IMPORTANCE of fungi
Act as decomposers.
cause disease in animals and plants.
essential for many industrial processes involving fermentation.
commercial production of many organic acids and certain drugs.
in manufacture of many antibiotics
acts as important research tools in the study of fundamental biological processes
Sunday, September 28, 2008
NUTRITION & METABOLISM
NUTRITION & METABOLISM
They grow best in dark moist habitats, but they are found wherever organic material is found. Most fungi are saprophytes. They release hydrolytic exoenzymes that digest external substrates, they then absorb the soluble products. They are chemo organoheterotrophs and derive carbon, oxygen and electrons from organic source.
They grow best in dark moist habitats, but they are found wherever organic material is found. Most fungi are saprophytes. They release hydrolytic exoenzymes that digest external substrates, they then absorb the soluble products. They are chemo organoheterotrophs and derive carbon, oxygen and electrons from organic source.
FUNGI STRUCTURE
Structure of Fungi
The main body of most fungi is made up of fine, branching, usually colourless threads called hyphae. Each fungus will have vast numbers of these hyphae, all intertwining to make up a tangled web called the mycelium.
The mycelium is generally too fine to be seen by the naked eye, except where the hyphae are very closely packed together. The picture on the left was taken through a microscope. The hyphae are magnified 100 times life size.
.
Some fungi, such as Honey Fungus, which is a parasite of woodland trees, have hyphae collected together into long cables, called rhizomorphs. Because there are so many hyphae packed together, they are easily seen, forming black 'bootlaces'. These can spread through a woodland infecting neighboring trees.
Fungal mycelium is mostly hidden from human view, not only because of its small size, but also as a result of its location. The tangled mycelial mass is usually hidden deep within its food sources, such as rotting matter in the soil, leaf litter, rotting wood, or dead animals. The mycelium remains undetected until it develops one or more fruiting bodies, containing the reproductive spores.
Fruiting bodies (such as mushrooms) are made up of thick collections of hyphae. They vary in size from small and insignificant, to large eye-catching structures.
They are usually produced at the surface of the food source, rather than hidden within it, to allow the spores to be shed and carried away by the wind, or by water, or animals. The fruiting bodies are usually the only indication we have that a fungus is present. Like icebergs, they represent a tiny fraction of the whole fungus, with most of it being hidden from view.
If you are looking for fungi in Britain, the best time to look is in the autumn. The fungal mycelia have then had a long, relatively warm period to grow extensively over the summer. With the coming of wetter weather in the autumn, damp warm conditions are created which are ideal for fungi to fruit. Many fungi have fruiting seasons in late summer to autumn. However, don't ignore the Spring. This is a good time to look for fungi such as Morels and other cup fungi. Little will be found in cold winter months with frosts, although Oyster mushrooms will continue to grow through most of the winter, as will Jew's Ear.
Fairy Rings
Fungal mycelia tend to grow in more or less circular shapes. This is because they grow by spreading out in all directions from a central point. This central point represents the location of the original spore which germinated to start the fungal mycelium. The mycelium will usually produce fruiting bodies on its outer edge. As a result, the fruiting bodies of fungi living hidden in the soil can sometimes be found in rings - the 'fairy rings' of children's stories.
The Clouded Agaric toadstool (right) is a good example of this. The fruiting bodies can often be found in rings, because they are produced on the outer growing edge of the circular, underground mycelium. The Clouded Agaric can be found in the leaf litter of both coniferous and deciduous woodland.
(image courtesy of Jeff Benn)
Another example is the 'Fairy Ring Toadstool' (Its scientific name is Marasmius oreades). This, in contrast to the Clouded Agaric, grows on grassy expanses such as lawns and golf courses. The fungus can be traced by the rings of dark green grass, with the mushrooms fruiting on the outer edge of the ring. The growth of the ring can be traced year on year. If there are no barriers, rings may grow outwards at up to 20 cm per year. This is a fungus which fruits early in the year, in the spring.
Giants of the natural world
Most people, if asked to name the largest organism on earth, come up with examples such as elephants, blue whales or giant trees, such as Redwoods. In fact, the largest living organism on this planet may well be a fungus!
In America, a single individual fungus, similar to the Honey Fungus already mentioned, was found to cover a staggering 2,200 acres of land. (This is 45 times bigger than the Woodland Education Centre in Offwell, for those of you who have visited the Centre before.)
Some of the oldest living organisms?
We tend to think of fungi as being short-lived, because the visible fruiting bodies which we see don't usually last very long. However, for the fungus to have reached this size, it must have been at least 2,400 years old. It would have been a mere youngster of 400 years old, at the time of Christ's birth.
The rings of Fairy Ring Toadstools found closer to home in Britain, possibly on your back lawn, can also grow to hundreds of meters in diameter, when they are likely to be at least a thousand years old.
Some lichens, which are a partnership between a fungus and an alga, have been estimated to be around 9000 years old!
Next time you find several fruiting bodies of one type of fungus growing on the ground, see if you can follow around in a circle and locate any more. The size of the circle will give you some indication of the size of the mycelium hidden beneath your feet. Bear in mind when you do this, that the fungus which is fruiting, will not be the only fungus hidden down there in the soil. In fact the ground beneath your feet will be absolutely seething with the mycelia of different fungi. We remain totally unaware of their presence simply because we can't see them.
The main body of most fungi is made up of fine, branching, usually colourless threads called hyphae. Each fungus will have vast numbers of these hyphae, all intertwining to make up a tangled web called the mycelium.
The mycelium is generally too fine to be seen by the naked eye, except where the hyphae are very closely packed together. The picture on the left was taken through a microscope. The hyphae are magnified 100 times life size.
.
Some fungi, such as Honey Fungus, which is a parasite of woodland trees, have hyphae collected together into long cables, called rhizomorphs. Because there are so many hyphae packed together, they are easily seen, forming black 'bootlaces'. These can spread through a woodland infecting neighboring trees.
Fungal mycelium is mostly hidden from human view, not only because of its small size, but also as a result of its location. The tangled mycelial mass is usually hidden deep within its food sources, such as rotting matter in the soil, leaf litter, rotting wood, or dead animals. The mycelium remains undetected until it develops one or more fruiting bodies, containing the reproductive spores.
Fruiting bodies (such as mushrooms) are made up of thick collections of hyphae. They vary in size from small and insignificant, to large eye-catching structures.
They are usually produced at the surface of the food source, rather than hidden within it, to allow the spores to be shed and carried away by the wind, or by water, or animals. The fruiting bodies are usually the only indication we have that a fungus is present. Like icebergs, they represent a tiny fraction of the whole fungus, with most of it being hidden from view.
If you are looking for fungi in Britain, the best time to look is in the autumn. The fungal mycelia have then had a long, relatively warm period to grow extensively over the summer. With the coming of wetter weather in the autumn, damp warm conditions are created which are ideal for fungi to fruit. Many fungi have fruiting seasons in late summer to autumn. However, don't ignore the Spring. This is a good time to look for fungi such as Morels and other cup fungi. Little will be found in cold winter months with frosts, although Oyster mushrooms will continue to grow through most of the winter, as will Jew's Ear.
Fairy Rings
Fungal mycelia tend to grow in more or less circular shapes. This is because they grow by spreading out in all directions from a central point. This central point represents the location of the original spore which germinated to start the fungal mycelium. The mycelium will usually produce fruiting bodies on its outer edge. As a result, the fruiting bodies of fungi living hidden in the soil can sometimes be found in rings - the 'fairy rings' of children's stories.
The Clouded Agaric toadstool (right) is a good example of this. The fruiting bodies can often be found in rings, because they are produced on the outer growing edge of the circular, underground mycelium. The Clouded Agaric can be found in the leaf litter of both coniferous and deciduous woodland.
(image courtesy of Jeff Benn)
Another example is the 'Fairy Ring Toadstool' (Its scientific name is Marasmius oreades). This, in contrast to the Clouded Agaric, grows on grassy expanses such as lawns and golf courses. The fungus can be traced by the rings of dark green grass, with the mushrooms fruiting on the outer edge of the ring. The growth of the ring can be traced year on year. If there are no barriers, rings may grow outwards at up to 20 cm per year. This is a fungus which fruits early in the year, in the spring.
Giants of the natural world
Most people, if asked to name the largest organism on earth, come up with examples such as elephants, blue whales or giant trees, such as Redwoods. In fact, the largest living organism on this planet may well be a fungus!
In America, a single individual fungus, similar to the Honey Fungus already mentioned, was found to cover a staggering 2,200 acres of land. (This is 45 times bigger than the Woodland Education Centre in Offwell, for those of you who have visited the Centre before.)
Some of the oldest living organisms?
We tend to think of fungi as being short-lived, because the visible fruiting bodies which we see don't usually last very long. However, for the fungus to have reached this size, it must have been at least 2,400 years old. It would have been a mere youngster of 400 years old, at the time of Christ's birth.
The rings of Fairy Ring Toadstools found closer to home in Britain, possibly on your back lawn, can also grow to hundreds of meters in diameter, when they are likely to be at least a thousand years old.
Some lichens, which are a partnership between a fungus and an alga, have been estimated to be around 9000 years old!
Next time you find several fruiting bodies of one type of fungus growing on the ground, see if you can follow around in a circle and locate any more. The size of the circle will give you some indication of the size of the mycelium hidden beneath your feet. Bear in mind when you do this, that the fungus which is fruiting, will not be the only fungus hidden down there in the soil. In fact the ground beneath your feet will be absolutely seething with the mycelia of different fungi. We remain totally unaware of their presence simply because we can't see them.
NEW FORM OF FUNGI
It's that time of year! Everything is coming up green, which means it's time to start planning your outdoor growing projects. Growing mushrooms on logs and stumps with our Mushroom Plug Spawn can be a great standalone project, or as a complement to any garden or landscape!
Our Outdoor Mushroom Patches™ are another great option for mushroom cultivation at home. We offer a wide variety of mushroom species (such as our Garden Giant Mushroom Patch™, pictured at right) for cultivation in beds of wood chips, soil or other material in your yard or garden.
2009 Seminar Dates Announced! We've put together our schedule of 2009 Cultivation and Mycorestorationsm Seminars and have made them available for signup. These classes will fill up fast—2008 is alread booked solid—so don't delay!
Our Outdoor Mushroom Patches™ are another great option for mushroom cultivation at home. We offer a wide variety of mushroom species (such as our Garden Giant Mushroom Patch™, pictured at right) for cultivation in beds of wood chips, soil or other material in your yard or garden.
2009 Seminar Dates Announced! We've put together our schedule of 2009 Cultivation and Mycorestorationsm Seminars and have made them available for signup. These classes will fill up fast—2008 is alread booked solid—so don't delay!
BACTERIA DRIVE AND MULTIPLY
Bacteria are all around us. Given good growing conditions, a bacterium grows slightly in size or length, a new cell wall grows through the center forming two daughter cells, each with the same genetic material as the parent cell. If the environment is optimum, the two daughter cells may divide into four in 20 minutes. Oh my! 1, 2, 4, 8, 16, 32, 64... Then why isn't the earth covered with bacteria?
The primary reason may be that conditions are rarely optimum. Scientists who study bacteria try to create the optimum environment in the lab: culture medium with the necessary energy source, nutrients, pH, and temperature, in which bacteria grow predictably.
LAG PHASE: Growth is slow at first, while the "bugs" acclimate to the food and nutrients in their new habitat.
LOG PHASE: Once the metabolic machinery is running, they start multiplying exponentially, doubling in number every few minutes.
STATIONARY PHASE: As more and more bugs are competing for dwindling food and nutrients, booming growth stops and the number of bacteria stabilizes.
DEATH PHASE: Toxic waste products build up, food is depleted and the bugs begin to die.
View a 520K time-lapse movie to see how two E. coli, given a suitable environment for growth, divide and form a colony of hundreds of bacteria in just a few hours. Or visit the CELLS alive! BioCam to see bacteria colony in "real time". A longer, larger, silent version of growing E. coli may be purchased and downloaded for classroom use.
Some Keywords:
exponential growth, binary fission, asexual reproduction, population dynamics, lag phase, log phase, stationary phase, death phase
The primary reason may be that conditions are rarely optimum. Scientists who study bacteria try to create the optimum environment in the lab: culture medium with the necessary energy source, nutrients, pH, and temperature, in which bacteria grow predictably.
LAG PHASE: Growth is slow at first, while the "bugs" acclimate to the food and nutrients in their new habitat.
LOG PHASE: Once the metabolic machinery is running, they start multiplying exponentially, doubling in number every few minutes.
STATIONARY PHASE: As more and more bugs are competing for dwindling food and nutrients, booming growth stops and the number of bacteria stabilizes.
DEATH PHASE: Toxic waste products build up, food is depleted and the bugs begin to die.
View a 520K time-lapse movie to see how two E. coli, given a suitable environment for growth, divide and form a colony of hundreds of bacteria in just a few hours. Or visit the CELLS alive! BioCam to see bacteria colony in "real time". A longer, larger, silent version of growing E. coli may be purchased and downloaded for classroom use.
Some Keywords:
exponential growth, binary fission, asexual reproduction, population dynamics, lag phase, log phase, stationary phase, death phase
NATURAL FUNGUS KINGDOM
The Fungus Kingdom
(Last modified: 3 Jan 2002)
In addition to the beauty of mushrooms, fungi provide a critical part of nature's continuous rebirth: fungi recycle dead organic matter into useful nutrients. Sometimes the fungus doesn't wait for the biomatter to die, in which case the fungus is called a parasite. Many plants, however, are dependent on the help of a fungus to get their own nutrients, living in a symbiotic relationship called a mycorrhizal association. Plants aren't the only ones, however, to enjoy fungi.
Fungi digest food outside their bodies: they release enzymes into the surrounding environment, breaking down organic matter into a form the fungus can absorb. Mycorrhizal associates benefit from this by absorbing materials digested by the fungi growing among their roots.
Fungi reproduce by releasing spores from a fruiting body. The fruit, called a mushroom, releases spores into the air, and the wind carries the spores off to start the next generation. Around 100,000 species of fungi are divided into five phyla, based largely on the characteristics of their reproductive organs.
Club Fungi (Basidiomycota)
When people think of mushrooms, the fruit of Basidiomycota probably comes to mind. Many mushrooms in this phylum look like umbrellas growing from the ground or like shelves growing on wood, but some, such as the latticed stinkhorn, look quite different.
Among the more famous families in this phylum are Agaricus -- including the supermarket variety of button mushrooms; Amanita -- including species that are deadly, delicious, or even hallucinogenic; Boletus -- best known for the King Bolete (called Porcini in Italy and Cepe in France); and Cantherellus -- known for the delicious and beautiful Chanterelle. These families include but a few of the mushrooms sought by collectors and gourmets from among the 25,000 species in this phylum.
Species in this phylum produce spores on a club-like structure called the basidium. The basidium may grow free or be attached to a surface called the hymenium.
Class: Homobasidiomycetae produce spores on a hymenium.
Subclass: Hymenomycetes
Produce spores on exposed surfaces -- releasing the spores gradually through structures such as pores or gills.
Orders: Agaricales, Aphyllophorales (3 examples)
Subclass: Gasteromycetes
Produce spores on concealed surfaces, releasing spores only after the cover ruptures. Pictured below are a puffball and earthstar of the Order Lycoperdales and two stinkhorns of the Phalales Order.
Class: Heterobasidiomcetae
Produce spores on the ends of inconspicuous threads. Examples include: jelly fungi (pictured), rusts, smuts
Sac Fungi (Ascomycota)
Ascomycota produce their spores in special pods or sac-like structures called asci. Included among the 25,000 species of this phylum are the prized Morel and Truffle mushrooms (class: Euascomycetae).
Other member of this class include Elfin Saddles (above/left), Morels, Cup Fungi, and Flask Fungi (below, left-to-right)
Another class of this phylum, Hemiascomycetae, is valued more for its activity than its beauty. Sacharomyces cerevisiae (Brewers, Bakers, and Nutritional Yeast) help us produce such popular staples as beer and bread.
Other Classes: Loculoascomycetae, Laboulbeniomycetae
Lichens (Mycophycophyta)
Once the beauty of mushrooms has enticed your greater scrutiny of the forest floor, you can't help but notice lichens as well.
Lichens are a symbiotic union between fungus and algae (or sometimes photosynthesizing bacteria). The algae provide nutrients while the fungus protects them from the elements. The result is a new organism distinctly different from its component species.
Though no longer considered a proper phylum, the radically different nature of these symbiots warrants separate treatment in this overview of the fungus kingdom.
Around 25,000 species of Lichens have been identified by scientists.
Conjugation Fungi (Zygomycota)
The best known of this phylum of around 600 species is black bread mold, such as Rhizopus stolonifer.
Imperfect Fungi (Deuteromycota)
Around 25,000 additional fungus species are grouped in this phylum -- these species are the "left-overs" that don't fit well into any of the other groups. Members include Trichophyton (Athlete's foot), Penicillium (Penicillin), and Candida albicans ("Yeast" infections).
Page 7 of 38
--------------------------------------------------------------------------------
Annotated Bibliography
Arora, David, Mushrooms Demystified (2nd edition), Ten Speed Press, Berkeley, 1986
This is the authoritative field guide to mushrooms of the Western United States. The book provides thorough keys for identifying mushrooms, as well as lively anecdotes and related information for the amateur and expert alike.
Alexopoulos, Constantine J., C. W. Mims, M. Blackwell, Introductory Mycology, (4th edition), John Wiley & Sons, New York, 1996
A college-level text on the world of fungi, organized according to the principles of classification.
Margulis, Lynn, Karlene Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (2nd edition), W. H. Freeman and Company, New York, 1988
An overview of the highest levels of Taxonomy. I have used the authors' nomenclature where available. Names, however, are constantly changing in the field of Taxonomy, and no doubt many of these names are disputed or have changed since 1988.
Margulis, Lynn, Diversity of Life: The Five Kingdoms, Enslow Publishers, Inc., New Jersey, 1992
Although billed as a children's book, this book is quite appropriate for the adult amateur. Dr. Margulis strikes an excellent balance between detail and brevity in this fact-filled book.
Milani, Jean P., et. al. Biological Science: An Ecological Approach (6th edition), Kendall/Hunt Publishing Company, Iowa, 1987
A high school textbook that devotes several chapters to Taxonomy and the diversity of life on our planet. The Appendix titled A Catalog of Living Things illustrates the phyla as well as many classes and families within the five kingdoms.
(Last modified: 3 Jan 2002)
In addition to the beauty of mushrooms, fungi provide a critical part of nature's continuous rebirth: fungi recycle dead organic matter into useful nutrients. Sometimes the fungus doesn't wait for the biomatter to die, in which case the fungus is called a parasite. Many plants, however, are dependent on the help of a fungus to get their own nutrients, living in a symbiotic relationship called a mycorrhizal association. Plants aren't the only ones, however, to enjoy fungi.
Fungi digest food outside their bodies: they release enzymes into the surrounding environment, breaking down organic matter into a form the fungus can absorb. Mycorrhizal associates benefit from this by absorbing materials digested by the fungi growing among their roots.
Fungi reproduce by releasing spores from a fruiting body. The fruit, called a mushroom, releases spores into the air, and the wind carries the spores off to start the next generation. Around 100,000 species of fungi are divided into five phyla, based largely on the characteristics of their reproductive organs.
Club Fungi (Basidiomycota)
When people think of mushrooms, the fruit of Basidiomycota probably comes to mind. Many mushrooms in this phylum look like umbrellas growing from the ground or like shelves growing on wood, but some, such as the latticed stinkhorn, look quite different.
Among the more famous families in this phylum are Agaricus -- including the supermarket variety of button mushrooms; Amanita -- including species that are deadly, delicious, or even hallucinogenic; Boletus -- best known for the King Bolete (called Porcini in Italy and Cepe in France); and Cantherellus -- known for the delicious and beautiful Chanterelle. These families include but a few of the mushrooms sought by collectors and gourmets from among the 25,000 species in this phylum.
Species in this phylum produce spores on a club-like structure called the basidium. The basidium may grow free or be attached to a surface called the hymenium.
Class: Homobasidiomycetae produce spores on a hymenium.
Subclass: Hymenomycetes
Produce spores on exposed surfaces -- releasing the spores gradually through structures such as pores or gills.
Orders: Agaricales, Aphyllophorales (3 examples)
Subclass: Gasteromycetes
Produce spores on concealed surfaces, releasing spores only after the cover ruptures. Pictured below are a puffball and earthstar of the Order Lycoperdales and two stinkhorns of the Phalales Order.
Class: Heterobasidiomcetae
Produce spores on the ends of inconspicuous threads. Examples include: jelly fungi (pictured), rusts, smuts
Sac Fungi (Ascomycota)
Ascomycota produce their spores in special pods or sac-like structures called asci. Included among the 25,000 species of this phylum are the prized Morel and Truffle mushrooms (class: Euascomycetae).
Other member of this class include Elfin Saddles (above/left), Morels, Cup Fungi, and Flask Fungi (below, left-to-right)
Another class of this phylum, Hemiascomycetae, is valued more for its activity than its beauty. Sacharomyces cerevisiae (Brewers, Bakers, and Nutritional Yeast) help us produce such popular staples as beer and bread.
Other Classes: Loculoascomycetae, Laboulbeniomycetae
Lichens (Mycophycophyta)
Once the beauty of mushrooms has enticed your greater scrutiny of the forest floor, you can't help but notice lichens as well.
Lichens are a symbiotic union between fungus and algae (or sometimes photosynthesizing bacteria). The algae provide nutrients while the fungus protects them from the elements. The result is a new organism distinctly different from its component species.
Though no longer considered a proper phylum, the radically different nature of these symbiots warrants separate treatment in this overview of the fungus kingdom.
Around 25,000 species of Lichens have been identified by scientists.
Conjugation Fungi (Zygomycota)
The best known of this phylum of around 600 species is black bread mold, such as Rhizopus stolonifer.
Imperfect Fungi (Deuteromycota)
Around 25,000 additional fungus species are grouped in this phylum -- these species are the "left-overs" that don't fit well into any of the other groups. Members include Trichophyton (Athlete's foot), Penicillium (Penicillin), and Candida albicans ("Yeast" infections).
Page 7 of 38
--------------------------------------------------------------------------------
Annotated Bibliography
Arora, David, Mushrooms Demystified (2nd edition), Ten Speed Press, Berkeley, 1986
This is the authoritative field guide to mushrooms of the Western United States. The book provides thorough keys for identifying mushrooms, as well as lively anecdotes and related information for the amateur and expert alike.
Alexopoulos, Constantine J., C. W. Mims, M. Blackwell, Introductory Mycology, (4th edition), John Wiley & Sons, New York, 1996
A college-level text on the world of fungi, organized according to the principles of classification.
Margulis, Lynn, Karlene Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (2nd edition), W. H. Freeman and Company, New York, 1988
An overview of the highest levels of Taxonomy. I have used the authors' nomenclature where available. Names, however, are constantly changing in the field of Taxonomy, and no doubt many of these names are disputed or have changed since 1988.
Margulis, Lynn, Diversity of Life: The Five Kingdoms, Enslow Publishers, Inc., New Jersey, 1992
Although billed as a children's book, this book is quite appropriate for the adult amateur. Dr. Margulis strikes an excellent balance between detail and brevity in this fact-filled book.
Milani, Jean P., et. al. Biological Science: An Ecological Approach (6th edition), Kendall/Hunt Publishing Company, Iowa, 1987
A high school textbook that devotes several chapters to Taxonomy and the diversity of life on our planet. The Appendix titled A Catalog of Living Things illustrates the phyla as well as many classes and families within the five kingdoms.
LIFE HISTORY OF FUNGI
Fungi exist primarily as filamentous dikaryotic organisms.
As part of their life cycle, fungi produce spores. In this electron micrograph of a mushroom gill, the four spores produced by meiosis (seen in the center of this picture) are carried on a clublike sporangium (visible to the left and right). From these spores, haploid hyphae grow and ramify, and may give rise to asexual sporangia, special hyphae which produce spores without meiosis.
The sexual phase is begun when haploid hyphae from two different fungal organisms meet and fuse. When this occurs, the cytoplasm from the two cells fuses, but the nuclei remain separate and distinct. The single hypha produced by fusion typically has two nuclei per "cell", and is known as a dikaryon, meaning "two nuclei". The dikaryon may live and grow for years, and some are thought to be many centuries old. Eventually, the dikaryon forms sexual sporangia in which the nuclei fuse into one, which then undergoes meiosis to form haploid spores, and the cycle is repeated.
Some fungi, especially the chytrids and zygomycetes, have a life cycle more like that found in many protists. The organism is haploid, and has no diploid phase, except for the sexual sporangium. A number of fungi have lost the capacity for sexual reproduction, and reproduce by asexual spores or by vegetative growth only. These fungi are referred to as Fungi Imperfecti, and include, among other members, the athlete's foot and the fungus in bleu cheese. Other fungi, such as the yeasts, primarily reproduce through asexual fission, or by fragmentation -- breaking apart, with each of the pieces growing into a new organism.
--------------------------------------------------------------------------------
Fungi are heterotrophic.
Fungi are not able to ingest their food like animals do, nor can they manufacture their own food the way plants do. Instead, fungi feed by absorption of nutrients from the environment around them. They accomplish this by growing through and within the substrate on which they are feeding. Numerous hyphae network through the wood, cheese, soil, or flesh from which they are growing. The hyphae secrete digestive enzymes which break down the substrate, making it easier for the fungus to absorb the nutrients which the substrate contains.
This filamentous growth means that the fungus is in intimate contact with its surroundings; it has a very large surface area compared to its volume. While this makes diffusion of nutrients into the hyphae easier, it also makes the fungus susceptible to dessication and ion imbalance. But usually this is not a problem, since the fungus is growing within a moist substrate.
Most fungi are saprophytes, feeding on dead or decaying material. This helps to remove leaf litter and other debris that would otherwise accumulate on the ground. Nutrients absorbed by the fungus then become available for other organisms which may eat fungi. A very few fungi actively capture prey, such as Arthrobotrys which snares nematodes on which it feeds. Many fungi are parastitic, feeding on living organisms without killing them. Ergot, corn smut, Dutch elm disease, and ringworm are all diseases caused by parasitic fungi.
--------------------------------------------------------------------------------
Mycorrhizae are a symbiotic relationship between fungi and plants.
Most plants rely on a symbiotic fungus to aid them in acquiring water and nutrients from the soil. The specialized roots which the plants grow and the fungus which inhabits them are together known as mycorrhizae, or "fungal roots". The fungus, with its large surface area, is able to soak up water and nutrients over a large area and provide them to the plant. In return, the plant provides energy-rich sugars manufactured through photosynthesis. Examples of mycorrhizal fungi include truffles and Auricularia, the mushroom which flavors sweet-and-sour soup.
In some cases, such as the vanilla orchid and many other orchids, the young plant cannot establish itself at all without the aid of its fungal partner. In liverworts, mosses, lycophytes, ferns, conifers, and flowering plants, fungi form a symbiotic relationship with the plant. Because mycorrhizal associations are found in so many plants, it is thought that they may have been an essential element in the transition of plants onto the land.
As part of their life cycle, fungi produce spores. In this electron micrograph of a mushroom gill, the four spores produced by meiosis (seen in the center of this picture) are carried on a clublike sporangium (visible to the left and right). From these spores, haploid hyphae grow and ramify, and may give rise to asexual sporangia, special hyphae which produce spores without meiosis.
The sexual phase is begun when haploid hyphae from two different fungal organisms meet and fuse. When this occurs, the cytoplasm from the two cells fuses, but the nuclei remain separate and distinct. The single hypha produced by fusion typically has two nuclei per "cell", and is known as a dikaryon, meaning "two nuclei". The dikaryon may live and grow for years, and some are thought to be many centuries old. Eventually, the dikaryon forms sexual sporangia in which the nuclei fuse into one, which then undergoes meiosis to form haploid spores, and the cycle is repeated.
Some fungi, especially the chytrids and zygomycetes, have a life cycle more like that found in many protists. The organism is haploid, and has no diploid phase, except for the sexual sporangium. A number of fungi have lost the capacity for sexual reproduction, and reproduce by asexual spores or by vegetative growth only. These fungi are referred to as Fungi Imperfecti, and include, among other members, the athlete's foot and the fungus in bleu cheese. Other fungi, such as the yeasts, primarily reproduce through asexual fission, or by fragmentation -- breaking apart, with each of the pieces growing into a new organism.
--------------------------------------------------------------------------------
Fungi are heterotrophic.
Fungi are not able to ingest their food like animals do, nor can they manufacture their own food the way plants do. Instead, fungi feed by absorption of nutrients from the environment around them. They accomplish this by growing through and within the substrate on which they are feeding. Numerous hyphae network through the wood, cheese, soil, or flesh from which they are growing. The hyphae secrete digestive enzymes which break down the substrate, making it easier for the fungus to absorb the nutrients which the substrate contains.
This filamentous growth means that the fungus is in intimate contact with its surroundings; it has a very large surface area compared to its volume. While this makes diffusion of nutrients into the hyphae easier, it also makes the fungus susceptible to dessication and ion imbalance. But usually this is not a problem, since the fungus is growing within a moist substrate.
Most fungi are saprophytes, feeding on dead or decaying material. This helps to remove leaf litter and other debris that would otherwise accumulate on the ground. Nutrients absorbed by the fungus then become available for other organisms which may eat fungi. A very few fungi actively capture prey, such as Arthrobotrys which snares nematodes on which it feeds. Many fungi are parastitic, feeding on living organisms without killing them. Ergot, corn smut, Dutch elm disease, and ringworm are all diseases caused by parasitic fungi.
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Mycorrhizae are a symbiotic relationship between fungi and plants.
Most plants rely on a symbiotic fungus to aid them in acquiring water and nutrients from the soil. The specialized roots which the plants grow and the fungus which inhabits them are together known as mycorrhizae, or "fungal roots". The fungus, with its large surface area, is able to soak up water and nutrients over a large area and provide them to the plant. In return, the plant provides energy-rich sugars manufactured through photosynthesis. Examples of mycorrhizal fungi include truffles and Auricularia, the mushroom which flavors sweet-and-sour soup.
In some cases, such as the vanilla orchid and many other orchids, the young plant cannot establish itself at all without the aid of its fungal partner. In liverworts, mosses, lycophytes, ferns, conifers, and flowering plants, fungi form a symbiotic relationship with the plant. Because mycorrhizal associations are found in so many plants, it is thought that they may have been an essential element in the transition of plants onto the land.
WAT IS BACTERIA
Bacteria are microscopic organisms whose single cells have neither a membrane-bounded nucleus nor other membrane-bounded organelles like mitochondria and chloroplasts. Another group of microbes, the archaea, meet these criteria but are so different from the bacteria in other ways that they must have had a long, independent evolutionary history since close to the dawn of life. In fact, there is considerable evidence that you are more closely related to the archaea than they are to the bacteria!
FUNGI
Introduction to the Fungi
Of athlete's foot, champignons, and beer. . .
The Kingdom Fungi includes some of the most important organisms, both in terms of their ecological and economic roles. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae, that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.
Fungi also cause a number of plant and animal diseases: in humans, ringworm, athlete's foot, and several more serious diseases are caused by fungi. Because fungi are more chemically and genetically similar to animals than other organisms, this makes fungal diseases very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage to crops. However, a number of fungi, in particular the yeasts, are important "model organisms" for studying problems in genetics and molecular biology.
Of athlete's foot, champignons, and beer. . .
The Kingdom Fungi includes some of the most important organisms, both in terms of their ecological and economic roles. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae, that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.
Fungi also cause a number of plant and animal diseases: in humans, ringworm, athlete's foot, and several more serious diseases are caused by fungi. Because fungi are more chemically and genetically similar to animals than other organisms, this makes fungal diseases very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage to crops. However, a number of fungi, in particular the yeasts, are important "model organisms" for studying problems in genetics and molecular biology.
BACTERIA
Bacteria are often maligned as the causes of human and animal disease (like this one, Leptospira, which causes serious disease in livestock). However, certain bacteria, the actinomycetes, produce antibiotics such as streptomycin and nocardicin; others live symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or on the roots of certain plants, converting nitrogen into a usable form. Bacteria put the tang in yogurt and the sour in sourdough bread; bacteria help to break down dead organic matter; bacteria make up the base of the food web in many environments. Bacteria are of such immense importance because of their extreme flexibility, capacity for rapid growth and reproduction, and great age - the oldest fossils known, nearly 3.5 billion years old, are fossils of bacteria-like organisms.
WORM
A worm is a computer program that has the ability to copy itself from machine to machine. Worms use up computer time and network bandwidth when they replicate, and often carry payloads that do considerable damage. A worm called Code Red made huge headlines in 2001. Experts predicted that this worm could clog the Internet so effectively that things would completely grind to a halt.
A worm usually exploits some sort of security hole in a piece of software or the operating system. For example, the Slammer worm (which caused mayhem in January 2003) exploited a hole in Microsoft's SQL server. "Wired" magazine took a fascinating look inside Slammer's tiny (376 byte) program.
Worms normally move around and infect other machines through computer networks. Using a network, a worm can expand from a single copy incredibly quickly. The Code Red worm replicated itself more than 250,000 times in approximately nine hours on July 19, 2001 [Source: Rhodes].
The Code Red worm slowed down Internet traffic when it began to replicate itself, but not nearly as badly as predicted. Each copy of the worm scanned the Internet for Windows NT or Windows 2000 servers that did not have the Microsoft security patch installed. Each time it found an unsecured server, the worm copied itself to that server. The new copy then scanned for other servers to infect. Depending on the number of unsecured servers, a worm could conceivably create hundreds of thousands of copies.
The Code Red worm had instructions to do three things:
Replicate itself for the first 20 days of each month
Replace Web pages on infected servers with a page featuring the message "Hacked by Chinese"
Launch a concerted attack on the White House Web site in an attempt to overwhelm it [Source: eEye Digital Security]
Upon successful infection, Code Red would wait for the appointed hour and connect to the www.whitehouse.gov domain. This attack would consist of the infected systems simultaneously sending 100 connections to port 80 of www.whitehouse.gov (198.137.240.91).
The U.S. government changed the IP address of www.whitehouse.gov to circumvent that particular threat from the worm and issued a general warning about the worm, advising users of Windows NT or Windows 2000 Web servers to make sure they installed the security patch. .
Reported Viruses
According to a report by Symantec published in September 2007, the company received more than 212,000 reports of viruses, worms and other threats during the first half of 2007, a 185% increase over the second half of 2006.
A worm called Storm, which showed up in 2007, immediately started making a name for itself. Storm uses social engineering techniques to trick users into loading the worm on their computers. So far, it's working -- experts believe between one million and 50 million computers have been infected [source: Schneier].
When the worm is launched, it opens a back door into the computer, adds the infected machine to a botnet and installs code that hides itself. The botnets are small peer-to-peer groups rather than a larger, more easily identified network. Experts think the people controlling Storm rent out their micro-botnets to deliver spam or adware, or for denial-of-service attacks on Web sites.
In the next section, we'll look at patching your system and other things you can do to protect your computer
A worm usually exploits some sort of security hole in a piece of software or the operating system. For example, the Slammer worm (which caused mayhem in January 2003) exploited a hole in Microsoft's SQL server. "Wired" magazine took a fascinating look inside Slammer's tiny (376 byte) program.
Worms normally move around and infect other machines through computer networks. Using a network, a worm can expand from a single copy incredibly quickly. The Code Red worm replicated itself more than 250,000 times in approximately nine hours on July 19, 2001 [Source: Rhodes].
The Code Red worm slowed down Internet traffic when it began to replicate itself, but not nearly as badly as predicted. Each copy of the worm scanned the Internet for Windows NT or Windows 2000 servers that did not have the Microsoft security patch installed. Each time it found an unsecured server, the worm copied itself to that server. The new copy then scanned for other servers to infect. Depending on the number of unsecured servers, a worm could conceivably create hundreds of thousands of copies.
The Code Red worm had instructions to do three things:
Replicate itself for the first 20 days of each month
Replace Web pages on infected servers with a page featuring the message "Hacked by Chinese"
Launch a concerted attack on the White House Web site in an attempt to overwhelm it [Source: eEye Digital Security]
Upon successful infection, Code Red would wait for the appointed hour and connect to the www.whitehouse.gov domain. This attack would consist of the infected systems simultaneously sending 100 connections to port 80 of www.whitehouse.gov (198.137.240.91).
The U.S. government changed the IP address of www.whitehouse.gov to circumvent that particular threat from the worm and issued a general warning about the worm, advising users of Windows NT or Windows 2000 Web servers to make sure they installed the security patch. .
Reported Viruses
According to a report by Symantec published in September 2007, the company received more than 212,000 reports of viruses, worms and other threats during the first half of 2007, a 185% increase over the second half of 2006.
A worm called Storm, which showed up in 2007, immediately started making a name for itself. Storm uses social engineering techniques to trick users into loading the worm on their computers. So far, it's working -- experts believe between one million and 50 million computers have been infected [source: Schneier].
When the worm is launched, it opens a back door into the computer, adds the infected machine to a botnet and installs code that hides itself. The botnets are small peer-to-peer groups rather than a larger, more easily identified network. Experts think the people controlling Storm rent out their micro-botnets to deliver spam or adware, or for denial-of-service attacks on Web sites.
In the next section, we'll look at patching your system and other things you can do to protect your computer
VIRUS HISTORY
Traditional computer viruses were first widely seen in the late 1980s, and they came about because of several factors. The first factor was the spread of personal computers (PCs). Prior to the 1980s, home computers were nearly non-existent or they were toys. Real computers were rare, and they were locked away for use by "experts." During the 1980s, real computers started to spread to businesses and homes because of the popularity of the IBM PC (released in 1982) and the Apple Macintosh (released in 1984). By the late 1980s, PCs were widespread in businesses, homes and college campuses.
The second factor was the use of computer bulletin boards. People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets and other productivity software. Bulletin boards led to the precursor of the virus known as the Trojan horse. A Trojan horse is a program with a cool-sounding name and description. So you download it. When you run the program, however, it does something uncool like erasing your disk. You think you are getting a neat game, but it wipes out your system. Trojan horses only hit a small number of people because they are quickly discovered, the infected programs are removed and word of the danger spreads among users.
Floppy disks were factors in the spread of computer viruses.
The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small, and you could fit the entire operating system, a few programs and some documents onto a floppy disk or two. Many computers did not have hard disks, so when you turned on your machine it would load the operating system and everything else from the floppy disk. Virus authors took advantage of this to create the first self-replicating programs.Early viruses were pieces of code attached to a common program like a popular game or a popular word processor. A person might download an infected game from a bulletin board and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. When the user runs the legitimate program, the virus loads itself into memory and looks around to see if it can find any other programs on the disk. If it can find one, it modifies the program to add the virus's code into the program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time the user launches either of those programs, they infect other programs, and the cycle continues.
If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to a bulletin board, then other programs get infected. This is how the virus spreads.
The spreading part is the infection phase of the virus. Viruses wouldn't be so violently despised if all they did was replicate themselves. Most viruses also have a destructive attack phase where they do damage. Some sort of trigger will activate the attack phase, and the virus will then do something -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, the number of times the virus has been replicated or something similar.
In the next section, we will look at how viruses have evolved over the years.
The second factor was the use of computer bulletin boards. People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets and other productivity software. Bulletin boards led to the precursor of the virus known as the Trojan horse. A Trojan horse is a program with a cool-sounding name and description. So you download it. When you run the program, however, it does something uncool like erasing your disk. You think you are getting a neat game, but it wipes out your system. Trojan horses only hit a small number of people because they are quickly discovered, the infected programs are removed and word of the danger spreads among users.
Floppy disks were factors in the spread of computer viruses.
The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small, and you could fit the entire operating system, a few programs and some documents onto a floppy disk or two. Many computers did not have hard disks, so when you turned on your machine it would load the operating system and everything else from the floppy disk. Virus authors took advantage of this to create the first self-replicating programs.Early viruses were pieces of code attached to a common program like a popular game or a popular word processor. A person might download an infected game from a bulletin board and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. When the user runs the legitimate program, the virus loads itself into memory and looks around to see if it can find any other programs on the disk. If it can find one, it modifies the program to add the virus's code into the program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time the user launches either of those programs, they infect other programs, and the cycle continues.
If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to a bulletin board, then other programs get infected. This is how the virus spreads.
The spreading part is the infection phase of the virus. Viruses wouldn't be so violently despised if all they did was replicate themselves. Most viruses also have a destructive attack phase where they do damage. Some sort of trigger will activate the attack phase, and the virus will then do something -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, the number of times the virus has been replicated or something similar.
In the next section, we will look at how viruses have evolved over the years.
CYBERCRIME
Over the last 18 months, an ominous change has swept across the Internet. The tools driving the new attacks and fueling the blackmarket are crimeware - bots, Trojan horses, and spyware
INTRODUCTION TO VIRUSES
In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.
Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabollically inert; examples of such forms are pictured below.
Viral micrographs : To the left is an electron micrograph of a cluster of influenza viruses, each about 100 nanometers (billionths of a meter) long; both membrane and protein coat are visible. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants.
When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers.
Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.
Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history of life. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral disease might have been responsible for mass extinctions, such as the extinction of the dinosaurs and other life forms. This theory is hard to test but seems unlikely, since a given virus can typically cause disease only in one species or in a group of related species. Even a hypothetical virus that could infect and kill all dinosaurs, 65 million years ago, could not have infected the ammonites or foraminifera that also went extinct at the same time.
On the other hand, because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering. Viruses also carry out natural "genetic engineering": a virus may incorporate some genetic material from its host as it is replicating, and transfer this genetic information to a new host, even to a host unrelated to the previous host. This is known as transduction, and in some cases it may serve as a means of evolutionary change -- although it is not clear how important an evolutionary mechanism transduction actually is.
The image of influenza virus was provided by the Department of Veterinary Sciences of the Queen's University of Belfast. The tobacco mosaic virus picture was provided by the Rothamstead Experimental Station. Both servers have extensive archives of virus images.
The Institute for Molecular Virology of the University of Wisconsin has a lot of excellent information on viruses, including news, course notes, and some magnificent computer images and animations of viruses.
Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabollically inert; examples of such forms are pictured below.
Viral micrographs : To the left is an electron micrograph of a cluster of influenza viruses, each about 100 nanometers (billionths of a meter) long; both membrane and protein coat are visible. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants.
When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers.
Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.
Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history of life. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral disease might have been responsible for mass extinctions, such as the extinction of the dinosaurs and other life forms. This theory is hard to test but seems unlikely, since a given virus can typically cause disease only in one species or in a group of related species. Even a hypothetical virus that could infect and kill all dinosaurs, 65 million years ago, could not have infected the ammonites or foraminifera that also went extinct at the same time.
On the other hand, because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering. Viruses also carry out natural "genetic engineering": a virus may incorporate some genetic material from its host as it is replicating, and transfer this genetic information to a new host, even to a host unrelated to the previous host. This is known as transduction, and in some cases it may serve as a means of evolutionary change -- although it is not clear how important an evolutionary mechanism transduction actually is.
The image of influenza virus was provided by the Department of Veterinary Sciences of the Queen's University of Belfast. The tobacco mosaic virus picture was provided by the Rothamstead Experimental Station. Both servers have extensive archives of virus images.
The Institute for Molecular Virology of the University of Wisconsin has a lot of excellent information on viruses, including news, course notes, and some magnificent computer images and animations of viruses.
virus
A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computer viruses are manmade. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems.
Since 1987, when a virus infected ARPANET, a large network used by the Defense Department and many universities, many antivirus programs have become available. These programs periodically check your computer system for the best-known types of viruses.
Some people distinguish between general viruses and worms. A worm is a special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.
Since 1987, when a virus infected ARPANET, a large network used by the Defense Department and many universities, many antivirus programs have become available. These programs periodically check your computer system for the best-known types of viruses.
Some people distinguish between general viruses and worms. A worm is a special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.
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