Lichen

Lichen (Figure 3.5) is an intimate, symbiotic association of a tangled mass of fungal hyphae (mycobiont) that holds a photosynthetic green or a blue-green algal partner (photobiont) resulting in a stable thallus of specific structure. Crustose lichens are composed of a flat and crust-like thallus. Fungi usually form the basal portion of the lichen, which may be differentiated into a stalk-like structure. The credit of establishing the combination of two dissimilar organisms living together as a single entity is due to the Swiss botanist Schwendener (1872), who said,

"As the result of my researches, the lichens are not simple plants, not individuals in the ordinary sense of the word; they are, rather, colonies, which consist of hundreds of thousands of individuals, of which, however one alone plays the master, while the rest in perpetual captivity prepare the nutriment for themselves and the master. This master is a fungus of the class Ascomycetes, a parasite which is accustomed to live upon other's work. Its slaves are green algae, which it has sought out, or indeed caught hold of, and compelled into its service. It surrounds them, as a spider its prey, with a fibrous net of narrow meshes, which is gradually converted into an impenetrable covering; but while the spider sucks its prey and leaves it dead, the fungus incites the algae found in its net to more rapid activity, even to more vigorous increase ..." (see Ahmadjian, 1967).

For nomenclatural purposes, names given to lichens are regarded as applying to their fungal component (mycobiont). Peltula polyspora is lichen (mycobiont) and its algal component (photobiont) is Anacystis montana, a blue-green alga. The photobiont in lichens is generally a green or yellow-green eukaryotic alga or sometimes a prokaryotic blue-green alga.

Approximately 20,000 species of lichens are known. Lichens grow very slowly— the increase in the radius of the colony of Xanthoria is about 2 mm a year. Lichens grow as skins or miniature bushes on rocks and the bark of trees and are colored red, blue, yellow, green and even black with some developing as long grey tufts hanging from branches. The outer layer is formed by fungal mycelium protecting the algal cells beneath from harmful ultra-violet radiation and desiccation. In the center is looser fungal tissue, termed medulla (Figure 3.6).

Lichens can survive in the most extreme and severe environments where neither plants nor fungi can exist alone. In the Himalayan mountains, they grow at altitudes of

Figure 3.5 Lichens. (A) Crustose lichen growing on rock. Photo: Fred Bruemmer, Montreal, with permission. (B) Lichen about an inch growing as miniature bush or thicket. Photo: Premaphotos Wildlife, with permission. (See color insert following page 140.)

up to 18,000 feet. On the Antarctic ice cap, they have been found on rocks within 300 miles of the South Pole where it is so cold that growth is only possible for a few days in the year. At the other end of the earth in the Arctic tundra, lichens grow with particular luxuriance. A bushy kind, called the "reindeer moss," forms ankle-deep carpets and provides the main food of reindeer in the winter. Lichens obtain their moisture from mists and find all the minerals they need dissolved in the rain. Lichens can also tolerate heat that would desiccate and kill most plants. They shrivel but remain alive and, when the opportunity comes, they take up moisture at extraordinary speed and in great quantities, absorbing as much as half their dried body weight in a mere ten minutes. Most species of lichens are extremely sensitive to sulfur dioxide in the air and are therefore indicators of air pollution.

Upper cortex

Algal layer

Medulla

Upper cortex

Algal layer

Medulla

Figure 3.6 Vertical sections of crustose (a) and fruticose (b) lichens. From Ahmadjian (1967). With permission of John Wiley and Sons.

Upper cortex

Algal layer

Medulla

Lower cortex

Rhizinae

Upper cortex

Algal layer

Medulla

Lower cortex

Rhizinae

Figure 3.6 Vertical sections of crustose (a) and fruticose (b) lichens. From Ahmadjian (1967). With permission of John Wiley and Sons.

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