For many years scientists considered fungi to be plants. Fungi have cell walls, which is a main distinction between plants and animals. However, fungi do not contain chlorophyll, and so they cannot make their own food. Instead, they absorb their nutrients. Their cell walls were found to be made of chitin, which is the material found in the hard covering of many animals. The characteristics of fungi thus make them very difficult to classify as either plants or animals. In a five kingdom classification system, therefore, fungi are placed in their own kingdom.
The Structure of Fungi
Most fungi consist of tubelike structures called hyphae that contain cytoplasm and nuclei. Hyphae may be divided into cells by cross walls, or they may be undivided. Fungi grow rapidly and within a day or two form a mat of interwoven hyphae called a mycelium. Growth Occurs only from the tips of hyphae. Mushrooms are made up of hyphae that are packed closely together. xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />xml:namespace prefix = o />xml:namespace prefix = o />xml:namespace prefix = o />xml:namespace prefix = o />
The cell walls of fungi consist of chitin, a polysaccharide
that is in the exoskeletons of animals such as insects and crustaceans. Recall that the cell walls of plants are made of cellulose and pectin. Fungi and animals store carbohydrates as glycogen. Plants store carbohydrates as starch.
Fungi reproduce asexually by means of spores or fragments of hyphae. Fungal spores are small, usually less than 25 microns in diameter, and can be carried on air currents or float on water for long distances. This is an important means of spreading fungi.
Many fungi also reproduce sexually. Fungal gametes are not motile, so water is not necessary for sexual reproduction. Male and female sex organs may grow toward each other, or the male gametes are carried to the female gametes by wind or insects. The diploid zygote undergoes meiosis immediately and forms haploid nuclei. Spores are formed as each nucleus and the cytoplasm around it is enclosed in a hard coat that is resistant to drying.
Some fungi do not form sex organs. They have different mating strains whose hyphae fuse and exchange nuclei. Mating strains have mycelia that look alike but are genetically different. The nuclei remain separate in the hyphae, but the genes of both of them are expressed. A fungus that has two or more genetically different nuclei in its mycelium is called a heterokaryon. Heterokaryons usually grow faster than homokaryons, which are fungi with only one type of nucleus, because they have a greater variety of genes.
Another characteristic of fungi is that their nuclear membrane does not disappear during mitosis and meiosis. At the end of nuclear division, the nuclear membrane pinches in and forms two daughter nuclei.
There are over 100,000 species of fungi. They grow practically everywhere and produce large numbers of spores. Fungal spores are in the air, in soil, in rivers and oceans, and on plants and animals, including humans. When a spore comes to rest on a moist surface such as a piece of food, it germinates and forms a large population within a few days.
Most fungi are microscopic, but some are easy to see with the naked eye. For example, mushrooms are fungi and so are molds that grow on bread and fruit and mildews that grow in damp places.
Fungi are important decomposers in nature because they secrete digestive enzymes through their cell walls. These enzymes break down organic matter outside the fungal body. Some of the products are absorbed by the fungi and used as food. Most of the molecules are released into the soil and atmosphere, where some of them are used by plants. In this way fungi have a role in the recycling of the elements in dead plants and animals. Fungi share their vital role as decomposers with bacteria. Organisms that obtain food from dead organisms are known as saprophytes.
The same properties that make fungi valuable decomposers in nature also make them serious pests. Fungi attack paint, leather, lumber, cloth, food, and many other products. The fungi either break them down or damage them with their enzymes. Fungi are particularly destructive in the tropics, where warm weather and humidity stimulate their growth. Humans add chemicals to food and refrigerate food in order to slow down the growth of fungi.
Thousands of fungi live as parasites on plants and animals. A parasite obtains its food from living organisms. Examples of fungal plant parasites are those that cause Dutch elm disease and chestnut blight. Fungi cause ringworm, some forms of asthma, athlete's foot, and many other infections in humans.
Other fungi have been used by humans for centuries in preparing food and beverages. It is a fungus, a yeast, that causes bread dough to rise and grapes to ferment and produce wines. Fungi are used in making products such as cheeses, and they also provide us with powerful antibiotics such as penicillin.
The Classification of Fungi
Fungi are classified according to the appearance of the fruiting body that produces the spores. There are four large phyla in the kingdom Fungi.
·Zygomycota multinucleate mycelium, no cross walls; asexual reproduction by means of spores borne on sporangia;sexual reproduction by conjugation to form a zygospore: Rhizopus
·Ascomycota mycelium with cross walls with one or more nuclei; asexual reproduction by means of conidia; sexual reproduction involves formation of an ascus: yeasts, molds, mildews
·Basidiomycota mycelium with cross walls; sexual reproduction results in basidiospores borne on a basidium: mushrooms, rusts, smuts
·Fungi Imperfectilack a sexual phase, or sexual phase has not been discovered; most reproduce asexually by means of conidia: Penicillium, Aspergillus
The Fungi Imperfecti is not a natural group of fungi and therefore is not considered by some scientists to be a phylum. Another name for this phylum is Deuteromycota.