The below mentioned article provides biology notes on fungi.
Fungi is the plural of the word fungus which is derived from the latin word fungour which means to flourish. The word was primarily used with reference to mushrooms which develop overnight. In usage, the meaning of the word has been expanded to include thallus like achlorophyllous plants such as the molds and other similar organisms related to mushrooms.
The fungi thus are a large group of simple thallus like plants which lack chlorophyll. The older botanists employed the term in a wider sense to include bacteria, slimemolds as well as true fungi. About 5100 genera and more than 50,000 species of fungi are known today. Familiar examples of the fungi are the yeasts, molds, toad stools mushrooms, polypores, puffballs, rusts and smuts.
The branch of botany that deals with fungi is called mycology (Gr. mykes means mushroom and logos means study of). Etymologically, mycology is the study of mushrooms. It deals with life histories, relationships and evolutionary tendencies of fungi. The scientist who is concerned with fungi is called a mycologist. He studies structure, reproduction, physiology and taxonomy of fungi.
In common language, the fungi may be defined as non-green, nucleated thallophytes. However, mycologists have defined fungi more scientifically. According to Alexopoulos (1962), the fungi include nucleated spore bearing achlorophyllous organisms that generally reproduce sexually and whose filamentous branched somatic structures are typically surrounded by cell walls containing cellulose or chitin or both.
A more technical definition of fungi was later given by Bessey (1968) which says that fungi are chlorophyll-less non-vascular plants whose reproductive or vegetative structures do not permit them to be assigned to positions among recognised groups of higher plants or algae.
On the basis of the above mentioned, the characteristics of fungi may be summarised as follows:
1. Habitat:
Ubiquitous i.e. found everywhere as saprophytes, parasites or hyper parasites and or symbionts.
2. Thallus:
On or in the substratum and plasmodial amoeboid; or unicellular or multicellular, filamentous; if filamentous septate or aseptate.
3. Cell wall:
Well defined; made up of chitin or of cellulose.
4. Nuclear status:
Eukaryotic; Uni-bi-or multi nucleate; homo-or heterokaryotic.
5. Nutrition:
Heterotrophic
6. Reproduction:
Asexual, sexual/or parasexual through spores.
7. Distribution:
Cosmopolitan.
The fungi grow in diverse habitats. In fact they are found in almost every available habitat on earth where organic material (living or dead) is present. They are, thus, universal in their distribution. Great many of them are terrestrial. They occur in soil which abounds in dead, decaying organic materials.
The terrestrial fungi thrive best in humus soil. They are considered more advanced. They produce non-motile reproductive cells which are dispersed passively by wind, water or animals. Some fungi attack living organisms. They live in tissues of plants and animals. Some fungi are aquatic.
The aquatic fungi are considered primitive. They live on decaying organic matter and living organisms found in fresh water and produce flagellate (motile) reproductive cells which swim to new localities. Many fungi grow on our foodstuffs such as bread, jams, pickles, fruits and vegetables.
Some fungi are found in drinking water. Thus, the fungi contaminate both our food and drinking water. They are present all the time in air that we breathe. Majority prefer to grow in darkness and dim light in moist habitats.
The fungi are plants of very varied habits. They lack chlorophyll and like animals are unable to manufacture their own food from carbon dioxide and water. In their mode of nutrition, the fungi thus differ from all green plants.
They obtain food readymade from an external source. All fungi are, therefore, heterotrophs. In their mode of nutrition, they are heterotrophic. However, like all other plants, they cannot ingest solid food but absorb it directly through cell membranes either by living as saprophytes (saprobes) or parasites.
Thus, according to their mode of nutrition, the fungi are classified into two categories, the saprophytes or saprobes and the parasites. The saprophytes grow where dead organic matter abounds in the substratum. This mode of life is called saprophytic (saprobic). The parasites live in or on the living bodies of other organisms (plants and animals) and obtain food from them.
This mode of life is called parasitic. The best examples of parasitic fungi are the rusts and smuts. The organism on which the parasite feeds is called the host or suspect. The presence of the parasite may cause an abnormal condition of the host which is called disease. The parasitic fungi are harmful organisms.
The thallus of a parasitic fungus may grow on the external surface of the host but usually it is hidden from view. The former are called the ectoparasites (Vine molds) and the latter endoparasites (Pythium debaryanum, Ustilago and rusts). In the endoparasites, the unseen but vitally active thallus grows in the tissues of the host plant.
There is, however, not always a sharp distinction between parasites and saprophytes.
Among the parasites, one can distinguish the following three degrees of parasitism:
1. Obligate parasites:
They can grow only upon suitable living host tissues. The best examples of obligate parasites are the downy and powdery mildews. So far all attempts to grow them on dead artificial media have failed.
2. Facultative saprophytes:
Normally they live as parasites and attain their best development as such. However, they are able to grow apparently indefinitely as saprophytes under emergent circumstances. To this category belong Taphrina deformans, the leaf curl fungi and some smuts. Phytophthora infestans is a parasite on potato. It causes potato blight diseases. In the laboratory, it can be grown as a saprophyte on cultures of oat meal agar.
3. Facultative parasites:
Normally they are saprophytes in their habit but may now and then become parasites. Some Fusarium species are good examples of this type. They live as saprophytes in the soil. When suitable host plants are sown in such a soil, they attack them and start living as parasites. Botrytis cinerea is another good example of a facultative parasite.
The fungi which live strictly as saprophytes are called the obligate saprophytes. They are incapable of infecting plants or animals. The common examples of obligate saprophytes are Mucor, many field toadstools and most of the species of Penicillium.
According to the damage done to the host plant, the parasitic fungi may be divided into two sorts, destructive parasites and balanced parasites. The destructive parasites cause the worst diseases of our field crops, garden crops and forest trees.
They kill the host cells or tissues or produce substances toxic to the host. The balanced parasites are so adapted in their demands upon the host that the latter is able to supply the needs of the parasite and continues to live itself.
In parasitism, advantages are all on one side i.e. to the parasite. The disadvantages to the host may vary from a trifling inconvenience to complete destruction. There is, however, another type of association in which the fungus enters into a partnership with another organism. In this case, the advantages are not all on one side.
Both the partners are mutually benefited. This type of association is known as symbiosis. The best example of symbiosis is furnished by lichens. The thallus of a lichen consists of a fungus living symbiotically with an alga. In this partnership, each member profits by the presence of the other.
Another example of symbiosis is furnished by mycorrhiza. In this case, a fungus establishes a relationship with the roots of certain higher plants particularly the forest trees. For example, in Pinus the fungal hyphae form a weft around the pine roots. The root hairs are absent in this region of the root.
It is suggested that the fungal hyphae perform the work of the root hairs. They absorb water and minerals in solution from the soil and transmit to the roots of the tree. The tree provides food to the fungus.
This type of mutually beneficial association between a fungus and the roots of a higher plant is called mycorrhiza. It is ectotrophic when the fungal hyphae live on the surface and endotrophic when they penetrate the root and grow inside the root tissues.
The life cycle of a fungus consists of two phases, the somatic or vegetative phase and the reproductive phase.
