Lichen are generally low-growing, vary in color from bright orange or yellow to gray or black, and are often found growing on rocks and tree bark. Although found throughout the world, they're unable to survive where the atmosphere is polluted so are good indicators of clean air.
The dual nature of lichens was first realized in 1867 by a Swiss naturalist called Schwendener.
Classification of lichenLichen are generally classified according to the way in which they grow. Three main types are recognized:
Reproduction and growth of lichen
Lichens are extraordinarily slow-growing, and are also among the longest-lived plants. This applies especially to the encrusting, or crustose, lichens that grow on rocks. The thallus of Rhizocarpon geographicum increases about 2 centimeters in diameter over 100 years; patches found growing on rocks in the arctic regions must, by their size, be many hundreds of years old. There seems no reason why they should not go on growing indefinitely. Although the rate of growth is very slow it is quite constant and this fact has been used to estimate minimum ages for glaciers, ancient monuments, and other objects.
Uses of lichensLichens are of great importance in soil formation. By breaking down the rock surfaces they pave the way for mosses and higher plants. In cold regions like the tundra they are important as food for animals such as reindeer and caribou. The Reindeer moss of Lapland is fruticose lichen of the genus Cladonia.
In the epast, lichens have been a valuable source of permanent dyes but this industry has been replaced by the synthetic dyes. Litmus, the indicator used to show acidity or alkalinity in the laboratory, was originally obtained from a lichen.
Lichens in spaceLichens are the most complex form of life known to have survived prolonged exposure to space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – Rhizocarpon geographicum and Xanthoria elegans – were sealed in the Foton M2 capsule and launched on a Russian Soyuz rocket on May 31, 2005, from Baikonur Cosmodrome. Once in Earth orbit, the lid of the container opened and the samples were exposed to the space environment for nearly 15 days before the lid resealed and the capsule returned to Earth. The lichens were subjected to the vacuum of space, unshielded solar ultraviolet radiation, and temperatures ranging from -20°C on the night side of the Earth, to 20°C on the sunlit side. Surprisingly, they survived apparently unscathed.
In space, the lichens became dormant and did not metabolize. However, back on Earth, they resumed their normal activity and their DNA appeared not to have been damaged. All of the lichen appeared to endure the ultraviolet radiation, even those receiving the most exposure. Lichens have a tough mineral coating that could shield them from UV rays. They are also made from individual organisms layered on top of one another, so outer layers may provide protection for underlying cells.
The experiment adds weight to the theory of panspermia – that life could be transported between worlds, perhaps by hitching a ride on an asteroid. It also indicates that organisms similar to lichens might be able to survive on the surface of Mars. Although the martian atmosphere is very thin, it is composed mainly of carbon dioxide, which is necessary for lichens' photosynthesis. The lichens might not survive on Mars for long, however, because of low oxygen levels in the atmosphere.
In the 1980s, experiments carried out on NASA's Long Duration Exposure Facility satellite showed that certain bacteria are hardy enough to endure space.
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