Lichen, << LY kuhn, >> is any of a group of living things that consist of a fungus and a simple organism growing together in a single unit. For most lichens, the fungi live together with algae. In some types, the fungi unite with single-celled organisms called cyanobacteria. Algae and cyanobacteria can make their own food with the help of sunlight. Fungi cannot make food, but they can absorb the water needed in the lichens’ food-making process.
Thousands of lichen species (kinds) grow around the world. Some grow on soil, but most occur on rocks or tree bark. Lichens often live in regions where few plants can survive. Some species inhabit the extreme cold of the Arctic. Others live in deserts or on mountains.
Lichens have no roots. They have an outer layer of fungal cells often colored orange, gray, green, brown, or yellow. This protective layer, called the upper cortex, covers a zone of algae or cyanobacteria. Below the algal or cyanobacterial cells lies a food storage zone called the medulla and a further protective layer called the lower cortex. Most lichens have rhizines (fungal strands) that attach the underside of the lichen to a surface.
Botanists recognize three major groups of lichens. Fruticose lichens generally have a shrublike appearance. They attach themselves to surfaces at one point or at a few points. Foliose lichens are leaflike in appearance. Crustose lichens are crustlike and attach themselves to surfaces with the whole underside.
How lichens grow.
Because lichens have no roots, they grow only when they absorb water from rain, dew, or moisture in the air. When a moist lichen absorbs sunlight, the algal or cyanobacterial part produces sugar by photosynthesis (see Photosynthesis ). Most sugar passes to the fungus, which uses it as food, thus enabling the entire organism to grow. Lichens do not grow when they are dry. Yet many kinds can survive for months in a completely dry state. Most lichens add about 1/8 to 1/4 inch (3 to 6 millimeters) a year to their radius, but a few species can grow much faster. Many crustose lichens grow only about 1/25 inch (1 millimeter) yearly. In Arctic regions, these lichens may live for hundreds of years.
Lichens reproduce in several ways. In most species, the fungus releases spores (microscopic cells) into the air. If a spore lands near a suitable alga or cyanobacterium, it grows around the organism and a new lichen is formed. Many lichens also reproduce by means of soredia. Soredia consist of several algal cells surrounded by a web of fungal strands. They form on the surface of a lichen and are carried away by wind or raindrops. If soredia get trapped in a crack of a tree or rock, they may begin to grow into new lichens. Some lichens produce isidia instead of soredia. Isidia are tiny, peg-shaped growths on the lichen’s surface. Like soredia, they are broken off and distributed by wind and water.
The importance of lichens.
Many animals benefit from lichens. In the Arctic, lichens provide winter food for reindeer and caribou. In other regions, many snails, slugs, and insects eat the organisms.
People have traditionally used lichens for many purposes. Lichens have served as human food during famine, and some cultures still consume certain lichen species. People have made drugs from lichens to treat various lung and skin disorders. They also have used lichens to make dyes.
Today, manufacturers process lichens called oakmoss and treemoss into a fixative for perfumes, after-shave lotions, and soaps. The fixative prevents flower fragrances from evaporating quickly. European countries use large quantities of lichens known as reindeer moss for decoration. Reindeer moss commonly appears in floral displays and as grave ornament. Another lichen, the canary weed, is the primary ingredient in litmus, a substance used to determine if a solution is an acid or a base (see Litmus ).
Lichens have long helped scientists determine the amounts of certain pollutants in the air. Sulfur dioxide and other poisonous gases kill lichens. Thus, scientists can estimate the amounts of such gases in the air by observing the number and type of lichens growing at a particular site. Lichens also absorb metals. By analyzing lichens that grow near factories and smelters, scientists can determine the type of metallic pollutants released.
