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Chapter 17: 15-22 Fungi

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Fungi: Classification and Characteristics

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Fungi (sing. fungus) include non-photosynthetic organisms with plant-like structures, but closer examination shows that the "roots" of fungi, their spores and life cycles, as well as the way that they gather nutrition, set them apart from the four classes of plants we've already studied.

Common Characteristics of Fungi

Fungi absorb their nutrients by injecting their soil or host with digestive chemicals that break down complex molecules from decaying organic matter into simpler components. Both the fungi and surrounding plant life then reabsorb these simpler molecules. The function of fungi as decomposers is essential for a healthy plant ecosystem.

Instead of real roots, most fungi have hyphae, long filaments of cells that form a mass called a mycelium. Some hyphae are divided into individual cells by cross walls, called septa. Fungi reproduce by creating spores that must be propelled by air. In some fungi, spores are created in fruiting bodies (the mushroom is a fruiting body). Gametes are produced from hyphae with two distinct nuclei (dikaryotic hyphae), so fungi are unusual in having cells with multiple nuclei.

Fungi are important as saprotrophs, decomposers which break down decaying organic material and return it to simple inorganic compounds that can be used again. While some are parasites that are harmful to their hosts, others form mutualist relationships where both fungi and host benefit. Perhaps the most important of these relationships is mycorrhizae, where fungi grow ont he roots of plants. The fungi break down materials in the soil for the roots to absorb; the roots provide complex substances like sugars and start to the fungi. Many plants could not live in their current habitats without the help of mycorrhizae structures. The mycorrhizae-organism communities can be quite complex, and increase diversity elsewhere in the plant kingdom. Indian Pipe, Pinesap, Candystick, and Pinedrops (pictured) are all angiosperms or flower and seed-bearing plants that draw their nutrients from mycchorizae in an indirect symbiotic relationship with the organisms that support the fungal mycchorizae. They are an example of a three-way symbiosis that includes the pine or fir trees, mycorrhizae, and a saprophytic (non-chlorophyll producing) angiosperm.

Fungi also flavor our foods, providing us with many of our most interesting cheeses. Yeasts provide fermentation for beer and wine, and the gasses which make bread doughs rise. Other fungi aid the fermentation of soy sauce. Edible mushrooms add variety to our diet (but not much nutrition). Some fungi provide us with drugs such as penicillin or with hallucinogens like lysergic acid. Fungi are also important as plant parasites in the form of rusts, smuts, blights, mildews, and scabs that are responsible for major crop losses every year.

Types of Fungi

Until about three years ago, there were three recognized phyla of fungi.

Fungi and Other Organisms

Strictly speaking, lichens are not fungi but are organisms which are symbiotic relationships between green algae, which use photosynthesis to produce food, and fungi which dissolve surface particles on rocks or in trees in order to extract the nutrients it needs.