Fungus

Above: a bracket fungus. On the basis of this photograph this fungus is determined to be Meripilus giganteus (Giant Polypore). This fungus forms a compound rosette of sporing bodies, each a brown fan-shaped cap, soft and pliable, with pores beneath. The caps originate from a common base and each may reach 10 to 30 cm in diameter. The upper surface is concentrically zoned light and dark brown with radial grooves and is almost flat and covered with fine scales. The intensity of the color is very variable. This fungus grows at the base of trees or stumps or from shallowly submerged roots and prefers beech (Fagus) but can occur on Oak (Quercus).

These fungi are usually found on the sides of trees and on fallen logs. This one is probably growing on a decomposing log beneath the soil surface. Fungi were once classed as plants, but they do not photosynthesize, but instead obtain nourishment from preformed organic materials, in much the same way that animals do. Many are saprotrophic - feeding off decomposing organic remains, such as dead wood. Some are parasitic and may feed off the living tissues of plants (and sometimes animals). Others exist in harmonious symbiosis with organisms that can photosynthesise. Most trees require such fungal symbionts if they are to grow properly (especially on poorer soils). These fungi associate with plant roots, forming mycorhizae (or mycorhizas, lit. 'fungus roots', sing. mycorhiza). The body of a fungus is an often large and diffuse mass of whitish hairlike threads that spread through soil, rotting logs or living wood in the case of parasites. This mass is called the mycelium. The part of the fungus that is usually visible are the sporocarps or sporing bodies, like those shown above. Periodically the mycelium puts out these structures into the open to shed spores. The mycelium is made up of fine hairlike fibers called hyphae. Hyphae have a very large surface area to volume ratio, which makes them exceptionally good at absorbing nutrients. The hyphae secrete enzymes (or rely on those secreted by other micro-organisms) into their food, digesting it and then absorbing the nutrients with their hyphae. In the case of mycelia, the hyphae are especially good at absorbing hard-to-get soil nutrients like phosphorus, indeed they are much better than plant roots at this. They give the plant roots some of these minerals in exchange for sugars manufactured by the plant during photosynthesis.

The diagram below shows the mycorhiza of the beech tree (
Fagus sylvatica). The mycelium forms a hairlike net that en-sheaths part of the root, this is called the Hartig net. Some of these hyphae penetrate between the root epidermal cells, which form the outer layer of root tissue.

Fagus mycorhiza

A single fungus may, in this way, connect the roots of several trees together. The roots of neighbouring trees may also naturally graft directly together, especially if the trees belong to the same species. Such grafted roots can exchange nutrients and this helps the weaker trees. If a tree is felled, the stump may regenerate (depending on the species) by putting out new shoots. Some of the nutrients for this growth may come from neighbouring trees through the interconnected root network.

The
largest organism on Earth is thought to be a fungus, whose mycelium covers some 37 acres, is estimated to be about 700 years old and to weigh over 100 tonnes. However, it is not easy to prove whether this mycelium remains continuous or whether it has fragmented into several smaller mycelia (which would make it a clone rather than a single organism).

Some fungi can form mycorhizas with a variety of different trees, but each woodland type, such as beech, birch, oak or pine, has its own characteristic set of mycorhizal fungi. For example, the fly agaric,
Amanita muscaria, will only grow under birch and pine. Likewise, each species of tree is often capable of forming mycorhizas with several different fungus species. These fungi may or may not contribute to leaf litter decomposition, but they are good at extracting nutrients like phosphate. Brushing aside the leaf-litter in a beechwood will reveal the tiny feeder roots that protrude vertically into the leaf-litter and these coral-like roots are the mycorhizas.

Orchids characteristically form mycorhizas of a type called
endomycorhizas - in this type there is no external hyphal sheath, or Hartig net, around the root, and the fungal hyphae penetrate and internally invade the cells in the outer layers of the root. Deeper in the root is a zone in which cells are digesting the fungal hyphae and the central zone is fungus-free. These fungi can digest cellulose and lignin (present in plant remains) and the relationship appears a complex and unsettled one. The orchids need the mycorhizas when they are young germlings and seedlings. The seeds will not germinate without their fungal partner, however, sometimes the fungus infects and consumes the seed, destroying it. This illustrates how these mutualistic symbioses cover a spectrum of relationships, with the fungus parasitising the plant on one hand, and the plant eating the fungus on the other. Some orchids are non-photosynthetic and  lack green chlorophyll and these depend on their fungal partner to provide them food throughout their life. In contrast to the mycorhizas of trees, in which the fungus provides minerals and the plant sugars, in orchid mycorhizas the fungus appears to be supplying the plant with sugars by digesting cellulose. In this way many orchids are able to feed saprotrophically as well as photosynthetically. Many other herbaceous plants rely on mycorhizas, but mycorhizas tend to be absent from grassland.

Fungal hypha

Key

Above: the growing tip of a fungal hypha. Hyphae grow at their tips. The fungus 'cell' is really a giant cell with many nuclei, though in some fungi cross-wall partitions occur at intervals along the hypha. The hypha is surrounded by a hyphal wall. the growing tip is rich in vesicles, which either bud from the Golgi apparatus (dictyosomes) if these are present, or else directly from the endoplasmic reticulum. Many of these vesicles fuse with the cell-surface membrane at the hyphal tip and discharge their cargo by exocytosis, which will include materials to extend the cell wall as the hypha grows. The vesicles also add to the cell-surface membrane.

Behind this zone is a region of intense metabolic activity - rich in ribosomes that synthesise proteins and mitochondria that provide the hypha with power.

Further from the tip, vacuoles of increasing size become increasingly numerous and large lipid bodies which store fuel.

The middle zone, rich in ribosomes, may be much longer than illustrated here.

Fungi (properly pronounced 'fun-gee' with a hard g as is 'geyser') take a very different approach to the cellular animals and plants, in that they build bodies from hyphal filaments. The hyphae often have cross-walls, with pores connecting the cytoplasm on either side, and so can in this case be considered as chains of cellular units. However, although we sometimes refer to fungal cells, with the exception of the cellular yeasts, it is not always easy to say what exactly a fungal 'cell' is: is it a hypha or one unit in a hypha partitioned by cross-walls.

Mushroom pseudoparenchyma

Above: a cross-section through part of stalk is made up of columns of hyphae, of variable size, packed together. In cross-section these hyphae resemble the parenchyma cells of plants, but in
longitudinal section are shown to be long filaments, unlike parenchyma, and so are sometimes called
pseudoparenchyma. This is an interesting alternative approach to building a body - building it from filaments. These sporing or fruiting bodies, such as toadstools, enable spores to be released above the boundary layer of still air that coats solid surfaces. In air the boundary layer is generally much thicker than in flowing water. In water microscopic filaments or biofilms may be sufficient
to break the boundary layer.

One of the most remarkable things about fungi is their hardiness. Fungi have been able to colonise extreme environments, such as inside rocks in the extremely cold and dry Antarctic deserts (the driest places on Earth) and the inside of fuel tanks. Even the mildew that may grow on walls and other damp surfaces seems to grow well with remarkably little. Many fungi are colonisers, some are adapted to grow on burnt soil in forests, and their spores will only germinate in high temperatures. Lichens can grow on bare rocks, even on wind-blasted and salty rocks that line the upper shores. Lichens are remarkable composite organisms - composed of two very different organisms growing in a single body. One is the fungus, the other is a photosynthetic cyanobacterium or a eukaryotic alga.

This bracket fungus, growing on an oak tree, is probably Ishnoderma resinosum or a related form, as it has the right shape and colour and characteristically yields drops of red resin (click photos to enlarge, the droplets are visible as spots). However, this fungus is said to grow on dead stumps from September to December, but here it is in June on a living oak tree. Of course it could be feeding off dead wood that is part of the tree.

Above: Daldinia (probably Daldinia concentrica) otherwise known as King Alfred's Cakes. This fungud grows on ash and occasionally on other deciduous trees. Here it is growing on a fallen branch, which does look like ash. The sporing body can be up to 5 cm (2 inches) across. Also sporulating on this log was a myxomycete or plasmodial slime mould.

More brackets on a living oak tree.

More Fungi

Types of Fungi

Fungi are classified scientifically into several groups and can also be classified more commonly by
general appearance. Scientifically we recognise three major divisions: 1) zygomycetes, 2) ascomycetes, and 3) basidiomycetes.

1) The
zygomycetes are the simplest of the true fungi, and many moulds belong to this group (moulds can also be bacterial or protoctistan in origin, see classification of organisms). These moulds consist of spreading masses of feeding hairlike-hyphae (the mycelium) and vertical aerial hyphae, such as the sporangiophores, bearing sporing bodies/capsules (sporangia, sing. sporangium) which contain a mass of spores. The mycelium consists of branching hyphae that generally lack dividing cross-walls (they are aseptate), so that they are multinucleate and the protoplasm is undivided. Cross-walls do form in old colonies, however, where they wall off dead air-filled hyphae from the still living hyphae.

Reproduction in Zygomycetes

Bread mould, Mucor, is a common example. Mucor is heterothallic, meaning it exists in two body types (heterothallic = more than one 'body' or thallus type) which are physically identical but of different sexual compatibilities: the + (plus) type of mycelium can only mate with a - (minus) type of mycelium and vice versa - mycelia of like types can not mate. Each spore produces either a plus-type or a minus-type mycelium, upon germinating. Each mycelium produces minute, more-or-less spherical, asexual sporangia, borne on sporangiophore stalks and the asexual spores are of the same genetic type as the parent mycelium (they are genetic clones of the parent). A hemispherical cross-wall, called the columella, forms the wall of the sporangium and divides the sporangiophore stalk from the sporangium.

mucor

Above: part of a Mucor mycelium magnified, showing an erect aerial and asexual sporangiophore bearing a spore-containing spherical sporangium. These sporing structures give the zygomycetes their common name of pin-moulds. The sporangium is about 100 micrometres in diameter.

Spore-dispersal may be primarily by wind or water, though the role of animals should not be underestimated. Wind-dispersed species, such as Mucor plumbeus, the outer surface of the
sporangium is studded with spikes of calcium oxalate, which make the surface water-repellent, keeping it dry. When ripe, the sporangial wall ruptures and breaks apart, exposing the dry, powdery spore mass which can be dispersed by air currents. Most species of
Mucor, however, rely primarily on rain-splash to disperse their spores. In these species, such as Mucor hiemalis and Mucor ramannianus, the sporangial wall dissolves, but it does not reveal a dry powdery mass of spores, instead the spore mass expands as it takes up water, forming a slimy sporangial drop, larger than the original sporangium. The slime dries, leaving the spores cemented to the columella. These spores cannot be easily blown away by the wind, but can be scattered by falling rain drops (an example of the use of gravitational potential energy to
disperse spores).
Splash-dispersal of this type is short-range, scattering the spores up to one metre from the parent sporangium.

Some pin-moulds that rely on rain-splash to disperse the sporangial spores, produce secondary asexual structures as side-branches branching off the main sporangiophore. Each branch ends in a small secondary sporangium, or
sporangiole, containing a few spores (4 spores per capsule in Thamnidium elegans). These sporangioles are small and very light and detach easily to be blown away by the wind. Cunninghamella only produces one type of asexual sporangium, called conidia (sing. conidium). These are interpreted as tiny sporangia, each containing only a single spore. Conidia are borne on single or branched aerial hyphae called conidiophores. Conidia easily detach to be blown away.

Each spore inside the sporangium contains one or more nuclei and each may germinate under certain conditions to form a daughter colony which is genetically identical to that of the parent. Both the parent and asexually produced daughter colonies are
haploid (that is they contain only one set of chromosomes).

The mating-type is determined by a single gene with two alleles (versions of a gene), one for plus, the other for minus. If a plus-type (+) encounters a minus-type (-) then mating and
sexual reproduction may occur. (Some mycelia may be neutral and these do not mate with either + or - types). When close, but not touching, the fungi sense the presence of the compatible other and both produce a type of aerial hyphae, called zygophores, which arch over towards the neighbouring mycelium. Each mycelium-type secretes its own mating hormone and this hormone triggers mycelia of the opposite type to produce trisporic acid. Two neighbouringh mycelia produce enough trisporic acid to trigger zygophore synthesis. Once synthesises, another set of volatile chemical messengers are produced, which diffuse through the air and stimulate the zygophores to grow towards the colony of opposite type (a process called zygotropism; tropism being growth in a specific direction triggered by a stimulus). When zygophores of neighbouring colonies rub against one-another (or at least come in close proximity?) they develop sexual structures. They often rub together on one side and here they produce lateral swellings, called progametangia (sing. progametangium). As the progametangia develop, a wall-partition grows, closing off a multinucleate mass of protoplasm in the end of each progametangium, this multinucleate compartment being called the gametangium (plural gametangia). Where the neighbouring plus-gametangium and minus-gametangium meet, their end walls break down and the protoplasts of the gametangia fuse into a single protoplasmic unit, which is the young zygospore. Plus and minus nuclei fuse in pairs within the young zygospore which becomes encased in a tough shell or coat when mature and is now called the zygospore (or zygosporangium). The zygospore is a resting stage, and after a period of dormancy it is able to germinate. A hypha emerges which develops into an aerial hypha, a sporangiophore bearing a spore-capsule at its tip called the germ-sporangium. This ruptures to release haploid spores which are all either plus or all minus (in some other zygomycetes, spores of both types are produced).

progametangia

Left: two progametangia borne on hyphae of compatible colonies growing toward one-another. In the case of mucor, the progametangia are borne on zygophores (aerial hyphae). In other forms they are simply short upright structures that grow together like a pincer from mycelial hyphae. In forms capable of self-fertilisation, the two aerial hyphae are borne on the same parent hypha.

gametangia

Gametangia develop where the progametangia meet. Subsequently the dividing walls between the two gametangia break down and the protoplasts of the two gametangia fuse. At least some of the haploid nuclei fuse in pairs, forming diploid nuclei.

zygospore

The fusion cell formed by the fusion of the two gametangia is the zygospore, which develops a black and warty outer wall, which is impregnated with melanin and sporopollenin. (Sporpollenin is also found in the walls of the pollen grains of plants). The zygospore wall is very impermeable and resistant. The zygospore protoplasm is rich in glycogen and lipid food reserves.

germ-sporangium

The zygospore is not a dispersive spore, but a resisting spore and does not germinate immediately. When conditions and/or timing are right and it does germinate, it gives rise to a germ tube which develops into an aerial sporangiophore, capped by a germ-sporangium. The germ-sporangium contains haploid spores, produced by meiosis. In Mucor, all the spores in a given germ-sporangium are of the same mating type (+ or -) though in some forms they may contain both mating types (or all three if a neutral type is also formed). This suggests that usually only one of the 4 haploid nuclei produced by meiosis survives. The spores of the germ-sporangium are dispersed and germinate to produce haploid mycelia of a given mating type.

Streaming of protoplasm is readily seen in Zygomycetes such as Phycomyces. The core of protoplasm in the centre of the hyphae undergoes rapid streaming at 20-40 micrometres per second. This rapid streaming is similar to the protoplasmic streaming seen in plant cells, but different in origin. In the fungal hypha it is driven by evaporation (transpiration) of fluid from the growing aerial hyphae - hyphae growing rapidly upwards to form sporangiophores. It is toward these aerial hyphae that the streaming eventually terminates, and such streaming presumably helps deliver the materials needed by these rapidly growing hyphae. In addition, however, there is much slower streaming of the cylinder of protoplasm in the hyphal periphery, adjacent to the cell membrane and wall. This slower streaming may be in the opposite direction to the main streaming and is probably driven by the cytoskeleton, as is the streaming in plant cells. Aerial hyphae initially grow at their tips (apical growth), but then this growth pauses as the aporangium begins to form and then extension of the hypha continues as it grows beneath the sporangium (sub-apical growth).

Fungal eyes

Another example of a zygomycete is Pilobolus kleinii which grows as a tiny hairy mould on cow-pats. This mould produces tiny aerial sporangiophores (each 5 mm or so in height) which bear a terminal swelling, about 1 mm long, which functions as an eye. On top of the eye is a dark spore mass, inside a black capsule (sporangial wall) forming the sporangium. Inside the eye is a lens and beneath it, in the adjoining sporangiophore is a yellow-orange doughnut-shaped ring, which is the light-sensitive retina. The lens focuses incident sunlight onto one spot on the retina, allowing the fungus to accurately determine the direction of the light source. (The mould has no brain and the eye does not serve in image formation,
merely as a light-direction sensor). The sporangiophore bends towards the light and when ready the black sporangium cap and its enclosed spore mass are jettisoned, being fired up to about 2 metres into the air! This disperses the spores clear of the cow pat, into the grass where they may be eaten by a cow and pass through to germinate on a fresh cow-pat.

Pilobolous mould with eyes, computer model rendered in Pov-Ray

Above: Pilobolus, showing four sporangiophores terminating in the eye-like structure with
lens and yellow ring-shaped retina underneath. A black spore mass sits on top of each
eye, waiting to be fired clear and so disperse the fungus.
The sporangium cap is fired by hydrostatic pressure - the sporangiophore protoplasm builds up some 5 atmospheres of osmotic pressure, swelling with liquid as it does so, until it splits along a pre-determined line of weakness behind the black cap which is squirted into the air as teh stretched elastic wall of the sporangiophore recoils. The spore mass or sporangium is discharged along with a droplet of liquid (mucilage) which sticks to a solid substrate, such as a blade a grass, and the unwettable sporangium floats to the surface of the droplet before the mucilage hardens to fasten the sporangium to the substrate. The spore mass is still covered by the dark wall of the sporangium, which probably protects the spores from UV radiation.

2)
Ascomycetes generally produce larger sporing bodies, typically from several millimetres to several centimetres in size. They are easily found in woods and forests. They include the disc-fungi and cup-fungi (discomycetes) such as the orange-peel fungus, Aleuria aurantia, whose irregular cups resemble orange-peel and may be up to 8 to 10 cm in diameter and is found growing on bare soil in woods. This group also includes various flask-fungi and the burnt-fungi, which may be encrusting, flattened or ball-shaped black structures typically found on dead wood, often dry and brittle when mature, as if wood had been burnt, such as Daldinia, shown above, and Hypoxylon; and also various pin-fungi,
resembling fields of pin-heads, often brightly coloured, such as
Nectria cinnabarina, which forms fields of
brownish-red 'pinheads' each 1 - 4 mm across on dead and dying wood.
Xylaria hypoxylon is the candle snuff fungus, often found in woodlands growing on dead wood. This fungus forms sporing bodes that are antler-shaped, 3-5 cm tall and have the appearance of snuffed-out candle wicks. The sporing body consists of one or many minute cylindrical or flask-shaped structures called asci (sing. ascus) each with a pore or ostium in its tip. The asci are modified hyphal hairs and are often accompanied by sterile hairs called paraphyses. The spores, called ascospores, develop inside these asci, and are released through the ostium. The spores are forcefully discharged by a build-up of hydrostatic pressure which ruptures the asci. For example, in the orange-peel fungus, many cylindrical asci, each about 200 micrometres tall and 10 micrometres wide, line the inside surface of the cup, each opens by an ostium, so the lining of the cup is porous and spores are fired from these pores, clear of the cup.

Lichens are composite organisms, consisting of a fungal body living along with a photosynthetic algal partner, which is enclosed in the fungal tissues. The alga may be a simple eukaryotic alga, or a cyanobacterium. Whilst the alga also occurs in nature living alone, the fungus can not live without its fungal host. The alga supplied the lichen with sugars produced by photosynthesis and, in the case of cyanobacteria, with fixed nitrogen. In return the alga is given a protected environment which is elevated higher above the substrate and so closer to the light. The fungus uses the sugars perhaps as a carbohydrate and energy source and also as an osmolyte to resist desiccation in those lichens growing in the splash zone of coastal habitats. The fungal partner is usually an ascomycete, and lichens can be seen to produce tiny cups which contain typical asci. However, there are so many lichens that they are usually considered separately.

The mycelium of ascomycetes consists of branching hyphae that also anastomose (fues) to form a 3D network. The hyphae have dividing cross-walls (they are septate) so that each hypha is divided into a chain of 'cellular' compartments, each containing one to several nuclei. Each cross-wall has a small central pore, which is large enough for mitochondria to squeeze through, and also nuclei 9albeit more slowly), so that the ptotoplasm is continuous throughout the hypha. Such cross-walls add mechanical strength to a hypha and this is likely their prime function.

3)
Basidiomycetes include those fungi with the largest sporing bodies, such as toadstools, mushrooms and the bracket fungi shown above. The largest brackets may be several feet across and are often tough, leathery or woody in texture. They also include some of the jelly-fungi, such as the oddly-named Jew's-ear fungus (Auricularia auricula-judae) which is gelatinous and translucent and resembles a brownish human ear.

In addition to these three groups a fourth, the
deuteromycetes, is a dumping-category for fungi that have only been observed in the asexual stage and either reproduce sexually only rarely or have lost the ability altogether. Since spore and sporocarp morphology determine whether a fungus is a zygomycete, ascomycete or basidiomycete, classifying these fungi is difficult. These are also known as the 'imperfect' fungi. Penicillium and Aspergillus belong to this group, though some forms do reproduce sexually and Penicillium is also an ascomycete. Aspergillus niger has never been seen to reproduce sexually. Penicillium is, of course, the original source of the first discovered antibiotic, penicillin, which the fungus secretes to inhibit the growth of competing bacteria, and ultimately to kill them, allowing the fungus to feed on their remains.

The spores, called basidiospres, are produced externally rather than inside a cell or hypha. Upon germination they produce haploid mycelia (haploid = containing half the set of genes, or one parental copy of the gene set) in which all the nuclei are of the same genetic type. This mycelium is called a
monokaryon. Monokaryons come in several mating types and when two compatible types meet, they fuse to produce a compartment containing two nuclei, one from each momokaryon. This compartment grows into the mature fungal mycelium, which contains an equal proportion of nuclei of each of the two types and so is said to be dikaryotic and is called a dikaryon. The dikaryon is diploid (containing the full genetic makeup of two parental sets of genes) even though the nuclei do not apparently fuse, so each nucleus remains haploid. The basidiospores are produced by compartments called basidia. The nucleus pair in each basidium fuse, forming a diploid nucleus, which then undergoes meiosis, that is they divide to form four haploid nuclei. These daughter nuclei form the basidiospores, so each basidium produces four spores (typically).

The mycelium of basidiomycetes consists of branching and anstomosing hyphae that form a 3D network. Cross-walls divide the hypha into a chain of compartments, each  compartment typically containing two nuclei in the dikaryon, one in the monokaryon. A large central pore traverses each cross-wall, so that the protoplasm throughout the hypha is continuous. The wall material around the pore is thickened, forming a barrel, the whole structure being called a
dolipore. Surrounding each end of the dolipore is a dome of perforated endoplasmic reticulum, called a parenthesome. A new cross-wall is laid down each time a pair of nuclei divide. Typically, there is a bulge in the cell wall, containing protoplasm, on one side of the hypha at each cross-wall in the dikaryon, called a clamp connection. Shortly after duplication of the pair of nuclei in each growing hyphal tip of the dikaryon, a new cross-wall is deposited, maintaining a pair of nuclei in each compartment. The clamp connection acts as a conduit for a nucleus as following each cross-wall formation it is essential to sort the nuclei so that the two new daughter compartments contain two nuclei, one of each genetic type. To prevent compartments forming containing two nuclei of one type only, the clamp connection acts as a conduit for a nucleus during re-assortment (otherwise the nucleus would have a hard time passing through the parenthosome and dolipore).

Proto-fungi

In the same way that algae ('protophytes') are no longer classified as plants, protozoa no longer classified as animals, there are some 'proto-fungi' that are no longer classified as fungi. These include the water-fungi or water-moulds (oomycetes) and the plant pathogen Phytophthora. These proto-fungi produce water-borne motile spores, called zoospores, at some stage in their life-cycle. these zoospores swim by means of one or more flagella, typically a tinsellated-flagellum (a flafgellum bearing side hairs) is held out in front and pulls the cell forward, whilst a typical eukaryotic smooth flagellum, trailing behind, pushes the cell forwards. Both flagella undulate, with waves passing  from the base to the tip. these prot-fungi were once grouped with the zygomycetes, as a group called the phycomycetes, but are now officially grouped (dumped!) with the algae and protozoa in the kingdom Protoctista. Other protofungi (sometimes classed as protozoa) are the cellular and plasmodial slime-moulds.

Sporocarps

As we have seen, the main body of a fungus is the white mass of very fine branching hyphae, called the mycelium, which lives beneath or within the substrate on which the fungus grows (such as wood or soil). The mycelium is the feeding and growing part of the fungus. It removes nutrients and water from the substrate, by secreting enzymes and then absorbing the released nutrients through its fine hyphae, which have very large surface area to volume ratios. The mycelium is exceptional at absorbing nutrients, which is why most trees and other flowering plants rely on mycorhizae for nutrients.

At certain times of the year, the mycelium puts out one or more 'fruit', more properly called sporing bodies or
sporocarps. The cups, discs, brackets and toadstools we have seen are all sporocarps. A medium-sized mushroom releases some 500 000 spores a minute over a several day period called spore fall. Most sporocarps die after spore fall, though some can live for ten years but only spore for a few weeks each year. Commonly, fungi are often classified by sporocarp type, as follows. This type of classification is descriptive.

1) Toadstools and mushrooms

Umbrela-shaped with a circular cap supported on a central stalk. Many basidiomycetes have this form. The cap produces spores, which fall from gill-like slits (as in Agaricus) or pores (as in Boletus), under gravity, into the turbulent air, which carries them away. The purpose of the stalk is to raise the cap above the layer of still air which naturally covers all surfaces, and into the turbulent air layer where the spores can be better dispersed. Toadstools have a spongy texture and are short-lived, lasting only a few days.

toadstool anatomy

Above: anatomy of a toadstool sporocarp (using a 3D computer model rendered in Pov-Ray). The cap or pileus contains gills: flat plates of soft tissue bearing characteristic cells called basidia (singular basidium). Each basidium initially bears four black spores (basidiospores). These spores are fired away from the gill surface to a distance of 0.1 to 0.3 millimetres and once clear of the gill they then fall from the cap, in-between the gill plates, under gravity. Spore discharge is accompanied by shedding of a drop of liquid from the spore and a weakening of the join connecting the spore to its basidium. Additionally either surface tension forces and/or electrostatic repulsion are thought to propel the spores from the gill surface. The stipe or stalk holds the cap a few centimetres above the surface, placing it above the most stagnant layer of air (stagnant boundary layer) which is typically a few centimetres, so that when the spores fall free of the cap they are caught by turbulent air and carried away and dispersed by the wind.

3D computer model of a toadstool rendered in Pov-Ray

3D computer model of a toadstool rendered in Pov-Ray

The annulus is a disc of tissue where the cap, originally a sphere in the young button-mushroom stage, was attached to the stipe. When the cap ripens and expands into its umbrella shape, it tears free of the stipe, leaving the annulus where the breakage occurred. The cap then continues to expand as it straightens out. Extending from the base of the stipe, into the substrate/soil are fine branching strands, called mycelial strands, which are bundles of hyphae. Mycelia often explore the substrate as mycelial strands and fan out into feeding hyphae where they encounter a food source. Bundling into mycelial strands helps the mycelium cover larger distances as it grows, and also provide stronger anchorage points for the sporocarp. The strands are also good at conducting nutrients over large distances from one part of the mycelium to another. The toadstool is short-lived, lasting a few days. Its light construction is ideal for its function. Click the thumbnails below for full-size unlabeled versions of the toadstool model.

computer model of mushroom gill modeled in Pov-Ray

Above: a vertical section through a mushroom gill (computer model). The gill comprises a sheet of vertical hyphae, called the trama. Either side of this is a sheet of convoluted hyphae, called the sub-hymenium, which are more-or-less horizontal and so appear as pseudo-parenchyma in section (that as sections through the width of the hyphae make it appear as if they are rounded cells, or parenchyma, when they are actually hyphal threads). Outermost are two layers of paraphyses (P) (fleshy hairs) with spore-bearing basidia (B) dispersed among them (in a hexagonal arrangement when the gill is seen face-on, typically with six paraphyses around each basidium, bottom right) forming the hymenium. Each basidium produces four basidiospores, each of which is borne on a small stalk (sterigma). When discharged the spores are fires out sideways in this perspective, before falling down under gravity. in at least some spaces, it is known that the sterile paraphyses are in fact developing basidia. The gill is about 150 micrometres thick and each pair of gills are about 100 micrometres apart. Large cells, called cystidia (sing. cystidium, not shown) emerge from the hymenium at intervals and span the gap between neighbouring gills and serve to keep the gills apart, so that the spores can fall freely.

computer model of mushroom gill modeled in Pov-Ray, without labels

toadstool gill model seen in surface view

Below: a section through the gills of the mushroom Agaricus (permanent preparation, stained and fixed).

Agaricus gills

Boletes, such as Boletus, are toadstools in which instead of gills, the cap has vertical tubes, each
opening by a pore on the underside of the cap. The spores fall from these pores.

2) Bracket Fungi (Shelf Fungi)

Basidiomycetes that grow attached to wood. Some are parasites growing and feeding on living trees.
Others grow on and feed off dead wood, which they help to rot. Sometimes they grow on buried logs and
may then emerge upright as large leathery trumpet-shaped structures that superficially resemble
toadstools. The sporocarps are tough, leathery or woody in texture and may live for 10 years in some
species; they are often large. They are often semicircular and protrude from the log or branch to which
they are attached, like brackets. They have no, or a very short, stalk. Pores on the underside release
spores, which fall under gravity to be dispersed by the wind. Growing attached to wood, they are
naturally elevated above the ground, often at considerable heights if the host tree is still standing, and
so need no stalk to lift them above the still air layers. Some species of bracket fungi are circular (an may
have short stalks) and do not project but are pressed against the wood. In this instance the spores are
released from the upper surface.

3) Stomach fungi

These basidiomycetes are also descriptively called gasteromycetes (literally: 'stomach fungi') and their
sporocarps are sac-like. A well-known example are the
puffballs. Puffball sporocarps consist of spongy
sacs full of spores. The spores are produced by brown tissue, called
gleba, inside the sac and which is
readily visible if a ripe puffball is cut open. There is a single large circular pore, or sometimes a tear, in
the top of the sac. When rain drops hit the sac, they deform it slightly, causing a dense cloud of spores
to be pumped out through the pore (think of a pair of bellows) to a height clear of the stagnant boundary
layer of air. The puffball is elastic and returns to its previous shape, until the next rain drop strikes it. The falling rain drops provide the energy to disperse the spores, which is due to gravity again.
Giant
puffballs
may be up to 40 cm (16 inches) across and can produce seven million million spores! In the
giant puffball, the spores are released from a tear.

Earth stars are another example of a gasteromycete. Like puffballs, these are sacs of spores and have
one or more pores in the top. The spores are also dispersed by rain-drops, but earthstars can also
move! Leaf-shaped structures or valves close around the sac in dry weather, protecting it and its spores, and open when wet to allow spore dispersal. These movements are passive and due to tensions set up in the tissues  which expand to different degrees when wet, expanding most on the outside, causing the valves to open.

Also belonging to the gasteromycetes are the
bird's nest fungi (Cyathus and Crucibulum) in which the
sporophore consists of a cup or open vase, about 1 centimetre in diameter, and containing disc-like
bodies (
peridiola, sing. peridiolum) or 'eggs' attached to the inner wall of the cup by a small stalk
(funiculus). The whole resembles a tiny bird's nest full of eggs. They occur on rotting wood but
Cyathus
stercoreus
occurs on herbivore dung. The dispersal mechanism is also splash dispersal - large
raindrops striking the inside of the cup are broken into smaller droplets and reflected out of the cup,
often carrying a peridiolum with them for a distance of up to one metre from the cup. The stalk breaks
away from the cup easily and accompanies the peridiolum. The stalk is sticky and attaches the
peridiolum to an object it lands upon. Each peridiolum consists of a wall of hyphal tissue surrounding
gleba (a mass of small chambers separated by tramal tissue and lined by basidia). The hyphae in the
wall may grow out and establish a new colony asexually, but if the peridiolum of
Cyathus stercoreus is
eaten along with vegetation by a herbivore, then the basidiospores pass through and germinate on the
dung.

Sphaerobolus forms tiny cups, only 1-2 millimetres in diameter, containing a single 'egg' or glebal mass
about 1 millimetre in diameter.The cup has two linings, joined at the rim, so that it is like one cup inside
another. The cup is designed to dry out in such a way that strains are set up, until the inner cup
suddenly flips out, bulging up and propelling the glebal mass several metres! The hyphal in the glebal
mass may produce a new mycelium asexually, or sometimes from the basidiospres. Often a new
mycelium grows from large asexual spores called
gemmae, which are also present in the glebal mass.
Again, this fungus may perhaps also be dispersed by herbivores eating the glebal mass and passing te
spores out with their faeces.

Bird's nest fungus courtesy of Nicholas Money

Above and below: The bird's nest fungus, e.g Cyathus, Crucibulum. The 'nest' or cup structure is about 1 cm across and the 'eggs'  are the peridiola - the gleba divides into these masses during development and each peridiolum is encased in a firm wall of interwoven hyphae. Each peridiolum is attached to the inner cup wall by a hyphal thread called the funiculus. Rain drops striking the cuip scatter off its inner surface and the larger droplet fragments may carry away a peridiolum up to a meter or so from the 'nest'. The funiculus is sticky and will entangle on any object that it strikes on its projectory. Some of these fungi, e.g. Cyathus stercoreus live on herbivore dung, and if the periodiolum comes to rest on grass it may be eaten by a herbivore and pass out in the dung, in which case the basidiospores will germinate with a ready-made food source! Other forms grow on rotting wood and sometimes new colonies are established by the growth of the hyphal case of the peridiolum, without basidiospores germinating.

Images courtesy of Nicholas Money, Professor of Botany at Miami University. Prof. Money is an expert on fungal movement and spore dispersal.

Dispersal in bird's nest fungus courtesy of Nicholas Money

4) Stink horns and cage fungi

These basidiomycetes are varied in shape, but many, such as Phallus, are phallus-shaped. They 'hatch'
from egg-like structures early in the morning and are covered in a sticky fluid which contains the spores
and smells like a rotting corpse. This attracts flies who carry away the sticky fluid and its spores,
dispersing them. By about midday there is no sticky fluid remaining on the stink-horn and it begins to
whither. Stink horns are classified with the gasteromycetes, as although they are not splash-dispersed,
the spores are released by tissue breakdown from a sac-like structure. In some forms, the spore-
producing body expands outwards, from a white egg-like structure, but develops into a lattice-like 'cage'.
These are the so-called 'cage fungi'.

5) Disc and Cup Fungi

The sporocarp is disc or cup-shaped in these ascomycetes. They are usually stalk-less, with the
exception of morels which consist of a wrinkled head on top of a stalk. They generally do not need
stalks, since they shoot their spores a short distance into the air (several millimetres to several
centimetres) just high enough to clear the layer of still air and enter the turbulent layer to be scattered
by the wind. The spores are fired from the upper surface of the disc, or from the inside of the cup. They
have no gills and no visible pores, but are smooth. However, the spore-releasing surface is covered by
microscopic pores. They become hard and brittle when dry, but remain alive, and fell rubbery when
moist. The ascomycete sporocarp (
ascocarp) is called an apothecium when disc or cup-shaped and a
perithecium when flask-shaped.

6) Burnt Fungi

These basidiomycetes also fire their spores into the air, but resemble nodules or raised patches of burnt
and hard wood. They are black, dry and hard to the touch. They grow on rotting wood.

7) Jelly Fungi

Feel like jelly, and are often translucent (like coloured glass). They come in a variety of shapes,
including brain-like masses and ear-like lobes (e.g. Jew's ear fungus). They usually grow on rotten wood.

8) Coral Fungi

Coral fungi, e.g. Clavaria, are basidiomycetes which grow upright, often branching, and so resemble
coral, candle wicks, tiny clubs, candelabras, tiny antlers or tiny flames. The spore-producing surface
covers the end regions of the sporocarp, being exposed as it covers the external surface.

9) Encrusting Fungi

In addition to the burnt fungi, many ascomycetes and basidiomycetes are encrusting, forming velvety
films, cushions or nodules on wood and other surfaces.

10) Truffles

These ascomycetes produce subterranean sporocarps and they belong to the discomycetes, even
though their spore-bearing surface is not open and exposed as in typical disc and cup-fungi. They seem
to rely on animals, such as rodents, finding and eating them in order to disperse them. The most prized
edible truffles belong to the genus
Tuber. The truffles appear to have evolved from typical
discomycetes, and some still have a structure suggestive of a closed cup. The paraphyses form a
convoluted branching network through the ascocarp (a pseudoparenchyma) which contains spherical
asci dotted about, each ascus containing one to four spores only. The asci in truffles do
not rupture to
disperse the spores.

11) Morels

Morels (Morchella) are ascomycetes that consist of a stalk, up to 15 cm tall, bearing a club-shaped
pitted surface, the pits being lined by asci. Like truffles they are prized by humans as food.

12) Rusts and Smuts

The rusts are basidiomycetes of the Uredinales group with over 100 genera and over 5000 species. All
are parasites of plants, being unable to grow in nature without a host (they are obligate parasites). The
smuts or Ustilaginales is another (smaller) group of plant parasites usually classed as basidiomycetes.
Fungal pathogens of plants will be considered in a separate article.

13) Water Moulds and Protofungi

The water moulds and related types, also called the oomycetes are protofungi (in a similar way that
protozoa are proto-animals and algae are protophytes). They produce aquatic spores, called
zoospores, which swim in water by means of flagella. These protofungi are grouped with the protozoa,
slime moulds and algae in the kingdom Protoctista and are no longer classified as fungi by most
biologists.

14) Slime Moulds

These are no longer classified as fungi and are considered a type of protozoan or protofungus and so
included in the Protoctista. There are two chief types:
cellular slime moulds, such as Dictyostelium, and
plasmodial slime moulds like Physarum. They are capable of motility, moving around slowly by crawling.

15) Yeasts

Yeasts are unicellular fungi that produce no sporocarp and typically no hyphae, occuring predominantly
as walled cells which reproduce by budding. Many yeasts are ascomycetes, but some are
basidiomycetes, whilst others have no sexual cycle and produce only asexual spores. Yeasts are
extremely common, accounting in large part for the dusty appearance of fruit like grapes (the yeast
growing on sugars that leak or exude from the grape) and some human infections. Pink films of 'mould'
are often due to yeast. More on
yeast.

More fungi:

  • See also deadwood for fungi that decompose plant remains and biodiversity for more examples of
    fungi.

Below: some mushroom graphics for general purpose use.

mushroom

mushroom

Bioluminescence

A number of fungi, particularly certain toadstools emit light. The light is frequently emitted by the sporocarp or spores and may attract nocturnal insects, such as moths, which disperse the spores. Sometimes the vegetative (non-sexually reproducing) mycelium luminesces. The exact functions of fungal bioluminescence are not known for certain. Luminescent toadstools are generally wood-eaters, capable of digesting lignin, a complex polymeric polyphenol which is hard to digest and extremely resistant to decay.  Since bioluminescence requires a lot of oxygen (requiring a lot of ATP generated by the mitochondria) it has been suggested that this protects the fungus by consuming reactive oxygen species (ions, atoms and molecules containing an oxygen radical) produced as toxic by-products of lignin digestion. The light produced is cold light, as is typical of bioluminescence in which some 90% of the energy is dissipated as light (compare this to about 10% for a tungsten-filament light-bulb in which most of the energy is radiated as heat) and the light is green or white or whitish-green.

One of the most well-known bioluminescent toadstools in the British Isles s the Honey-Tuft (Honey) fungus,
Armillaria mellea, a well-known parasite of trees which can also parasitise shrubs and herbaceous plants. In North America is to be found the famed Jack o' Lantern: Omphatolus illudens and Omphatolus olearius, formerly Clitocybe illudens, Omphatolus olearius also occurs on the European
continent where it grows preferentially on olive trees (formerly
Pleurotus olearius). Although these fungi emit light bright enough to navigate by, and sometimes to read and write by, at night, others emit a much dimmer light. Some species may also have luminous and non-luminous strains and the full extent of bioluminescence in fungi is not established. So, what is the function of bioluminescence in fungi? Production of light by living organisms consumes large amounts of chemical energy and requires oxygen. The fact that it is so energetically expensive and relatively common suggests that it has a definite advantage to the fungus. In some fungi the sporing bodies emit light as long as they are shedding spores and it has been suggested that they attract night-flying insects to disperse the spores. This may well be the case in these species, but in others, including the Honey tuft fungus, it is the feeding and growing mycelium which emits light, causing rotting wood to glow. Similarly, fungal hyphae have been seen to cause rotting leaves to glow. Indeed, tree roots infected by fungi have been seen to glow. In subterranean roots the light cannot be functioning to attract spore dispersers. Another distinct possibility is that the light is acting as a signal to synchronise fungal activity and growth. Recent research suggests that light produced in deep sea sponges may perform a similar function: conveying signals from one part of the organism to another to synchronise activity during development. Cells in the mammalian body, including human cells, also respond to light and it has been suggested that infra-red light might be sued as a synchronising signal in cell-to-cell communication in mammals.

Fungal Habitats

Some toadstools found on a damp grass lawn in autumn:

Click images to enlarge.

These toadstools were found on a damp lawn in Autumn. Marasmius forms classic 'fairy-rings' - rings of toadstools growing in a lawn, appearing in the same place each subsequent year, but age can be estimated by their diameter). They occur because as the mycelium depletes nutrients in the soil it grows outwards to colonise new soil. Boletes form mycorrhizae with trees and are grow in association with tree roots, which were presumably running beneath the lawn.

In toadstools not all the gills are of the same size. All the gills extend from the cap margin inwards, but
they are of different lengths. Typically some of the gills extend more-or-less all the way from the margin
to the stipe, these are the largest or primary gills. Between each pair of primary gills is a secondary gill,
which is shorter, extending most of the way towards the stipe, and between each primary and
secondary gill is a short tertiary gill.

Another common toadstool of lawns is the
ink cap (lawyer's wig, shaggy ink cap, or shaggy mane)
Coprinus comatus. This toadstool, which may reach 30 cm in height, is unusual in that the gills, initially
white, turn pink and then black with spores as they drip black ink-like droplets from the gill-margin which
slowly dissolves. These droplets are laden with spores. This happens as the gills undergo
autolysis, or
self-digestion, dissolving into liquid spore masses. They will auto-digest rapidly once picked. In most
toadstools, any given region of a gill will contain basidia in different stages of development, but in the
ink cap they develop in a definite pattern, with those at the margin of the cap being the most mature,
thus the maturing gills grade from white near the stipe, to pink and through to black at the margin. The
caps therefore dissolve from the perimeter inwards.

Fungi on Decaying Wood

Fallen tree limbs, dead trunks and stumps are valuable habitats for fungi (and in turn for the creatures
that depend on them, such as fungus-eating insects). (See
Deadwood). Brown rots are caused by fungi
that digest the hemi-cellulose and cellulose components of wood, whilst leaving the lignin intact, causing
the wood to darken. White rots decompose all components of the wood, including lignin. Lignin is an especially tough compound, essentially a complex polymer of phenolic compounds, it is very resilient physically and chemically and few organism can digest it. The wood has to be moist enough to support the growth of these fungi.

Fallen limbs, stumps and dead trunks of
beech trees (Fagus) support an especially rich diversity of
fungi. Some of these have a strong preference for beech wood, e.g.
Pholiota adiposa, a large toadstool
with golden-yellow caps and rust-coloured spores, and which also occurs at the bases of living beech
trees. Others are more general, occurring also on the wood of other species, e.g. the sulphur-tuft
(
Hypholoma fasciculare) a medium-sized toadstool of sulphur-yellow (sulfur-yellow) caps. In the final
stages, when the wood is crumbling the puff-ball
Lycoperdon pyriforme can be seen growing on it (it is
unusual for puffballs to grow on wood). A beech stump takes a few years to rot away completely.

Oak (
Quercus) supports fewer fungi, but some are specific to oak. Fistulina hepatica, the beef-steak
fungus, a large bracket which feels and looks like red meat and resembles meat when cut, grows on
living oaks, though it feeds on the dead heartwood. It is a brown rot, darkening the hard wood to a rich
brown colour without destroying the lignin which gives it its structure and such darkened heartwood is
prized by cabinet makers.

Birch wood decomposes especially rapidly, often leaving behind empty cylinders of bark, whilst conifer
wood tends to decompose more slowly, with yew wood persisting for many years.

Fungi on Decaying Leaf Litter

When leaves begin to senesce, whilst still on the tree, they leak materials from their aging cells, which
supports the growth of microscopic 'phylloplane' ('leaf-plane') fungi, particularly
yeasts, including
basidiomycete yeasts (such as
Sporobolomyces) and black yeast, which is part yeast-like, part
mycelial. These fungi initiate decomposition when the leaf falls, but larger fungi, such as toadstools
soon develop and feed on the
leaf-litter (and also bacteria and invertebrates, such as insects, mites
and worms). These toadstools may be general, occurring in a variety of broadleaf and/or coniferous
woods, or they may be more specific, preferring a certain type of leaf-litter. Many toadstools growing
amongst the leaf litter are also mycorrhizae and these are often quite specific, e.g. the fly agaric,
Amanita muscaria, which is found only under birch and pine with which it forms mycorrhizae.

Eventually, only the more indigestible parts of leaves and wood litter and other organic matter remain -
the lignin (from plants), chitin (from animals and fungi) and keratin (from animals) to form
humus.

polypore fungus

Polyporus squamosus (Dryad's Saddle) bracket fungi growing on an old sycamore tree in Fredville Park. This fungus is parasitic but is also found on stumps. This tree had lost much of its crown but was still very much alive. It is not clear whether this fungus is growing on the living or dead wood. This fungus is found most often on beech, elm and sycamore.


More Fungi


More Fungi - part 2


Fungi
Oomycetes (water-moulds)        Plasmodial slime moulds        Cellular slime moulds        Lichens        Yeast