Panellus Stipticus

''Panellus stipticus'', commonly known as the bitter oyster, the astringent panus, the luminescent panellus, or the stiptic fungus, is a species of fungus in the family Mycenaceae, and the type species of the genus '' Panellus''. A common and widely distributed species, it is found in Asia, Australia, Europe, and North America, where it grows in groups or dense overlapping clusters on the logs, stumps, and trunks of deciduous trees, especially beech, oak, and birch. During the development of the fruit bodies, the mushrooms start out as tiny white knobs, which, over a period of one to three months, develop into fan- or kidney-shaped caps that measure up to broad. The caps are orange-yellow to brownish, and attached to the decaying wood by short stubby stalks that are connected off-center or on the side of the caps. The fungus was given its current scientific name in 1879, but has been known by many names since French mycologist Jean Bulliard first described it as ''Agaricus stypticus'' in 1783. Molecular phylogenetic analysis revealed ''P. stipticus'' to have a close genetic relationship with members of the genus ''Mycena''.

''Panellus stipticus'' is one of several dozen species of fungi that are bioluminescent. Strains from eastern North America are typically bioluminescent, but those from the Pacific coast regions of North America and from other continents are not. The luminescence is localized to the edges of the gills and the junction of the gills with the stem and cap. Bioluminescence is also observable with mycelia grown in laboratory culture, and the growth conditions for optimal light production have been studied in detail. Several chemicals have been isolated and characterized that are believed to be responsible for light production. Genetic analysis has shown that luminescence is controlled by a single dominant allele. The luminescent glow of this and other fungi inspired the term foxfire, coined by early settlers in eastern and southern North America. Modern research has probed the potential of ''P. stipticus'' as a tool in bioremediation, because of its ability to detoxify various environmental pollutants.

Taxonomy and phylogeny

The species was first named ''Agaricus stypticus'' by the French botanist Jean Bulliard in 1783, and later sanctioned by Elias Magnus Fries under this name in 1821. Fries later changed the genus as well as the spelling of the epithet and called it ''Panus stipticus''. The species has had an extensive taxonomic history and been shuffled to a number of genera by various authors, resulting in several synonyms: ''Agaricus flabelliformis'' (Johann Friedrich Gmelin, 1792), ''Pocillaria stiptica'' (Otto Kuntze, 1898), ''Rhipidium stipticum'' (Karl Friedrich Wilhelm Wallroth, 1833), ''Crepidopus stipticus'' (Samuel Frederick Gray, 1821), ''Pleurotus stipticus'' (Paul Kummer), ''Lentinus stipticus'' (Joseph Schröter, 1885), and ''Merulius stipticus'' (Jean-Baptiste Lamarck). It was Finnish mycologist Petter Karsten who in 1879 assigned its current name. ''Panellus stypticus'' is still used in the literature as a variant spelling.

''Panellus stipticus'' is the type species of the genus ''Panellus'', and, in Rolf Singer's authoritative 1986 classification of the Agaricales, it is also the type species of subgenus ''Panellus'', an infrageneric (below the taxonomic level of genus) grouping of ''Panellus'' species characterized by the absence of cystidia on the sides of the gills. More recently, phylogenetic analyses of the sequences of their ribosomal large subunit genes have concluded that ''Panellus stipticus'' is closely related to the poroid mushroom ''Dictyopanus pusillus''. The molecular analysis supports a previous assessment by mycologists Harold Bursdall and Orson K. Miller, who in 1975 suggested merging ''Dictyopanus'' into ''Panellus'' based on similarities in spore shape, stem structure, and the ability of dried fruit bodies to revive when moistened. Formerly grouped in the family Tricholomataceae, a wastebasket taxon of gilled mushrooms with white spores, ''P. stipticus'' is now classified in the Mycenaceae, after a large-scale phylogenetic analysis revealed "a previously unsuspected relationship between ''Mycena'' and ''Panellus'' (including ''Dictyopanus'')".

The fungus is commonly known as the bitter oyster, the luminescent panellus, the astringent panus, or the styptic fungus. The specific epithet ''stipticus'' refers to its purported value in stopping bleeding. Etymologically, it is a Greek equivalent to the Latin word ''astringens'', deriving from στυπτικός (''styptikós''), itself from the verb στύφειν (''styphein''), "to contract".

Description

The fungus normally exists unseen, in the form of a mass of threadlike vegetative cells called a mycelium, inhabiting rotting wood; only when suitable environmental conditions of temperature, moisture, and nutrient availability are achieved does the fungus produce the reproductive structures known as fruit bodies, or mushrooms. The cap of the fruit body is kidney- or clamshell-shaped, convex to roughly flat, with dimensions of by . The edges of the caps are scalloped with small rounded teeth, and curved slightly inward. The cap surface is dry, with a pattern of block-like areas similar to cracked dried mud; the surface is also covered with small fine hairs that give it a somewhat woolly consistency. It may have several concentric ridges or zones. Fresh fruit bodies range in color from yellowish-orange to buff to cinnamon; when dried they may be various shades of tan, brown or clay. The faded colors of dried fruit bodies tend to revive when moistened. On the underside of the cap, the gills are narrow and spaced closely together, often forked, buff-colored, and with numerous interconnecting cross-veins. Holding the cap in position is a stem that is long by thick, and has an off-center attachment to the cap, either at or near the cap side. The dull-white stem is covered with minute silk-like fibers, and is narrower at the base where it attaches to the substrate. Fruit bodies do not have any distinctive odor. The flesh is thin and tough, and dark yellow-brown to cream-colored.

Microscopic features

Various microscopic characteristics may be used to help identify the fungus from other morphologically similar species. A spore print of ''P. stipticus'', made by depositing a large number of spores in a small area, reveals their color to be white. Viewed with a microscope, the spores are smooth-walled, elliptical to nearly allantoid (sausage-shaped), with dimensions of 3–6 by 2–3  µm. Spores are amyloid, meaning that they will absorb iodine and become bluish-black when stained with Melzer's reagent, but this staining reaction has been described as "relatively weak".

The basidia (the spore-bearing cells) are 15–20 by 2.5–3.5 µm, club-shaped, and clamped at the base. The spores are attached to the end of the basidia by four projections called sterigmata that are 1–3.5 µm long. Cystidia are hyphal cells in the hymenium that do not produce basidiospores; they also make up the surfaces of the fruit body (the pelli or "cuticles"). They are usually structurally distinct from the basidia, and their features may be used as microscopic characters to help distinguish and differentiate similar fungi. The cheilocystidia are found on the gill edge; in ''P. stipticus'' they are narrowly club-shaped, cylindrical, spindle-shaped to bifurcate at the apex. They are also thin-walled, hyaline (translucent), abundant and crowded, and measure 17–45 by 3.5–6 µm. The pleurocystidia, located on the gill face, are 17–40 by 3–4.5 µm, spindle- or club-shaped, sometimes bifurcate at the apex, thin-walled, and hyaline. They are scattered or in dense clusters, mostly embedded in hymenium, occasionally protruding up to half the width of the hymenium.

The flesh of the cap consists of a number of microscopically distinct layers of tissue. The cuticle of the cap (known as the pileipellis) is between 8–10 µm thick, and is made of a loose ''textura intricata'', a type of tissue in which the hyphae are irregularly interwoven with distinct spaces between them. The cuticle hyphae are thick-walled to thin-walled, with scattered inconspicuous cystidia measuring 40–55 by 3.5–5.5 µm. These cystidia located in the cap (pileocystidia) are cylindrical, thin-walled, yellow in Melzer's reagent, hyaline in KOH, sometimes with amorphous dingy brown material coating the walls. Beneath the cuticle layer is a zone 54–65 µm thick, made of very loosely entwined, thin-walled hyphae, 2–3 µm in thickness, with clamps at the septa. Below this is a zone 208–221 µm in thickness, in which the densely compacted hyphae, 3–8 µm in diameter, have swollen, gelatinized walls, and often more or less a vertical orientation. This in turn is followed by a layer 520 µm in thickness, formed of loosely interwoven hyphae, 2–8 µm in width, some of which have thin walls with clamps at the septa, whilst others have somewhat thickened gelatinized walls. The flesh of the cap has a layer of upright hyphae bending into a lower layer of interwoven hyphae with diameters of 2.5–8 µm. The flesh of the gills is similar to that of the lower cap.

Similar species

Species of ''Crepidotus'' having a similar shape can be distinguished by their brown spore print, compared with the white spore print of ''P. stipticus''. ''Schizophyllum commune'' has a densely hairy white to grayish cap and longitudinally split gill-folds on the underside. The ruddy panus mushroom ('' Panus rudis'') is larger, has a reddish-brown cap that fades to pinkish-tan, and shows lilac tinges when young, fresh, and moist. Some ''Paxillus'' species may have a similar appearance, but they have yellow-brown spore prints.

Uses

''Panellus stipticus'' is considered too small and bitter to be edible, but is considered nonpoisonous. Its taste has been described as acidic, acrid, or astringent. One 1992 study reported that the taste of the fruit bodies varies across the geographic range of the species. Those from eastern North America have a mildly acrid taste that takes time to develop, and cause uncomfortable drying in the mouth. In contrast, specimens from Japan, New Zealand, and Russia produce no sensation in the mouth, but cause significant constriction and a nauseating taste in the throat. The fruit bodies are reputed to have been used in traditional Chinese medicine as a styptic to staunch bleeding, and also as a "violent purgative".

Fruit body development

Fruit bodies first appear as tiny white knobs less than a cubic millimeter in size. In a day or two the knobs grow into a horizontal pyramidal mass, increasing in height as the hyphae lengthen. This is soon followed by the formation of a minute cap, and lengthening of the stem. The stem is about 1 mm long when the cap first begins to form. The hyphae that comprise the stem gradually cease to grow at their ends, and then start to branch, with many of the branches growing in a horizontal direction. This growth, indicated by the flattening and broadening out of the top of the stem, gives rise to the cap. The horizontally aligned hyphae grow vertical branches which remain more or less parallel, ultimately forming the dorsal tissue of the cap. Other similar downward-growing branches form the fertile hymenium, which can be seen when the cap is about 2 mm in diameter.The young cap is spherical and its growth is at first epinastic, its margin being curved inwards and pressed against the stem. In this way, the hymenium begins its development within a special enclosed chamber. As the hymenial surface increases and keeps pace with the growth of the dorsal tissue of the cap, the latter expands and exposes the gills. The gills are formed by the continual downward growth of some of the hyphae. The gills are exposed before the cap is completely developed, and before the spores are mature. Spores can be produced by fruit bodies as small as broad, and liberation of the spores continues until the fruit body is fully grown—a period of one month to three months, depending on the conditions of temperature and moisture. The mature spores are disseminated by the wind. When the fruit body is nearing maturity, some of the terminal portions of the hyphae of the dorsal surface of the cap separate, and as a consequence, the upper surface of the fruit body becomes granular in appearance.

The fruit body projects out horizontally from the growing surface. If the position of a log is altered after young fruit bodies with the beginnings of gills have appeared, the stems of these attempt to readjust themselves in order to place the cap in a horizontal position. The cap are sometimes zonate