There are many different kinds of Fungi (sing. = fungus). They have many different forms and they occur all around. Look at some of the common fungi and usage, below. Do you recognize any of these species?
|Tremella mesenterica, fruiting body of a "jelly fungus"||Unleavened and leavened (requires yeast) bread||Glomus sp., spore of an arbuscular mycorrhiza fungus|
|Metatrichia vesparium, sporulating stage of an acellular slime mold||Pseudoplectania sp., fruiting bodies of a discomycete||Blue Cheese made from Penicillium roquerforti|
|Peltigera venosa, a lichen, which represents two organisms, an alga and a fungus||Cyathus, sp, fruiting bodies of a "bird's nest fungus"||Rhizopus stolonifer, zygospore stage of "bread mold fungus"|
|Geastrum, fruiting body of an earth star||Athlete's Foot can be caused by several different species of molds||Aseroe rubra, fruiting bodies of a "stink horn"|
If you have never seen the above fungi before it would not be that unusual. Some of the structures pictured above can only be observed through a microscope or at least will require the use of a 10X hand lens. However, most of the above species form large fruiting bodies that can readily be seen with the naked eye. For example, in Hawai‘i, Aseroe rubra is probably one of the more common species that occur. Its fruiting bodies can usually be found quite commonly from October to January and with its bright red coloration and its fetid odor, it would seem to be difficult to miss. Yet, I have shown this picture to numerous local people who have lived here all their lives that have never seen this species. Why is this? My guess is that they are overlooked because we often do not see what we are not looking for. Also, fungi are very diverse. Would you necessarily recognize that all of the above species are fungi if you happen to come upon it? For example, did you know that athlete's foot is caused by a fungus? That in order to make blue cheese a fungus is required? So, one of the goals in this class is to make you aware of the fungi that are around you and perhaps appreciate what they are doing in your environment. However, before we can do this we must give you some background that will help you in understanding what a fungus is.
What is a fungus
First, just what is a fungus? Over the last 40 years the way that we have defined "fungi" has changed several times. In the early 20th. Century, until around the 1950's, botanist used the term fungi to include all members of the "plant kingdom" that did not have stems, roots, leaves and chlorophyll (Alexopoulos, 1952). By this definition, bacteria would even be included with the fungi. However, by the 1950's botanists, slowly, began classifying bacteria as a distinct group, separate from the fungi, but still classified as plants. In 1952, Constantine Alexopoulos, the author of the popular mycology text book, Introductory Mycology, defined fungi, in the first edition of his textbook, as being "nucleated, achlorophyllous organisms which typically reproduce sexually and asexually, and whose usually filamentous branched somatic structures are surrounded by cell walls". I have bolded the terms that are still emphasized when defining what a fungus is. By the time I took mycology, in 1969, there was only a slight change to this definition. In 1962, when the second edition of Alexopoulos' Introductory Mycology was published he specified that the cell walls of fungi contained cellulose or chitin or both. In addition, Alexopoulos stated that the slime molds, the organisms in which he had considerable expertise, to be "closely related to the true fungi.." I mention this because in the first edition of his text book, Alexopoulos did not believe the slime molds to be related to the fungi. The point of this story is that ideas as to what constitutes a fungus has changed a number of times since the last half of the 20th. Century and is quite different than the the concept that we currently have. Currently, most mycologists define fungi as those organisms that are nucleated, achlorophyllous, typically reproduce sexually and asexually by spores, and whose somatic structure is composed of filamentous branched or yeast, which are surrounded by cells walls composed of chitin. Let us go over this definition, in detail, so that you may understand the above terms.
This term indicates means that fungi cannot produce their own food. Such organisms are called heterotrophs. This is important because it is commonly believed that fungi are plants, and once upon a time fungi were classified in the plant kingdom. Heterotrophs can be divided into several categories:
that derives its food
from non-living organic carbon sources. Many old text books use the term
literally meaning "rotting plant" (showing the botanical origin of this word). These types of fungi are very
important as nature's recyclers. They consume dead organic material and break down
into their most basic components, i.e. minerals, which can then be utilized by plants to produce more food.
So even though we say that these fungi cause "rot" and "decay", that
isn't necessarily a bad thing. This is a very important process in nature's effort to recycle. The
down side of decomposition is that fungi do not distinguish between fallen branches and logs, and your
furniture. To a fungus, wood is wood, regardless of the source. We
will talk more on this topic later in the semester.
Heterotroph that derives its food from the
living cells of another organism referred to as the host. Many fungi fit into this
category, but not all, and not even most. This is just one of the many biases that we have
of fungi, i.e., the common belief that most fungi are parasites.
Parasite: Heterotroph that is primarily a saprobe, but when opportunity
presents itself, can be a parasite.
Saprobe: Heterotroph that is primarily a parasite, but when opportunity
presents itself, can become a saprobe.
Symbiont (used here in the mutualistic sense): Heterotroph that derives its food from another living organism, but the relationship is mutually beneficial to both organisms involved, e.g. lichens = fungus and alga. As you'll see later in the semester, fungi that are in this category are very important, i.e. if they did not exist, the world as we know it also would not exist
The latter two categories of fungi are of more concern to plant pathologists than those that are obligate parasites. An obligate parasite will normally weaken its host, but not kill it since killing the host would almost certainly guarantee the death of the parasite. However, a facultative parasite or facultative saprobe, when they are in their parasitic mode are more likely to be aggressive parasites that will kill their host.
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