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Symbiotic Associations of Fungi

 

    

      A close association of two or more different organisms is called symbiosis. Parasitism and mutualism are both forms of symbiosis. Many fungi have symbiotic associations with plants. Some fungi associate with green algae or blue-green bacteria in a parasitic relationship called a lichen. Other fungi have a mutualistic relationship with the roots of trees that is called mycorrhiza.

                 

Lichens

      

          

        About 20,000 species of fungi, mostly ascomycetes, form lichens. The association of fungus and alga (or blue-green bacterium) produces a new type of struc­ture called the lichen thallus. A thallus is the body of a lichen. It consists of an upper cortex, algal layer, medulla, and usually a lower cortex. Rhizoids grow out from the lower cortex and attach the thallus to soil, bark, or rock.

Examples of foliose Lichens

                 

 

One way to classify lichens is according to the nature of their thallus. Crustose lichens have crustlike thalli that grow closely against rocks, trees, or soil. They appear to be painted on these surfaces. Foliose lichens have leafy thalli that are attached loosely to their substrate. Fruticose lichens have thalli that grow upright. Filamentous lichens have thalli that hang down from the branches of trees.

 

Lichens are pioneer organisms. They survive in places where few others exist. They grow on the highest mountains, in the Arctic and Antarctic, in the tropics, and in deserts. In spite of this hardiness, lichens are harmed by sulfur dioxide pollution and are rare in cities and near industries. Because they are sensitive to sulfur dioxide, lichens are used as bioindicators of air pollution.

 

The fungi and algae of lichens live together in a controlled parasitism. The fungus obtains food from the algae but does not kill the algal cells immediately. The harmful effects of the parasitism are felt so slowly that the algal cells often live for years before they eventually die.

 

The Fungi
Zygomycota
Ascomycota
Basidiomycota

 

Examples of foliose Lichens

 

Mycorrhizae ("fungus-roots")

      

      The association of a fungus and the roots of a plant to form a mycorrhiza is an example of mutualism. Both partners benefit from the association.

 

        

        

      The plant provides sugars for the fungus, and the fungus absorbs minerals and water for the plant. The roots of almost all land plants have mycorrhizal associations. Mycorrhizal fungi grow over a wide area of soil. They help a plant absorb more water than it could by itself.

 

 


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