Ch 21 Reading PDF

# 21-1 The Kingdom Fungi In spring, if you know where to look, you can find one of the most prized of all foods - the common morel - growing wild in woodlands throughout the United States. Its ridged cap is often camouflaged by dead leaves that collect in abandoned orchards or underneath old oaks o...

# 21-1 The Kingdom Fungi In spring, if you know where to look, you can find one of the most prized of all foods - the common morel - growing wild in woodlands throughout the United States. Its ridged cap is often camouflaged by dead leaves that collect in abandoned orchards or underneath old oaks or tulip poplars. Some morels grow alone, but others grow in groups. They appear suddenly, often overnight, and live for only a few days. What are these mysterious organisms? How do they grow so quickly? ## What Are Fungi? Like mushrooms and molds, morels are fungi. The way in which many fungi grow from the ground once led scientists to classify them as nonphotosynthetic plants. But they aren't plants at all. In fact, fungi are very different from plants. - Fungi are eukaryotic heterotrophs that have cell walls. - The cell walls of fungi are made up of chitin, a complex carbohydrate that is also found in the external skeletons of insects. - Recall that heterotrophs depend on other organisms for food. - Unlike animals, fungi do not ingest their food. - Instead, they digest food outside of their bodies and then absorb it. - Many fungi feed by absorbing nutrients from decaying matter in the soil. - Others live as parasites, absorbing nutrients from the bodies of their hosts. ## Structure and Function of Fungi Except for yeasts, all fungi are multicellular. Multicellular fungi are composed of tiny filaments called hyphae (HY-fee; singular: hypha). Each hypha is only one cell thick. In some fungi, cross walls divide the hyphae into cells containing one or two nuclei, as shown in Figure 21-1. In the cross walls, there are tiny openings through which the cytoplasm and nuclei can move. Other hyphae lack cross walls and contain many nuclei. ## Reproduction in Fungi Most fungi reproduce both asexually and sexually. Asexual reproduction takes place when cells or hyphae break off from a fungus and begin to grow on their own. Some fungi also produce spores, which can scatter and grow into new organisms. You may recall that a spore is a reproductive cell formed by mitosis that is capable of growing into a new organism. In some fungi, spores are produced in structures called sporangia (spoh-RAN-jee-uh; singular: sporangium). Sporangia are found at the tips of specialized hyphae called sporangiophores (spoh-RAN-jee-oh-fawrz). Sexual reproduction in fungi usually involves two different mating types. Because gametes of both mating types are about the same size, they are not called male and female. Rather, one mating type is referred to as "+" (plus), and the other is referred to as "-" (minus). When the hyphae of opposite mating types meet, each hypha forms a gametangium (gam-uh-TAN-jee-um; plural: gametangia), or structure that forms gametes. Then, the two gametangia join, and the haploid nuclei contained in them fuse to form a diploid nucleus, or zygote. After the zygote forms, meiosis takes place, producing haploid nuclei that dominate the remainder of the life cycle. In most fungi, the zygote is the only cell in the entire life cycle that is diploid. ## How Fungi Spread Fungal spores are found in almost every environment. This is why molds seem to spring up in any location that has the right combination of moisture and food. Many fungi produce dry, almost weightless spores, as shown in Figure 21-4. These spores scatter easily in the wind. On a clear day, a few liters of fresh air may contain hundreds of spores from many species of fungi. If these spores are to germinate, they must land in a favorable environment. There must be the proper combination of temperature, moisture, and food so that the spores can grow. Even under the best of circumstances, the probability that a spore will produce a mature organism can be less than one in a billion. Other fungi are specialized to lure animals, which disperse fungal spores over long distances. Stinkhorns smell like rotting meat, which attracts flies. When they land on the stinkhorn, the flies ingest the sticky, smelly fluid on the surface of the fungus. The spore-containing fluid will pass unharmed out of the flies' digestive systems, depositing spores over many kilometers.