Most of you are familiar with basidiomycetes. Mushrooms, bracket fungi, and puffballs are basidiomycetes and so are plant pests such as rusts and smuts.
Characteristics of the Basidiomycetes
Basidiomycetes produce basidiospores that are attached by short stalks to clublike structures called basidia (singular, basidium). Each basidium usually has four spores. Basidiomycetes are classified according to the structure of their basidium. In some groups the basidium consists of one cell; other groups have a four-celled basidium. Basidiospores are exposed to the air and are discharged explosively when mature. There are about 25,000 species.
When a basidiospore germinates, it gives rise to a primary mycelium. This mycelium has hyphae that have one nucleus in each cell. There are different mating strains of basidiomycetes. When two different strains come together, their hyphae fuse and produce a secondary mycelium. Each cell of the mycelium has two nuclei, one from each parent strain. After a rainy period in the spring or fall, the secondary mycelium of some basidiomycetes produces fruiting bodies, which develop quickly.
Life cycle of a mushroom
Mushrooms and Bracket Fungi
A mushroom is the fruiting body of a basidiomycete. It consists of an umbrellalike cap and a stalk or stipe. The underside of the cap in many mushrooms consists of rows of gills, which are lined with thousands of basidia. Some mushrooms have pores instead of gills on the undersurface of the caps. The pores are also lined with basidia. Each basidium contains two nuclei. These fuse and the cell becomes a zygote. The diploid nucleus undergoes meiosis, and four haploid nuclei are produced. These migrate into extensions of the basidium, which become the basidiospores.
Some mushrooms are poisonous, and that is a good reason not to eat any wild mushrooms. It is not easy to tell the difference between a poisonous mushroom and a harmless one. Even a small piece of the "death angel" mushroom, which is fairly common on lawns and in woods, may be fatal. In Mexico and Central America, certain mushrooms that cause hallucinations when eaten are used in religious ceremonies. Some mushrooms are a good food source. In the United States about 65,000 tons of the common field mushroom are grown commercially each year for human consumption.
Bracket fungi are basidiomycetes that are usually woody and can be very large.They grow on trees that are old or injured, and their platelike fruiting bodies project from the tree trunks. Their mycelium grows inside the tree and breaks down the cellulose in the tree's cell walls with enzymes. Bracket fungi live for many years. Each year the fruiting body adds another layer to the old structure. These layers appear as growth rings.
The Fung; Imperfect; (Deuteromycetes)
Imperfect fungi include those whose sexual phase is either lacking or has not yet been discovered. There are about 25,000 species of deuteromycetes. They reproduce asexually by forming conidia. Imperfect fungi include species of Penicillium, a blue mold of food products, and the black mold Aspergillus. Some species of Penicillium and Aspergillus have a sexual phase and are classified with the ascomycetes. When a sexual phase is discovered in the imperfect fungi, it usually is of the ascomycete type.
Species of Penicillium produce penicillin, a valuable antibiotic. Other species add flavor to Roquefort and Camembert cheeses. Aspergillus is used to make citric acid and soy sauce.
Imperfect fungi are classified by the shape of their conidia and how the conidia are produced. Penicillium and Aspergillus have long rows of beadlike conidia that are produced at the ends of fingerlike branches.
Bracket fungi. Notice the dark bands. They are growth rings. Each ring represents a year's growth.
Rusts and Smuts
The fungi known as rusts cause many plant diseases. About 5,000 species of rusts attack ferns, conifers, and flowering plants. Rusts are of concern to humans because they cause millions of dollarsrorth of damage to cereal crops such as wheat, barley, and oats. The reddish-brown spores of these fungi make the infected plants look rusty. Rust fungi have complex life cycles. They must infect two unrelated plants in order to complete their life cycle, and they produce four different types of spores. The spores of rusts are not borne in fruiting bodies like those of other basidiomycetes. The spores are produced in groups called sori (singular, sorus). The bestknown rust fungus is Puccinia graminis, which causes black stem rust of wheat and other cereals. As scientists develop new strains of resistant wheat, more virulent strains of black stem rust fungi arise by mutations. A solution to this problem is to eliminate the common barberry plant which is the other plant needed by this fungus to complete its life cycle. This has been tried, and millions of barberry bushes have been destroyed. But spores of rust fungi are small and can be carried long distances by air currents.
Smuts are also destructive pests of cereal crops. Like rusts, the smuts do not produce fruiting bodies. There are over 1,000 species of smuts. They form black, powdery masses of spores on the plants they infect.