts and forms a branching mass of thread-like filaments, the protonema, that looks much like a piece of felt and may cover several square centimeters. There are reddish brown filaments at the base that serve to anchor the protonema to the wood, stone or ground.
Growing a Moss Plant
As the protonema grows, buds form within it that soon sprout into the stems and leaves of the gametophyte, the moss plant. One spore, by this process, can form a substantial patch of moss.