There is considerable variation in the structure, size, and complexity of various fungal species. For example, fungi include the microscopic yeasts, the molds seen on contaminated bread, and the common mushrooms.
Molds consist of long, branching filaments of cells called hyphae (singular, hypha). A tangled mass of hyphae visible to the unaided eye is a mycelium (plural, mycelia). In some molds, the cytoplasm passes through and among cells of the hypha uninterrupted by cross walls. These fungi are said to be coenocytic fungi. Those fungi that have cross walls are called septate fungi, since the cross walls are called septa.
Yeasts are microscopic, unicellular fungi with a single nucleus and eukaryotic organelles. They reproduce asexually by a process of budding. In this process, a new cell forms at the surface of the original cell, enlarges, and then breaks free to assume an independent existence.
Some species of fungi have the ability to shift from the yeast form to the mold form and vice versa. These fungi are dimorphic. Many fungal pathogens exist in the body in the yeast form but revert to the mold form in the laboratory when cultivated.
Reproduction in yeasts usually involves spores. Spores are produced by either sexual or asexual means. Asexual spores may be free and unprotected at the tips of hyphae, where they are called conidia (Figure 1 ). Asexual spores may also be formed within a sac, in which case they are called sporangiospores.