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3.5. Fungi

Fungi are eukaryotic organisms; most species are multicellular. The cell walls of most fungi contain a hard substance called chitin. Chitin is found only in fungi and in the hard outer skeletons of insects.

The body of a typical fungus consists of many individual filaments called hyphae. Hyphae contain cytoplasm and one or more nuclei. Hyphae secrete enzymes that digest food. The fungus then absorbs the nutrients from the food through its cell walls.

Intertwined hyphae form the body of the fungus, or mycelium. Most of a fungus lives under the sub­strate, or material in which the fungus is growing. The visible part contains the spore-producing structures and is called the fruiting body. Saprophytic fungi feed on dead matter. Parasitic fungi feed on living organisms.

Nutrition. Fungi are nonmotile organisms that obtain food by decomposing organic matter. Fungi were once considered plants, but studies later revealed that fungi have none of the characteristics that plants possess. Unlike most plants, fungi lack chloroplasts and cannot carry out photosynthesis. Neither do fungi have many animal characteristics. Because of their unusual combination of traits, fungi are classified in a separate kingdom.

3.5.1. Terrestrial molds

About 600 species of terrestrial molds make up the class Zygomycetes. The fuzzy part of the mold actually consists of the specialized hyphae that produce tiny spores in structures called sporangia. These hyphae are called sporangiophores.

The common black bread mold, Rhizopus stolonifer, shows the development of a typical terrestrial mold. An airborne spore that lands on a piece of bread forms hyphae, which branch out over the bread’s surface. These surface hyphae, called stolons, form short extensions that penetrate into the bread. These extensions are called rhizoids. Rhizoids anchor the mold to its food supply, secrete digestive enzymes and absorb the nutrients.

Sexual reproduction occurs in terrestrial molds when contact occurs between hyphae from two genetically different molds, called mating strains. The mating strains are called plus (+) and minus (-) rather than male and female, because they have identical shapes and functions. The tip of each mating hypha contains nuclei that fuse and form a diploid zygote. In unfavorable conditions the zygote may become a durable zygospore. In favorable conditions the zygote undergoes meiosis and produces spores that form new hyphae. Terrestrial molds and other fungi do not go through alternation of-generations.

3.5.2. Water molds

More than 500 species of water molds make up the class Oomycetes. Most are water-dwelling saprophytes. Water molds are the only fungi whose cell walls consist mainly of cellulose rather than of chitin. Unlike all other fungi, water molds produce distinctly male and female gametes. The differentiation of gametes into distinctly male (sperm) and female (egg) forms is called oogamy.

A water mold called Phytophthora infestans ruined potato crops in Ireland between 1845 and 1847. Potatoes were Ireland’s main crop, and the resulting famine caused the death of more than 2 million people. Other water molds cause disease of fish, a common problem in aquariums.

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