GROWTH AND REPRODUCTION OF LICHENS

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Lichens lack roots, but they are usually firmly attached to the surfaces where they grow by hyphae. Lichens grow excruciatingly slowly, adding a few millimeters to their length or diameter in a year. The fastest growing varieties may add no more than 30 mm (1.2 in) to their length in a year. Lichens may have long life spans—a lichen found in West Greenland in the Arctic is more than 4,500 years old. Scientists have used the sizes of large specimens to estimate how long it has been since glaciers covered arctic and mountain areas. 

During prolonged dry periods, lichens survive by retaining a small amount of water and reducing their growth and metabolic processes to the barest survival levels. After a rain, the fungal partner is able to soak up water like a sponge, absorbing two to three times its weight in water. A moist internal environment is critical for the alga, which needs water, along with carbon dioxide and sunlight, to manufacture food through photosynthesis. During dry periods when other food may be lacking, the fungus benefits by absorbing sugars and nitrogen-rich compounds produced by the alga during photosynthesis. In some cases the fungal partner is parasitic, using special fungal hyphae called haustoria to penetrate the algal cell and absorb nourishment. Since the fungal partner usually does not kill the alga, some scientists call this relationship a controlled parasitism of the alga by the fungus.

In lichen reproduction, the fungus undergoes sexual reproduction and the alga undergoes asexual reproduction, each independently of its partner. In the fungus, the hyphae contains two mating strains, called plus and minus strains because there are no anatomical distinctions between them. The nuclei from these two mating strains fuse within a special, multicellular reproductive structure on the surface of the lichen. The fused nuclei divide several times and produce spores, which are carried away by the wind and may germinate to form a new mass of hyphae. These new fungal hyphae are free of algae and typically must link with an alga in order to survive. The algal cells within the lichen reproduce through mitosis, a process in which a single cell divides into two genetically identical cells.

Lichens as a unit may also undergo asexual reproduction. The lichen may slough off small clumps of soredia, intertwined fungal hyphae with a few algal cells, which disperse to new habitats. Alternatively, the lichen may produce small, fingerlike buds on the surface called isidia, which break off and form new lichens.

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