Extensive surface area and rapid growth adapt fungi for absorptive nutrition. Yeasts are single-celled fungi. Most other species of fungi are multicellular. The vegetative bodies of most fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium. Fungal mycelia can be huge, but they usually escape notice because they are subterranean. Fungal hyphae have cell walls. These are built mainly of chitin, a strong but flexible nitrogen-containing polysaccharide identical to that found in arthropods. Most fungi are multicellular with hyphae divided into cells by cross walls, or septa. These generally have pores large enough for ribosomes, mitochondria, and even nuclei to flow from cell to cell. Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei. This results from repeated nuclear division without cytoplasmic division. Parasitic fungi usually have some hyphae modified as haustoria, nutrient-absorbing hyphal tips that penetrate the tissues of their host. Some fungi even have hyphae adapted for preying on animals. The filamentous structure of the mycelium provides an extensive surface area that suits the absorptive nutrition of fungi. One cubic centimeter of rich organic soil may contain 1 km of fungal hyphae with a surface area of more than 300 cm2. A fungal mycelium grows rapidly. Proteins and other materials synthesized by the entire mycelium are channeled by cytoplasmic streaming to the tips of the extending hyphae. The fungus concentrates its energy and resources on adding hyphal length and absorptive surface area. While fungal mycelia are nonmotile, by swiftly extending the tips of its hyphae it can extend into new territory. Concept 31.2 Fungi produce spores through sexual or asexual life cycles Fungi reproduce by producing vast numbers of spores, either sexually or asexually. The output of spores from one reproductive structure can be enormous. Puffballs may release trillions of spores. Dispersed widely by wind or water, spores germinate to produce mycelia if they land in a moist place where there is food.
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