Structure of Fungi

Most fungi are composed of threadlike tubular filaments called hyphae. Each individual hypha is surrounded by a fairly rigid wall usually made of chitin—the same material that forms the exoskeletons of insects. Hyphae that are partitioned by dividing cross walls are called septate hyphae, and hyphae without cross walls are called nonseptate hyphae. Fungal cells contain cytoplasm, which is a mixture of internal fluids and nutrients. Cytoplasm flows freely within the hyphae, providing nutrients wherever they are needed.

Hyphae grow by elongation at the tips and by branching to form an interwoven mat known as the mycelium. As the mycelium develops, it may produce large fruiting bodies or other structures that contain reproductive spores. Fruiting bodies are often the most visible structure of a fungus, usually growing above the soil or other surfaces so that the spores can be dispersed by air currents or other mechanisms. In contrast, the mycelium is usually hidden beneath the surface of the plant, animal, or other material it is decomposing. For example, a mushroom mycelium is typically buried beneath the soil surface, while its fruiting body, the familiar umbrella-shaped structure, sprouts from the ground.

Reproduction of Fungi

The wide variety of fungi demonstrate many reproductive methods. In general, most fungi reproduce by making tiny spores. Fungi typically produce large numbers of spores. A giant puffball, for example, produces an estimated 7 trillion spores.

Fungi typically follow a reproductive cycle that involves the production of sexual spores. These spores contain one or more nuclei and are usually haploid—that is, their nuclei contain one set of chromosomes. When environmental conditions are favorable, the spores germinate and develop into a mycelium that produces fruiting bodies with enormous numbers of sexual spores, which repeat the reproductive cycle. Some fungi produce asexual spores directly from hyphae, which then germinate to produce additional mycelium. The mycelium spreads rapidly, aiding the fungus in dispersal and colonization.

Many fungi can reproduce by the fragmentation of their hyphae. Each fragment develops into a new individual. Yeast, a type of unicellular fungus, reproduce by budding, in which a bump forms on the yeast cell, eventually partitioning from the cell and growing into a new yeast cell.