Fungi and plants are sessile (immobile). Unlike animals, they cannot walk or fly to new habitats. Their immobility generally leaves only two ways for fungi and plants to extend their range: they can grow into an adjoining area, or disperse spores or seeds. Most fungal spores are single cells. They can travel beyond the physical limits of their parent into more distant territory.
An organism's physical growth for a single season usually limits yearly dispersal by growth to short distances. The maximum outward growth rate of fairy rings in the soil is only 8 inches (20 cm) per year. In plants, outward growth is also slow. Aspen clones, for example, are created by the roots of the parent trees sending up new stems on the edge of the clone. This type of growth results in a dome-shaped cluster of trees because the older trees in the center are taller. Fairy rings and aspen clones can become enormous, but it takes hundreds to many thousands of years. An aspen clone in Utah covers 17.2 acres (43 hectares) and is estimated to be one million years old.
Many plant seeds depend upon wind to increase the range of dispersal. Some seeds are modified to increase the chances of long range dispersal. If the seeds are heavy, or the wind light, the seeds will land close to the parent. Seeds with "wings" (maples) or "parachutes" (milkweed) will stay aloft longer and be dispersed farther from the parent.
The spores of fungi are smaller and lighter than all plant seeds, but fungi encounter more barriers than plants do in achieving successful dispersal. A major problem is that many fungi do not grow tall enough to clear the "boundary layer" of still air next to the ground. Most plants grow through the boundary layer. Fungi have adapted to the problem posed by the boundary layer by either shooting their spores through it, or evading it entirely by utilizing vectors (animals or water or wind) for dispersal. Once spores are caught by the wind they can be carried very long distances. Spores of a wheat rust have been reported to have been dispersed 1,243 miles (2000 km) by the wind.
Spore dispersal is a two-step process. The first step is spore discharge or release. The second step is dispersal away from the parent. Fungi have evolved a number of different mechanisms for spore discharge and dispersal.
Solutions for dispersal can be grouped into passive and active mechanisms.
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