In the past, the true fungi, slime molds, and bacteria were all placed in a single division (phylum) of the Plant Kingdom. Once the fundamental differences between prokaryotic and others (mycorrhizal fungi) have a mutualistic relationship with plants Chytrids and water molds are also funguslike, but their classification is controversial because, like most protists, their reproductive cells have flagella.
However, chytrids, like other fungi, have chitin in their cell walls, and recent molecular research suggests the chytrids are probably more closely related to the true fungi than they are to members of Kingdom Protista. The water molds, however, which have cellulose but no chitin in their cell walls, continue to be more appropriately included in Kingdom Protista. The members of Kingdom Fungi are placed in five phyla. With the exception of some chytrids and all yeasts, they are filamentous. Most, but not all, fungi lack motile cells. Filamentous fungi produce hyphae that grow at their tips. Structures such as mushrooms are formed from hyphae tightly interwoven and packed together. The cell walls of true fungi consist primarily of chitin, a material also found in the shells of arthropods (e.g., insects, crabs). Fungi exhibit a variety of forms of sexual reproduction. The food substances that most fungi absorb through their cell walls are often broken down with the aid of enzymes secreted to the outside by the cells. Because of the great variety of form and reproduction throughout Kingdom Fungi, a neat pigeonholing of all the members into distinct groups is difficult. Broad groups can, however, be recognized, based mainly on vegetative and reproductive structures.
Fungi are organisms without chlorophyll, and therefore, are dependent on other organisms for nutrition. A saprophytic fungus obtains food from dead organic matter. A parasitic fungus feeds upon a living organism, the host. Symbiotic fungi live in a mutually beneficial relationship with a host. A fungus associated with roots of a higher plant, called a mycorrhizal association, may be parasitic or symbiotic. In a symbiotic relationship both the plant and the fungus benefit. The fungal associate on plant roots produces digestive enzymes (proteases/peptidases) that release amino acids from proteins and absorb extra amounts of phosphorus from the soil, thence to the plant. The fungus benefits by using carbonóbasal metabolites (e.g., sugars, amino acids, vitamins) from the plant roots.
Fig. 22.1. Fungi
Fungi range in size from one-celled, microscopic organisms to masses of cells strung together in long filamentous strands. Each strand is called a hypha. The mass of hyphae that make up a fungal body is called mycelium. Cell walls are made up of chitin, which is hard and resists water loss.
Reproduction. All fungi bear spores that germinate into strands of hyphae. The microscopic lower fungi mainly live within a host and reproduce mostly by asexual spores. Higher fungi have elaborate fruiting bodies, composed of hyphae, in which spores are produced. Sexual reproduction, used as the basis of fungal classification, commonly occurs once a year.