What is a fungus?

What is a fungus?
The growing mycelium
Finally....the answer to: What is a fungus?
Lifestyles
Habitats and distributions
Substrates
Fungi - good guys or bad guys?

What is a fungus?

To answer this question, let's start with something familiar: a common mushroom. Everyone has seen mushrooms in gardens or fields and knows that they mostly appear in autumn, with each mushroom lasting for a short time before rotting away to a sloppy mess. Typically, there's a stem, a cap and gills under the cap.

When you look at such a mushroom growing out of the ground, you are looking at just part of a fungus - not the whole organism. The rest of the organism (often 90% or more) is underground and consists of a network of microscopically thin "threads" which spread through the soil. An individual thread is called a hypha and the network of hyphae is called a mycelium. The mycelium is there throughout the year, feeding and expanding. You will often see the mycelium referred to as the vegetative part of the fungus.

Under suitable conditions the underground mycelium will produce mushrooms, which are also composed of hyphae. The function of a mushroom is to produce and disperse spores, from which new fungi can develop.

One way to put this into a more familiar framework is to make an analogy with an apple tree. You know that the apple tree in your garden is there throughout the year but apples are produced for just a short time. So the mycelium is there throughout the year but mushrooms are produced for just a short time. The role of the apples is to produce seeds from which new apple trees can develop, analogous to the role of a mushroom, as explained above. Your apple tree typically produces many apples, so a single mycelium can produce many mushrooms. In some years your apple tree may produce no apples, because of poor conditions - but even in such years your apple tree is still there. So, in some years a mycelium may produce no mushrooms, because of poor conditions - but the mycelium is still there, in the ground. The comparison between mushrooms and apple trees is a useful explanatory analogy, but it can't be pushed too far because plant seeds and fungal spores are fundamentally different.

In a mushroom, the spores are produced on the gills that are on the underside of the cap. When mature, the spores are released from the gills, fall down under the force of gravity and, when clear of the bottom of the cap, are then carried away by air breezes. Mushroom spores are tiny, typically less than a hundredth of a millimetre long, and so are easily dispersed by even the slightest of breezes. The role of the mushroom stalk is to raise the cap above the grass, twigs or stones that are close to the ground. If the cap is raised a suitable distance, the spores released from the gills have a good chance of being carried away a substantial distance - rather than getting trapped by obstacles such as the grass, twigs or stones mentioned above.

The growing mycelium

Underground, each hypha grows in length, but not width, and may branch. Each branch then grows in length, may branch again and so on. In addition, connecting hyphae may form between neighbouring hyphae. Over time, the habit of repeatedly branching and lengthening means that the mycelium can explore a large volume of soil.

There is no photosynthesizing green pigment (chlorophyll) in the mycelium, so the organism doesn't make it's own food (as plants do). The hyphae cannot swallow food fragments (as animals do). Rather, enzymes are secreted from the tips of the hyphae in the mycelium and these enzymes break down the complex molecules found in organic matter in the soil, into smaller molecules, which are then absorbed through the hyphal walls near the growing tips.

As the mycelium exhausts its food supply it grows outward, seeking more food sources. There is little point in maintaining the inner mycelium, since the food in that area has been exhausted. Therefore the inner mycelium dies, with the organism extracting from there whatever nutrients it can re-use and transporting those outward.

As you can see, the mycelium does not expand as a circle but as a ring, with most of the activity near the ring's outer edge. Not surprisingly, mushrooms often appear to grow in a ring, the so-called fairy ring (see fairy rings in MYCELIUM SECTION), that reflects the underground mycelial presence. While the steadily expanding, ring-like growth is theoretically the ideal growth pattern for the mycelium, there are many factors which can disrupt this perfectly symmetrical growth. For example, one part of the expanding mycelium may encounter an inhospitable or nutrient-poor area, perhaps a river, a mass of solid rock or soil that's too acidic or too alkaline. Or during construction work, someone runs a bulldozer through a mycelium, leaving only a couple of isolated mycelial patches intact. Whenever there is some such disruption, the mycelium (or what's left of it) will still expand into new areas but the expansion may become quite irregular - perhaps stopping completely in some directions or being faster in one direction than another.

There is more about mycelia in the MYCELIUM SECTION.

Finally....the answer to: What is a fungus?

The descriptions of the mushroom and the underground mycelium encapsulate the essence of all the (macro) fungi that are the subject of this website. Basically, there's an out-of-sight mycelium and visible spore producing structures - called fruiting bodies or sporocarps.

A mushroom is just one type of sporocarp and in the TYPES OF FUNGI SECTION you'll see examples of various types of sporocarps. You'll already be familiar with some of them. For example, most people have seen puffballs on the ground and bracket fungi growing out of trees. But remember whenever you see a mushroom growing from wombat dung, a puffball growing on a grassy oval or a bracket fungus growing from a gum tree - you're seeing just the sporocarp. Its sole function is to produce and disperse spores (more of this in the DISPERSAL SECTION). Out of sight there's a mycelium in the wombat dung, under the grassy oval or in the trunk of the gum tree - secreting enzymes to break down organic matter in the dung, soil or tree trunk.

Strictly speaking, it is incorrect to call a mushroom a fungus, since we are only looking at part of the fungus, but people commonly refer to mushrooms, puffballs, etc. as 'fungi' - which is understandable since these are the only parts we usually see.

Lifestyles

All fungi need existing organic matter for their food. A fungus that feeds on dead organic matter is called a saprotroph and one that feeds on living organisms is a parasite. While there are species that are always parasitic and others that are always saprotrophic there are also those which may feed on either live or dead organic matter and so change from parasitic to saprotrophic behaviour (or vice versa), depending on what food sources are available.

A large group of fungi (called mycorrhizal fungi) form symbiotic associations with plants and obtain some of the food that the plants produce by photosynthesis. However, unlike parasites, the mycorrhizal fungi do not harm plants. Quite the contrary, they are in fact useful to plants - in fact often essential for plant survival. The bulk of the world's plants have such fungal partners. To give just one Australian example, every eucalypt tree that you see (whether in park, garden or forest) will have its underground mycorrhizal partners. There's more about this in the MYCORRHIZA SECTION.

Habitats and distributions

Macro and micro fungi are found throughout the world. While many species of microfungi are found on land and in water, the macrofungi (which are the ones dealt with in this website) are basically terrestrial.

Some plants or animals are found in very restricted habitats while others live happily in a variety of habitats. The same is the case with the fungi. Many people seem to think that they are always associated with well-watered areas, but you can find many species even in the deserts of the world. For example Podaxis pistillaris is widespread in the arid areas of the world.

This delicate Coprinus plicatilis (the cap is about 2 centimetres across) was photographed near the edge of a salt lake in the dry Mallee area of north-west Victoria.

There are many species which are either found only in burnt areas or at least have a strong preference for burnt areas. These are called phoenicoid fungi, after the Phoenix - a mythical bird that arose afresh from a fire every 500 years. An example of this is Anthracobia sp. .

Human environments are not free from fungi. Some years ago the Australian National Botanic Gardens displayed sub-antarctic plants from Macquarie Island in its visitor centre. These were held in a refrigerated cabinet, which leaked onto the carpet, and soon a good crop of Peziza appeared.

Everyone knows that some plants and animals have a very wide geographical distribution, while others have extremely limited ranges. The same applies to fungi. For example, Schizophyllum commune, is found on all continents except Antarctica, from seaside to montane areas - while Gymnopilus norfolkensis is known only from Norfolk Island, halfway between Australia and New Zealand. There is more about the distribution of fungi in the MYCOGEOGRAPHY SECTION.

Substrates

Fungi can be found growing on all sorts of organic matter. This page started with a common mushroom, the mycelium being in the ground and numerous other species are also ground inhabiters. Of course, just as plants have preferences for the soils they grow in, so fungi have likes and dislikes. For example, ammonia is toxic to many fungi but there are species that can tolerate relatively high levels of ammonia. You can find such species growing near old carcases, since a rotting carcase releases ammonia into the underlying soil. Until that carcase has rotted away and the ammonia levels dropped, the ammonia-tolerant fungi will be favoured. An Australian example of this can be seen on the Fungimap web site: Hebeloma aminophilum [Fungimap link].

However, wherever there is organic material, there's undoubtedly a fungus that will use it and here are a few examples.

Wood is host to numerous fungal species and offers numerous habitats. For example, some fungi will grow only on dead wood, while others are found only on live plants. Not even all dead wood is the same. Coniferous wood is quite different to non-coniferous wood and you will find some species able to survive only on coniferous wood, others never on coniferous wood. There are also differences between the environments offered by recently dead wood, partly rotted wood and well-rotted wood.

Schizophyllum commune is a wood-rotting fungus, found on dead or dying wood from a vast variety of tree species. It has also been found on whalebone and the mycelium has been extracted a number of times from human tissue. On at least one occasion the mushroom-like fruiting body was found in a human sinus. While basically a wood-rotting fungus, it appears to be an opportunistic invader of humans with greatly weakened immune systems. By contrast, Cyttaria gunnii grows only on living trees in the genus Nothofagus.

Numerous fungi grow on dung and nowhere else. What was said about wood can also be said about dung. Dung from herbivores is different to dung from carnivores. The fungi found on fresh dung are quite different to the fungi found on old dung.

A fungus such as this Cordyceps gunnii parasitises the larval stage of Ghost Moths in the genus Oxycanus. When the mycelium has extracted all the nutrients that it can from its host (killing it in the process) the fungus produces a fruiting body, releasing spores that will infect other larvae. The mycelium of the Cordyceps cannot grow outside the larval body . There are many species of Cordyceps around the world, parasitising the juvenile or adult stages of various butterflies, moths, beetles, spiders and ants. This link [http://www.mycolog.com/Clavicipitaceae.htm] shows exquisite paintings of various fungi of this type.

Fungi - good guys or bad guys?

This is a question that comes up often enough. Some people look on fungi as slimy, yucky, horrible things - with no redeeming features. Others pursue them with great joy - either for digestive delight or because of their inherent beauty. What is the truth? Well, some of them are slimy and yucky. Just look at this species of Hygrocybe, its slimy, gelatinous coat glistening in the camera's flashlight. But imagine a mass of these carpeting the forest floor - surely a delight to the eye.

Fungi have numerous interactions with other organisms. Parasitic and mycorrhizal lifestyles have already been mentioned. Obviously the parasite is bad for the host while mycorrhizae are beneficial to both plant and fungus. So fungi are inherently neither good nor bad. This page gives just a very brief introduction to the fungal world and elsewhere in this site there's more detail about the roles fungi play. Whether a particular interaction is good or bad will sometimes be a matter of perspective.

If you're a forester, then a wood-rotting fungus that destroys the heartwood in a tree is going to be a serious pest in your eyes because there's an economic loss. From the tree's point of view, the same fungus may be neither harmful nor beneficial. The heartwood is dead wood, with the living tissue confined to a relatively thin skin under the bark. As long as the fungus is not harming that living skin the tree can go on living quite happily. In fact, there are numerous old, healthy, hollowed-out trees in existence. Moreover, an empty cylinder (such as a hollowed trunk) can resist some stresses better than a solid cylinder (such as a solid trunk). If you're a possum or a parrot, then you'd probably look very favourably on that fungus because it is helping to create potential nesting hollows.

So here we have three different views on the same fungus.

Classification and identification

Though this website does not deal with identification the CLASSIFICATION page looks at some aspects of fungal classification. That page should help you get an idea of how mycologists have gone about studying fungi over the past two centuries and there will be a few comments about the naming of fungi. However, there won't be a detailed discussion of the fine points of fungal classification - such as how different species or genera are defined.