3.5.5. Sac fungi

About 30,000 species belong to the class Ascomycetes. Members of this class include the gourmet delicacies morels and truffles as well as the single-celled yeast used in making bread. Ascomycetes are called sac fungi because sexually produced spores form in an ascus, or "little sac". An ascus begins to develop when two gametes or two mating strains fuse. Nuclei divide as the hyphae grow, resulting in a row of haploid ascospores within the ascus. In sac fungi and club fungi, hyphae are divided by cross walls. Nuclei and cytoplasm flow through pores in these cross walls as the hyphae grow.

Yeasts

Yeasts are unusual sac fungi. They contain chitin and reproduce sexually by forming ascospores, but they are unicellular and do not form hyphae. Yeasts also reproduce asexually by budding. Many types of yeasts grow most rapidly in environments with high sugar content. In bread dough, yeast cells feed on carbohydrates. As yeast cells grow, they produce carbon dioxide gas by fermentation. The process of fermentation causes bread dough to rise and creates the bubbles in beer.

Parasitic Sac Fungi

The powdery mildews are among the most destructive of the parasitic sac fungi. The mycelia of these fungi form a white powder on the leaves of apples, roses, grapes, and other economically important plants. The growth of the powdery mildew destroys the tissues of the host plant. This process hinders the plant’s ability to carry out photosynthesis, and further damage results.

Dutch elm disease is caused by another sac fungus. The hyphae of the fungus grow into the wood of an elm tree and clog the tissues that carry water and nutrients from the soil up to the leaves. Dutch elm disease threatens to wipe out all American elms. Chestnut blight, caused by a related fungus, poses a similar threat to chestnut trees.

3.5.6. Imperfect fungi

The class Deuteromycetes includes about 25,000 species. Fungi in this class are called "imperfect" fungi because they do not reproduce sexually or because their sexual life cycles are not fully understood. The most familiar imperfect fungi belong to the genus Penicillium. These fungi are used to produce penicillin. Other imperfect fungi cause diseases including ringworm and thrush.

3.5.7. Fungi and habitats

Fungi have adapted to almost every environment where organic material and moisture are available. They flourish in forests, grasslands, and other areas where dead wood and leaves are abundant. Some species of fungi live in deserts. Others live high atop mountains. Certain marine fungi live on the remains of dead bacteria and plankton trapped in polar icecaps. You may have seen molds — small, fuzzy growths of fungi on fruit, bread or other foods.

Though nonmotile, fungi can reach these diverse environments by means of spores that drift in the wind. A single fungus may produce millions or even trillions of spores at a time. Many of these spores land in unsuitable environments and perish. However, many others will survive and germinate. Most kinds of fungi rely on their spores to disperse the species and to find new food sources.

Modem land plants probably evolved from green algae. About 400 million years ago, the sea was teeming with life, but the land was mostly barren rock. Algae and other marine organisms began to grow near shore because of the availability of direct sunlight and of minerals washed of the shore. As competition for recourses in the sea increased, some of the algae started growing on land.