The different taxonomic groups of fungi have different levels of cellular organization. Some groups, such as the yeasts, consist of single-celled organisms, which have a single nucleus per cell. Some groups, such as the conjugating fungi, consist of single-celled organisms in which each cell has hundreds or thousands of nuclei. Groups such as the mushrooms, consist of multicellular, filamentous organisms which have one or two nuclei per cell. These multicellular fungi are composed of branched filaments of cells called hyphae. The hyphae, in turn, often mass together to form a tissue called mycelium.
Mycology, the study of fungi, has traditionally included groups such as the cellular slime molds, plasmodial slime molds, water molds, chytrids, and several other groups of fungus-like organisms. Most modern biologists consider these groups as diverse assemblages of organisms unrelated to the true fungi considered here. However, it should be emphasized that biologists are very uncertain about the evolutionary relationships of these other groups and the true fungi.