Fungi grow everywhere. You can find them on rotting fruit, between your toes as athlete’s foot, or on the damp ground of a forest. Fungi are plantlike in many ways, but they are not green. They have

Fungi

Fungi grow everywhere. You can find them on rotting fruit, between your toes as athlete’s foot, or on the damp ground of a forest. Fungi are plantlike in many ways, but they are not green. They have cellulose cell walls but lack chlorophyll and cannot make their own food. They are heterotrophs, which means, they must depend on other organisms for their food. Fungi use both animals and plants as sources of food. In fact, many species of fungi compete with humans for the same foods. If two oranges are on a table, and one is slightly moldy, which will you choose to eat? If you leave the moldy orange, the fungus wins. Farms have had to learn how to deal with fungi that threaten their crops. Farmers plant fungus-resistant crops and even fumigate the soil in an effort to prevent fungal disease. Mycologists, scientists who study fungi, are employed to find new ways of protecting both animals and plants from fungal diseases such as ringworm, Dutch elm blight, potato blight, and some forms of asthma.

Although many fungi are troublesome, many others are helpful. Baker’s yeast is a fungus used in making bread. Many cheeses owe their flavor to fungi. The powerful antibacterial agents called antibiotics are derived from fungi. Some fungi, such as wheat rust and corn smut, are parasites and live off living organisms. Most fungi are saprophytes. They live off the bodies of dead organisms or nonliving products, such as wood. The most important role of fungi, essential to all life on earth, is as decay organisms. In the decay process digestive enzymes are secreted by fungi. Complex molecules are broken down into simpler once. Some of the molecules are absorbed by the fungi and used as food, but many of these simple molecules are released into the soil and atmosphere. These molecules can then be taken in and used by green plants as nutrients. Thus saprophytic fungi supply nutrients to living plants. The fungi, together with decay bacteria, serve as a bridge between the dead and living. They enable organic matter to be recycled continually. In this way they are absolutely vital to life on earth.

All of the true fungi are classified in the phylum Eumycophyta. These organisms have cell walls but no chlorophyll, and are usually composed of threadlike filaments that secrete digestive enzymes. The true fungi are simple in basic structure. Yeasts, such as the dried baker’s yeast you use in making bread, are unicellular. All of the other of these fungi form, long filaments of cells called hyphae. If you hold a piece of moldy bread or moldy orange ring up to the light, you can see these thin, glistening filaments. A single hypha is a long tube containing the living cytoplasm of the fungus. Sometimes the tube is hollow along its entire length, and sometimes it has cross walls. The cell wall of a hypha is made of cellulose, so some structures of certain fungi can be hard and woody.

A thick mass of interwoven hyphae filaments is called a mycelium. Sometimes the filaments are pressed so closely together that they form the solid bodies of mushrooms or the bracket, or shelf, fungi you sometimes see growing on rotting logs or tree trunks. These solid structures are “fruiting bodies”. They contain the spore-producing organs of the fungi. Fungi produce spores by the billions, and the wind distributes them all over the earth. The method of spore production is an important feature in the life cycle of any fungus. In fact, fungi are classified according to the type of reproduction bodies they produce.

One of the most familiar molds is Rhizopus nigricans, or bread mold. If you rub dust on a piece of moist bread, put the bread in a covered dish and wait a few days, bread mold will form. The mold can form from a single spore on the moist bread. The hyphae grow very quickly into a fluffy white mycelium.

When you look through a microscope you will see several different kinds of hyphae in the mold. Those that spread over the surface of the food supply are called stolons. At intervals along the stolons, clusters of shorter hyphae reach down into the food supply like roots. These rhizoids secrete enzymes act on the sugar and starch in the bread. The digested foods, along with water, are then absorbed by the rhizoids. The flavor, odor, and color spots produced by bread mold, result from the action of its enzymes.

After a few days, black knobs appear among the hyphae of bread mold. Each knob is a spore case, or sporangium. More than 50,000 spores develop in each sporangium. When a sporangium matures it breaks open and releases its spores. The spores are carried away by air currents. If a spore lodges on a food supply in a suitable environment, it germinates. That is, the spore becomes active and forms hyphae that branch out and form a new mycelium.

Bread mold also reproduced sexually by a form of conjugation. Although the reproductive hyphae all look alike, they actually are two different mating types. These types can be called plus and minus. In sexual reproduction the hyphae form short, specialized side branches. The tip of a plus branch meets the trip of a minus branch during conjugation. Cross walls form behind the end tips, leaving each tip as a gamete cell. The two gametes then fuse the form a zygote. These zygote develops a thick wall and matures into a zygospore, which enters a dormant period. After from one to several months, if conditions permit, the zygospore germinates. In some cases it produces a single stalk that bears a sporangium and spores. In other cases, a short, branching hypha grows from the zygospore. A cluster of rhizoids grows, from this hypha sporangium, and spores then develop.

Дата добавления: 2015-11-04