The Open Door Web Site
Part XIII : Plant Dispersal
Moss growing on a forest floor
The Spore-producing Plants
Plants which do not produce seeds produce spores to reproduce. Spores are microscopic reproductive cells which can be easily transported by wind.
Bryophytes: Mosses and Liverworts
These are small plants which never grow more than a few centimetres tall. Their roots, if they have any at all, are very small and simple. The bryophytes are always found growing where conditions are wet or where the climate is wet for a part of the year. This is important because it helps them to reproduce.
The liverworts are plants which are very delicate. They can easily dry out and die. Their bodies are flat and broad and they grow in dense mats over rocks beside streams and waterfalls.
In the early spring, as the amount of daylight increases and the temperature rises, liverworts start to grow a long stem with a ball shaped end to it. This ball will gradually turn black or brown and, eventually, it will burst open. The ball is called a spore case or sporangium (pl. sporangia). When the spore case bursts open it releases thousands of microscopic spores into the air. The liverworts produce spores to be carried in the air currents. When the spore lands on the soil, if the conditions are suitable it will grow into a new liverwort plant.
The spore case which grows out of the liverwort is the product of fertilization. One part of the liverwort produces male reproductive cells while another part produces egg cells. The male reproductive cells swim across the surface of the liverwort in a film of water. This explains why liverworts need to be near water or covered in water a large part of the time. Without water they cannot reproduce. So that the sperm cells can find the egg cells they are attracted to the egg cells by a chemical.
Mosses also belong to the bryophyte division of the plant kingdom. They are less sensitive to drying out than liverworts. Some mosses can dry out and will revive once they become wet again. It will become clear, however, that water is essential for their reproduction.
Mosses grow in spreading mats or in tightly packed cushions. Their bodies consist of a stem covered in tiny leaflets. It is at the end of these stems that the reproductive cells are produced. This usually happens in early springtime. The male reproductive cells and the eggs are not always produced on the same stem. In the case of some mosses these cells are produced by different plants; in other words there may be male and female moss plants. When the moss plants are showered with drops of water from a rain storm or from a waterfall, the male reproductive cells swim through the water to the stems which support the eggs.
Moss plants with spore cases
After fertilization, a spore case grows out of the moss on a stem. The spore case is often protected by a cap which blows off when the case ripens and dries out. The spores are microscopic and are carried away on the wind. If they land in a suitable habitat they will eventually germinate and grow into new moss plants.
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