Adaptations to life on land
Adaptation is the process by which a species gradually becomes better able to survive in a given environment. The specialized structures of land plants are adaptations that allowed water-dwelling algae to overcome the problems of living on land and to best use land resources.
Most of the problems of a land habitat result from the lack of surrounding water. Algae absorb water and minerals directly by diffusion across the cell membrane. In some land plants, pores evolved. Pores absorb moisture from the environment. Other plants developed specialized structures that draw water and minerals from the soil.
Ecological and economic roles
Fungi help to perform the important ecological function of decomposing dead organic matter. This process not only helps clear dead plants and animals from the environment but also returns nitrogen, phosphorus, and other nutrients to the soil. Fungi arc also economically important. They are used directly as food or in making such foods as bread and cheese. Fungi also produce medically valuable antibiotics, such as penicillin and streptomycin. However, fungi can be extremely destructive when they attack crops. Saprophytic fungi destroy millions of dollars worth of food crops each year. Some fungi also cause certain diseases in animals, including humans.
Fungi include two main groups of organisms. One group, the true fungi, account for about 81,500 species. The other group consists of 600 species called slime molds. Slime molds possess a mixture of traits found in fungi and in protozoa. Individual species within both groups are further classified by their means of reproduction.
Lichens are organisms that consist of fungus and alga. The fungus and the alga live in a symbiotic relationship. The organisms resulting from these combinations are completely different from those of either the alga or the fungus growing alone. In our case fungal hyphae give lichen its internal structure and characteristic shape. Algal cells are embedded in the micellium. The fungus also protects alga from excessive sunlight and provide for water that the alga needs for photosynthesis. The alga secretes carbohydrates that the fungus absorbs as food. The alga allows the lichens to survive on bare rocks where the fungus alone cannot grow.
The lichens are widespread in nature; they occur from desert regions to the arctic, and grow on bare soil, tree trunks, sunbaked rocks, fence posts and alpine peaks all over the world. They are often the first colonists of bare rocky areas. Lichens are considered to be pioneer species because they grow on rocks – thy secret chemicals that cause the rock to break and crack. This breakdown of rock creates soil, from which small plants can grow.
Lichens do not need in organic food source. They require only light, air and a few minerals. They apparently absorb some minerals from their substrate, but most often minerals enter the lichen through the air or rainfall.
Because lichens rapidly absorb substances from rainwater, lichens are among the first organisms to suffer from air pollution. Because lichens absorb water directly, they absorb more dissolved toxic substances then plants. Plants use water that has first been filtered through the ground. They are very sensitive to airborne toxic compounds, thus the presence or absence of lichens is a sensitive index of air pollution.
Some lichens are the food for animals, such as reindeer moss. “Reindeer moss”, actually a lichen, is a principal food source for large herbivores – namely, reindeer and caribou of the treeless arctic plains.
The algae found in lichens also occur as free-living species. The lichen fungi are generally found in nature only in the lichens. For these reasons lichens are generally classified according to the fungus species with which they are associated.
There are about 17,000 species of lichens, that is, of lichen fungi. Most are ascomycetes or Fungi Imperfecti, although a few are basidiomycetes. Algae from 30 different genera are found in symbiotic combination with these fungi.
Lichens reproduce most commonly by the breaking off of fragments containing both fungal hyphae and algae. New lichens are formed by the capture of an alga by a fungus. Sometimes the alga is destroyed by the fungus, in which case the fungus also dies. If it survives, a lichen is produced.
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