The cell walls and septa of fungi are comprised mostly of a nitrogen containing chemical, chitin , which is structurally similar to a polysaccharide. At one time, fungi were considered to be plants. It is significant that the structure of the fungus body confers an enormous surface area, since nutrition is accomplished via absorption through the cell walls of hyphae. The life cycle of Neurospora , which has been studied extensively, illustrates this phenomenon. Mating types are genetically dissimilar but can form either male or female structures- hence, they are not different sexes.
Classification is based on the sexual structures exhibited.
These fungi have no septa between adjacent cells. An example is Rhizopus , bread mold. This group includes such forms as cup fungi , brewing yeast, pathogenic yeasts, the causative agent of Dutch Elm Disease , truffles, morels, and many types which are associated with algae to form lichens. There is no asexual reproduction in members of this group. A fourth group of fungi, tentatively called Fungi Imperfecti, contains fungi for which no sexual stage is yet known.