Classification of Agaricales | Fungi

In this article we will discuss about the classification of agaricales.

Family Agaricaceae of Agaricales:

The family Agaricaceae includes fungi, commonly known as agarics or gill-fungi, mushrooms or toadstools, produce conspicuous basidiocarps. Mushroom is a general term applied to the fleshy agarics. A mushroom may be edible, poisonous, unpalatable, or otherwise. But popular usage applies the term ‘mushroom’ to edible ones, calling the others ‘toadstools’.

What is ordinarily called the mushroom is simply the basi­diocarp. This structure is not the whole plant but merely a spore-producing part. The vegetative part of the-plant body is the mycelium composed of septate branched hyphae which grow saprophytically in soil rich in organic matter, in or on dead wood, in or on rotting logs or stumps.

The hyphae spread in every direction of the substratum and absorb food from it. The individual hyphae are usually not isolated, but woven together into strands which may develop into root-like structures often termed rhizomorphs. These fungi are mostly saprophytic growing commonly in lawns, pasture and gardens.

Some of them are Agaricus campestris, Lepiota morgani. But some are also facul­tative parasites on the roots of trees and elsewhere. Very common one is Armillariella mellea which is known as ‘honey agaric’.

It causes root rot of Larch. Again some are involved in mycorrhizal associations like Amanita muscaria and Lactarius rufus. The vegeta­tive mycelium buried in the substratum sends up basidiocarps at irregular intervals of months or years, usually after soaking rains.

A mycelium established at a single point tends to ramify radially in all directions and to die out at the centre with the depletion of the available food supply. Successive crops of basidiocarps may thus be produced in successively larger rings, often called fairy rings (Fig. 283) based on the ancient belief that these circular rings marked the path of dancing fairies.

The fairy rings or fungus rings are very frequent in grassland, and not uncommon in woods.

They are of three chief types:

(i) Those in which the development of the basidiocarps has no effect on the vegetation, e.g., Lepiota morgani;

(ii) Those in which there is increased growth of the vegetation, e.g., Tricholoma personatum; and

(iii) Those in which the vege­tation is damaged very badly, e.g., Agaricus praerimosus. Rings of the third type are frequently made up of outer and inner zones in which the growth of the vegetation is strong with a zone of dead or badly damaged vegetation in between where the fungus ring is developed.

Many of the agarics are edible and are considered great delicacies but others extremely poisonous. The edible species are popularly known as mushrooms. Where­as, the poisonous ones are the toadstoods.

Some of the edible agarics are: Agaricus campestris, Agaricus bisporus, Coprinus comatus, species of Lepiota, Marasmius, Tricholoma; and the poisonous ones are: Amanita muscaria, A. phalloides, A. verna.

Spores that are produced in the basidiocarps on germination produce germ tubes which develop into monokaryotic mycelia. The monokaryotic mycelia, in many species, produce at the surface of the substratum single-celled uninucleate struc­tures usually called oidia (sing, oidium). The oidia may be distributed by wind or insects.

An oidium which comes into contact with a cell of compatible hypha, fuses with that cell initiating the dikaryotic stage by spermatization. In some members of the Agaricaceae the oidium germinates to produce a new mycelium.

The oidia may thus behave as gametes or spores. Besides spermatization the dikaryotic condition may be initiated by somatogamy, i.e., by the fusion of cells of compatible hyphae. Two compatible mycelia or mycelium and oidium thus are involved in the establishment of the dikaryotic condition.

Once the initial dikaryotic cell has been formed, a new dikaryotic mycelium may then be derived directly and solely by growth from this cell, or the existing mycelium may be converted to the dikaryotic condition by migration and mitotic divisions of the proper nuclei.

The newly introduced nucleus in- the dikaryon initial divides mitotically, and one of the daughter nuclei passes through the pore in the septum into the adjoining cell, where it again undergoes mitosis and the daughter nucleus passes in the adjoining cell and the process continues when finally all the cells become dikaryotic.

The basidiocarps usually arise from a dikaryotic mycelium which may be short-lived or perennial (Fig. 284).

Their deve­lopment is controlled by the available supply of food, moisture, temperature, and other factors. Under favourable conditions little knots or swellings (Fig. 285A) appear at one or many places on the dikaryotic mycelial threads. These swellings increase in size until they project outside the substratum in which they started to grow.

Each one of these swellings is destined to develop into a full-grown basidiocarp. The swellings gradually develop as small balls, which constitute the button stage (Fig. 285B). A longitudinal section through a swelling from top to bottom shows an undifferentiated mass of fungal tissue enclosed in a membrane or veil which is known as universal veil or volva.

Gradually the swelling grows in the vertical direction and a depression appears all-round more or less near the central region (Fig. 285G & D). A vertical section of it indicates the presence of a stalk or stipe and a small very immature umbrella-like cap or the pileus together with a thin membrane, the partial or inner veil, which connects the margins of the pileus to the upper part of the stipe.

Growth of the pileus and stipe ruptures the volva part of which, in some species, forms a cup at the base of the stipe and the rest breaks up into small pieces which remain on the roof of the pileus (Fig. 285E to H). The partial veil, also ruptures, in most cases, dis­appears entirely. Whereas, in others part of it persists as a collar or little ring around the stipe and is known as an annulus (Fig. 285H).

In still others, fragments of the partial veil may be found hanging from the margin of the pileus.

The presence (Fig. 286A) or absence (Fig. 286D & E) of volva and annulus or any one (Fig. 286B & G) of them are features used to identify the members of the Agaricaceae. In some cases along with the development of basidiocarp, the individual hyphae may be aggregated into mycelial strands or rhizomorphs which pass deep into the soil.

Some shade of tan or brown is the commonest colour of the basidiocarps. But there is scarcely any colour that does not occur among them. A mature basidiocarp usually consists of a stalk or stipe and an umbrella-like cap called the pileus. The tissue of the pileus is like that of the stipe, but rather denser.

On the underside of the pileus there are immense number of plates called gills or lamellae (Figs. 285H and 287 A & D) which radiate from the stipe toward the margin of pileus and are covered by the hymenium. The tissue of the basidiocarp is pseudoparenchymatous, consisting of an aggregation of hyphae. The hyphae are usually thin-walled and dikaryotic (gene­rative hyphae), and may or may not bear clamp connections.

The basidiocarp expands due to inflation of the cells. Although no differentiation into skeletal or binding hyphae occurs, specialized tissues or cells may arise. In a study of the fine structure of the basi­diocarp of Agaricus campestris, Manccha (1965) has shown that the stipe contains two kinds of cells: wide inflated cells and narrower thread-like cells. A similar differentia­tion is, found in Coprinus cinereus.

Two main types of gill structure in a basidiocarp of the Agaricaceae may be dis­tinguished:

(i) Aequi-hymeniiferous (aequihymenial):

Most genera have gills of this type. In longitudinal section the gills are wedge-shaped. The aequi-hymeniiferous refers to the fact that the hymenium develops in an equal manner all over the surface of the gill, i.e., basidial development is not localized at any one point on the gill. The wedge- shaped section may be an adaptation to minimize wastage of spores should the basi­diocarp be tilted from the vertical.

(ii) Inaequi-hymeniiferous (inaequihymenial):

The gills are not wedge- shaped in section, but parallel-sided. The term inaequi-hymeniiferous refers to the fact that the hymenium develops in an unequal manner, with basidia ripening in zones.

The hymenium consists of a layer of very many basidia intermingled with paraphyses (Fig. 287B & G). In some cases, certain specialized structures of taxonomic impor­tance are also present in the hymenial layer. The basidia are aseptate, commonly longer than thick, with four terminal ellipsoid basidiospores obliquely set on the sterigmata (Fig. 287E).

Mature basidiospores are forcibly discharged from the basidia. Under favourable conditions the basidiospores germinate to produce monokaryotic mycelia. The basidiocarps die and decay soon after the liberation of basidiospores. The mono­karyotic mycelia subsequently becomes dikaryotic from which successive crops of basidiocarps are developed. In general, the basidiocarp is stipitate, i.e., with a stipe.

The stipe is usually attached to the tinder surface of the pileus at its centre and file basidiocarp is known as centrally stipitate. When the stipe is attached to the margin it is marginally stipitate and when laterally, eccentrically stipitate. The stipe may be slender or thick, long or short, hollow, pithy or solid. The stipe may also be absent, when the basidiocarp is attached to the substratum directly by its margin (Fig. 286F & G).

The pileus may be leathery (coriaceous), hairy (tomentose), with or without scales.

When dry the pileus may change colour hygrophanous. The pileus may be bell-shaped (campanulate); cone-shaped (conic), inverted cone-shaped (obconic), with edge turned up (revolute), with edge turned downward and inward (involute); funnel-shaped (infundibuliform) with a pit or small depression at the centre (umbilicate), with a knob protruding from the centre (umbonate); with radiating marks or furrows near the edge (stristulate or striate); and split at the edge, with weave edge, smooth, scaly (squamose or squamulose).

The pileus may be large having four or more inches in diameter, medium of one to four inches in diameter and small with less than one inch in diameter.

In form, gills may be broad or narrow, varying with different species. An important distinction is made between gills that extend to the stem known as adnexed, those that are attached broadly to it adnate and others that do not reach it but are said to be free. Gills that run down the stem are termed decurrent, those that are notched in their edge near the stem are known as sinuate or emarginate.

Basidiocarps and mycelium of some of the genera of the Agaricaceae, such as species of Panus and Pleurotus, and Clitocybe illudens glow in the dark which is known as phosphorescence and the basidiocarps are phosphorescent. In Armillariella mellea, mycelium in wood when exposed to air gives out light.

The fungi that give out light sometimes cause the attacked wood or leaves to become luminous, they are also known as luminescent fungi. Again some species of Laclarius possess milky juice in their basidiocarps.

Whereas, some genera of the Agaricaceae are commonly known as inky-caps for having typical pileus. In certain genera, particularly in Coprinus when the basidiocarp is mature, a process of autodigestion or autodeliquescence begins. The pileus becomes liquid resulting in the formation of inky-drops, until only the stipe remains (Fig. 288).

The spores around the margin of the pileus of the basidio­carp of the Agaricaceae are the first to be discharged and are carried away by air current, after which this area is digested. The spores lying higher on the gills are released next, after which the zone where they were formed liquefies and so on, until the entire pileus has disappeared.

The spores vary in size and shape in different species. The characteristics used to distinguish one species of agaric from another include, not only external features but also the colour of the spores. The spore colour is not necessarily correlated with the colour of the gills, which often changes with age, darkening as the pileus matures.

Individual spores are too small to be seen with the naked eye. Ordinarily, spore colour is determined by bringing the fungi in the laboratory and making a spore print (Fig. 289). The stipe is removed from the basidiocarp close to the pileus.

The pileus is placed, gill side down on a sheet of white paper. Since some spores are white or very light-coloured, a duplicate pileus is often placed upon black paper as well. The pileus is placed under a bell jar to prevent the spores from being distributed by air currents.

After several hours, the pileus is carefully lifted and the spores which have fallen are seen in a radiating pattern which duplicates the arrangement of the gills. The colour of the spores may be observed very easily. The spore colours of the various genera of the Agaricaceae are white, yellowish, brown, pink, purple, purplish- brown, black, etc.

A simplified key to the genera of the family Agaricaceae is presented below. On the basis of spore colour, the genera are put together into five groups which are again split up into respective genera considering different other characters.

Genus Agaricus:

The genus Agaricus includes many species that live on the decaying matter in the soil, some of which are edible. Again many of the species belonging to the genus Agaricus are the causes of parasitic diseases and saprophytic rots. There are both homo­thallic as well as heterothallic species of the genus Agaricus. This genus includes fungi whose basidiocarps are soft. They are stipitate.

Stipe is readily separating from the pileus. The basidiocarp is characterized by the production of radiating lamellae (gills). Lamellae are not deliquescing. When young, the lamellae are covered by a veil that extends from the stipe to the margin of pileus. During growth of the basi­diocarp the veil breaks away from the edge of the pileus as this expands or spreads open.

Part of it remains as a collar or little ring around the stipe and forms the annulus. Volva is absent in the basidiocarp. The lamellae are pink in colour, but turning brown with age. They are free from the stipe. Club-shaped basidium usually produces four elliptic basidiospores, whose number may also be two. The spores are purple-brown in mass.

Some Indian species of Genus Agaricus:

Agaricus arvensis Schacff.; A. campestris L. ex. Fr.; A. exaltatus Berk.; A. latipes Berk.; A. pratensis Schacff.; A. squalidus Massee.

Agaricus Campestris L. Ex Fr:

Agaricus campestris, commonly known as medow or field mushroom grows on ground in grassy places, in pastures, on lawn and manured ground, never in thick forests. Basidiocarps occur singly or in groups. This is one of the common represen­tatives of the family Agaricaceae. It is one of the those edible mushrooms most often seen on the tables of restaurants.

This is a homothallic saprophyte whose vegetative mycelium is composed of many interwoven septate hyphae which ramify through the soil just beneath the surface. The basidiocarp producing hyphae are dikaryotic which usually escape notice. The reproductive phase is initiated by the formation of small knob-like swellings at different points of interwoven mycelial strands.

These swellings increase in size and break through the surface of the substratum as small balls constituting the button stage. These ball-like swellings increase in size and each one of them is destined to develop into a full-grown basidiocarp.

Since the growth of the mycelium follows more or less a regular pattern of centrifugal extension starting from a particular point and accompanied by a death and decay of old hyphae in the central portion this may ultimately under favourable conditions, result in the development of a circular ring of basidiocarps which is known as fairy ring. The basidiocarps develop rhizomorphs.

A matured basidiocarp is white or whitish in colour and consists of a thick short stipe with an annulus rather more than half-way up. The stipe supports the pileus which appears as a hat-like expansion. The pileus is broadly expanded or nearly flat, white or with a tendency to become pinkish when cut.

On the underside of the pileus are immense number of radiating gills or lamellae, pink when young, but purple-brown when mature. The pink colouration of the young gills is due to cytoplasmic pigment in the spores. Later the gills turn purple-brown due to the deposition of dark pig ments in the spore wall. The gills are free from the stipe. The gills are formed by an extension of the hyphae of the pileus.

A cross-section of a gill shows that the bulk of the tissue is composed of compact hyphae. The middle part of each gill is formed of hyphae coming down from the pileus, and following on .the whole a longitudinal course, their lateral branches, however, diverging towards the two surfaces. This central tissue of the gill is called the trama.

Towards the free surfaces, the cells of the diverging hyphae are shorter and more closely packed, forming the subhymenial layer, and beyond this again is the hymenial layer (Fig. 287B), which is composed of the terminal cells of the same hyphae constituting the trama and subhymenial layer.

In this layer the hyphae have diverged from their original direction to such an extent that they now stand at right angles to the surface of the gill.

The hymenial layer is composed of a palisade-like club-shaped cells rich in protoplasm. A few of these are somewhat more slender than the rest, and remain sterile, they are known as para- physes (sing, paraphysis) (Fig. 287C).

The others are stouter at maturity, and are the elements which produce spores, these are the basidia (Fig. 287C). Each basidium bears four basidiospores on sterigmata (sing, sterigma) (Fig. 287E).

The young basidiospores are pink, but when ripe are purple-brown in mass. They contain oil, and have each two or four nuclei. The basidia do not develop simulta­neously on all parts of the gills. In the pink zones the basidia are nearly ready to shed their spores.

Whereas in the paler zones the basidia are only just beginning to form their spores. At maturity the spores are violently shot from the sterigmata. After spore discharge the basidia collapse, and this part of the gill again turns pale. Subsequently, the same area once more becomes pink owing to the development of another crop of basidia.

At a later stage the whole of the gills of the basidiocarp become brownish or purplish black on account of darkening of the cells in the matrix of the gills. Ultimately the basidiocarp falls to the ground and decays. The spores give rise to fresh mycelium from which new basidiocarps are developed.

Life cycle of a heterothallic species of Agaricus is presented in Figure 290.

The Scientific name:

E.M. Fries (1794-1878) placed Agaricus campestris under the genus Agaricus but within a tribe which he called Psalliota. The tribe Psalliota was later elevated to the rank of genus. Consequently, books not directly influenced by modern taxonomy and not concerned with the international rules of nomenclature, named Agaricus campestris as Psalliota campestris.

The name Psalliota campestris is taxonomically and nomenclaturally wrong, and should disappear from the literature. The genus Agaricus which has been split into two or more genera, for one of them, according to the International Rules of Botanical Nomenclature, Article 51, the original generic name Agaricus should have been retained. But very few have followed this rule.

According to this rule, Agaricus is the valid name for the genus of Agaricus campes­tris. Instead of naming the genus correctly it is often called Psalliota which actually is a synonym of Agaricus. In spite of this, a small group of mycologists wished to retain the name Psalliota against the rules of priority creating a confusing situation. This was rejected at the International Congress in Paris in the year 1954.

Cultivated and medow mushrooms:

Agaricus campestris L. ex. Fr. as des­cribed by Linnaeus and accepted by Fries, is a very common fungus of the open pas­tures and grasslands. Its basidia are four-spored. Whereas, the commonly cultivated white mushroom whose correct name is Agaricus bisporus (Lange) Sing, (also called Agaricus campestris bisporus) has basidia with two spores.

For some time the situation was very confusing, when both Agaricus campestris and Agaricus bisporus were thought to be same considering the possibility that the four-spored Agaricus campestris, once taken into cultivation, changes the type of basidia under the new conditions, and becomes two-spored.

Nobody has ever experimentally shown that this can happen. The other possibility is, a change from four-spored to two-spored basidium takes place as a mutation occurring under the influence of environmental conditions, not as a consequence of adaptation.

But all recent attempts to grow Agaricus campestris from wild strains under the same conditions as those prevailing in the cultivation of the cultivated white mushroom Agaricus bisporus have failed. Besides the difference in the number of spores, this also indicates the specific difference between Agaricus campestris and the cultivated white mushroom Agaricus bisporus.

Series Gasteromycetes:

All members of the Gasteromycetes are saprobic, and grow on such substrate as soil, rotting wood and other vegetation, and on dung. Rhizopogon, which forms sub­terranean basidiocarps, and Scleroderma can form mycorrhiza with forest trees.

The fungi included in the Gasteromycetes produce large, often very large basidio­carps which either remain closed, or open only after the spores are mature. Some of them have hypogeous basidiocarps that emerge as they approach maturity; others possess hypogeous; while a third type develops entirely epigeous basidiocarps.

The surrounding wall of the basidiocarp composed of sterile tissue is the peridium, which may be composed of one to three layers known as the endoperidium (on the inside), the mesoperidium, and the exoperidium (on the outside). The peridium may be firm and hard or soft and papery or may be so delicate that it may disappear during the development of the basidiocarp.

Enclosed within the peridium is the fertile tissue known as gleba.

The gleba consists of more or less fleshy mycelial growth anastomosing to form numerous chambers or cavities lined by a layer of basidia. In some cases instead of glebal chambers with a definite layer of basidia being formed, clusters of basidia appear here and there in the gleba.

Again there may develop in the gleba a network of veins or interglebal tissue performing functions of support and food conduction. Whereas in others, a sterile axial tissue or columella may be developed which behaves like a stipe.

Both veins and columella may also be developed in the same basidiocarp. In many of the Gasteromycetes, the gleba undergoes autodigestion after the basidiospores have been formed.

Again in some others, after autodigestion of the gleba, only the spore mass remains which at time is interspersed with stiff, thick-walled thread-like structures, the capillitia (sing, capillitium), they help spore dissemination. In the Gastero­mycetes, the gleba may be fleshy to cartilaginous, slimy, powdery, or waxy. The nature of gleba has great taxonomic importance.

Once basidiospores have developed, the gleba undergoes autodigestion or con­version into dried-up, thread-like strands. The tissue between the glebal chambers constitutes the trama and serves, in different species, for conduction, mechanical support or even dehiscence in some cases. There are variations in the origin of glebal tissue in the basidiocarps of the Gasteromycetes.

These variations are as follows:

(a) The lacunar type where, in the extreme case, a single cavity or more usually, cavities lined with hymeniun, arise within the basidiocarp, encountered in the Nidula- riaceae (Fig. 291 A).

(b) The coralloid type, in which a single cavity arises in the upper half of the basidiocarp. This is enclosed above by tramal tissue which may unite with the inner side of the peridium but, at the base of the cavity, branching masses of hyphae grow upwards and outwards to form ‘coral-like’ branches covered by a continuous hymenium (Fig. 291B).

(c) The multipileate type is one in which a number of tramal branches grow rapidly, reach the inner surface of the peridium and there spread out over it. Coral­like branches then grow from these main branches into the spaces into which the basidiocarp has been divided, exhibited by the genus Clathrus (Fig. 291C).

(d) The unipileate type resembles the multipileate but here a single tramal branch develops as a column of tissue from base to apex and spreads out in a pileus- like manner, on the inner surface of the peridium. Hymenium develops on coralloid branches on the underside of the pileus-like structure, encountered in the genus Phallus (Fig. 291D).

In the Gasteromycetes the basidia may be clavate or cylindrical, but in majority of cases the basidia are ovoid or globose with short or long sterigmata. The sterigmata may also be lacking (Fig. 297B). The basidia are mostly four, occasionally 2 to 3 or 6 to 8-spored. Very rarely a basidium may produce up to 12 basidiospores.

The basidiospores are placed symmetrically at the tips of the sterigmata and are not forcibly discharged as encountered among the Hymenomycetes.

In case of basidium with long sterigma either the entire sterigma or a portion of it remains attached to the spore. In the Gasteromycetes there is tetra-polar heterothallic condition.

Sexual compatibility is governed by two pairs of factors Aa Bb situated on different chromosomes and segre­gating independently. Four types of basidiospores (AB, Ab, aB, ab) are produced which give rise to four types of primary monokaryotic mycelia with different dikaryotizing possibilities. Basidiocarps are produced from dikaryotic mycelium.

Dissemination of spores among the Gasteromycetes usually takes place only upon the disintegration of the peridium and through the action of external agents such as, wind, flies, raindrops, etc. The spore mass of a large number of Gasteromycetes is dry and powdery and is adapted for dissemination by air currents, through a natural opening (stoma) of the peridium.

The most common example is the genus Lycoperdon (Fig. 292A). The strong and penetrating odour in the glebal portion of the members of the order Phallales, commonly known as stinkhorns, attracts insects, which help dissemina­tion of spores (Fig. 292B).

In the genus Sphaerobolus, the entire gleba is discharged by a special hurling mechanism through the activities of the different layers of the peridium (Fig. 308A-G). Whereas, in the genus Scleroderma, the spores are dispersed by irregularly rupturing of the thick and firm peridium.

The masses of glebat tissues (peridioles) (Fig. 292D) of the Nidulariales, known as the ‘bird’s nest fungi’, are disseminated by rain splash. The peridioles on being displaced by raindrops adhere to nearby objects by the sticky bases of the funicular cords, haptera (sing, hapteron). Simultaneously with this, the funicular cords warp closely round the objects against which the peridioles strike (Figs. 292E to G and 312A to D).