| Fig. 20.—Agaricus mucidus. Portion of hymenium. s, Sporidia; st, sterigmata; g, sterile cells; c, cystidium, with operculum o. |
Hymenomycetes are a very large group containing over 11,000 species, most of which live in soil rich in humus or on fallen wood or stems, a few only being parasites. In the simplest forms (e.g. Exobasidium) the basidia are borne directly on the ordinary mycelium, but in the majority of cases the basidia are found developed in layers (hymenium) on special sporophores of characteristic form in the various groups. In these sporophores (such as the well-known toadstools and mushrooms where the ordinary vegetative mycelium is underground) we have structures specially developed for bearing the basidiospores and protecting them from rain, &c., and for the distribution of the spores—see earlier part of article on distribution of spores (figs. 19 and 20). The underground mycelium in many cases spreads wider and wider each year, often in a circular manner, and the sporophores springing from it appear in the form of a ring—the so-called fairy rings. Armillaria melleus and Polyporus annosus are examples of parasitic forms which attack and destroy living trees, while Merulius lacrymans is the well-known “dry rot” fungus.
Gasteromycetes are characterized by having closed sporophores or fruit-bodies which only open after the spores are ripe and then often merely by a small pore. The fruit-bodies are of very various shapes, showing a differentiation into an outer peridium and an inner spore-bearing mass, the gleba. The gleba is usually differentiated into a number of chambers which are lined directly by the hymenium (basidial layer), or else the chambers contain an interwoven mass of hyphae, the branches of which bear the basidia. By the breaking down of the inner tissues the spores often come to lie as a loose powdery mass in the interior of the hollow fruit-body, mixed sometimes with a capillitium. The best-known genera are Bovista, Lycoperdon (puff-ball) Scleroderma, Geaster (earth-star, q.v.). In the last-named genus the peridium is double and the outer layer becomes ruptured and spreads out in the form of star-shaped pieces; the inner layer, however, merely opens at the apex by a small pore.
The most complex members of the Gasteromycetes belong to the Phalloideae, which is sometimes placed as a distinct division of the Autobasidiomycetes. Phallus impudicus, the stink-horn, is occasionally found growing in woods in Britain. The fruit-body before it ruptures may reach the size of a hen’s egg and is white in colour; from this there grows out a hollow cylindrical structure which can be distinguished at the distance of several yards by its disgusting odour. It is highly poisonous.
Physiology.—The physiology of the fungi comes under the head of that of plants generally, and the works of Pfeffer, Sachs, Vines, Darwin and Klebs may be consulted for details. But we may refer generally here to certain phenomena peculiar to these plants, the life-actions of which are restricted and specialized by their peculiar dependence on organic supplies of carbon and nitrogen, so that most fungi resemble the colourless cells of higher plants in their nutrition. Like these they require water, small but indispensable quantities of salts of potassium, magnesium, sulphur and phosphorus, and supplies of carbonaceous and nitrogenous materials in different stages of complexity in the different cases. Like these, also, they respire oxygen, and are independent of light; and their various powers of growth, secretion, and general metabolism, irritability, and response to external factors show similar specific variations in both cases. It is quite a mistake to suppose that, apart from the chlorophyll function, the physiology of the fungus-cell is fundamentally different from that of ordinary plant-cells. Nevertheless, certain biological phenomena in fungi are especially pronounced, and of these the following require particular notice.
Parasitism. —Some fungi, though able to live as saprophytes, occasionally enter the body of living plants, and are thus termed facultative parasites. The occasion may be a wound (e.g. Nectria, Dasyscypha, &c.), or the enfeeblement of the tissues of the host, or invigoration of the fungus, the mycelium of which then becomes strong enough to overcome the host’s resistance (Botrytis). Many fungi, however, cannot complete their life-history apart from the host-plant. Such obligate parasites may be epiphytic (Erysipheae), the mycelium remaining on the outside and at most merely sending haustoria into the epidermal cells, or endophytic (Uredineae, Ustilagineae, &c.), when the mycelium is entirely inside the organs of the host. An epiphytic fungus is not necessarily a parasite, however, as many saprophytes (moulds, &c.) germinate and develop a loose mycelium on living leaves, but only enter and destroy the tissues after the leaf has fallen; in some cases, however, these saprophytic epiphytes can do harm by intercepting light and air from the leaf (Fumago, &c.), and such cases make it difficult to draw the line between saprophytism and parasitism. Endophytic parasites may be intracellular, when the fungus or its mycelium plunges into the cells and destroys their contents directly (Olpidium, Lagenidium, Sclerotinia, &c.), but they are far more frequently intercellular, at any rate while young, the mycelium growing in the lacunae between the cells (Peronospora, Uredineae) into which it may send short (Cystopus), or long and branched (Peronospora Calotheca) haustoria, or it extends in the middle lamella (Ustilago), or even in the solid substance of the cell-wall (Botrytis). No sharp lines can be drawn, however, since many mycelia are intercellular at first and subsequently become intracellular (Ustilagineae), and the various stages doubtless depend on the degrees of resistance which the host tissues are able to offer. Similar gradations are observed in the direct effect of the parasite on the host, which may be local (Hemileia) when the mycelium never extends far from the point of infection, or general (Phytophthora) when it runs throughout the plant. Destructive parasites rapidly ruin the whole plant-body (Pythium), whereas restrained parasites only tax the host slightly, and ill effects may not be visible for a long time, or only when the fungus is epidemic (Rhytisma). A parasite may be restricted during a long incubation-period, however, and rampant and destructive later (Ustilago). The latter fact, as well as the extraordinary fastidiousness, so to speak, of parasites in their choice of hosts or of organs for attack, point to reactions on the part of the host-plant, as well as capacities on that of the parasite, which may be partly explained in the light of what we now know regarding enzymes and chemotropism. Some parasites attack many hosts and almost any tissue or organ (Botrytis cinerea), others are restricted to one family (Cystopus Candidus) or genus (Phytophthora infestans) or even species (Pucciniastrum Padi), and it is customary to speak of root-parasites, leaf-parasites, &c., in expression of the fact that a given parasite occurs only on such organs—e.g. Dematophora necatrix on roots, Calyptospora Goeppertiana on stems, Ustilago Scabiosae in anthers, Claviceps purpurea in ovaries, &c. Associated with these relations are the specializations which parasites show in regard to the age of the host. Many parasites can enter a seedling, but are unable to attack the same host when older—e.g. Pythium, Phytophthora omnivora.
Chemotropism.—Taken in conjunction with Pfeffer’s beautiful discovery that certain chemicals exert a distinct attractive influence on fungus hyphae (chemotropism), and the results of Miyoshi’s experimental application of it, the phenomena of enzyme-secretion throw considerable light on the processes of infection and parasitism of fungi. Pfeffer showed that certain substances in definite concentrations cause the tips of hyphae to turn towards them; other substances, though not innutritious, repel them, as also do nutritious bodies if too highly concentrated. Marshall Ward showed that the hyphae of Botrytis pierce the cell-walls of a lily by secreting a cytase and dissolving a hole through the membrane. Miyoshi then demonstrated that if Botrytis is sown in a lamella of gelatine, and this lamella is superposed on another similar one to which a chemotropic substance is added, the tips of the hyphae at once turn from the former and enter the latter. If a thin cellulose membrane is interposed between the lamellae, the hyphae nevertheless turn chemotropically from the one lamella to the other and pierce the cellulose membrane in the process. The hyphae will also dissolve their way through a lamella of collodion, paraffin, parchment paper, elder-pith, or even cork or the wing of a fly, to do which it must excrete very different enzymes. If the membrane is of some impermeable substance, like gold leaf, the hyphae cannot dissolve its way through, but the tip finds the most minute pore and traverses the barrier by means of it, as it does a stoma on a leaf We may hence conclude that a parasitic hyphae pierces some plants or their stomata and refuses to enter others, because in the former case there are chemotropically attractive substances present which are absent from the latter, or are there replaced by repellent poisonous or protective substances such as enzymes or antitoxins.
Specialization of Parasitism.—The careful investigations of recent years have shown that in several groups of fungi we cannot be content to distinguish as units morphologically different species, but we are compelled to go deeper and analyse further the species. It has been shown especially in the Uredineae and Erysiphaceae that many forms which can hardly be distinguished morphologically, or which cannot be differentiated at all by structural characters, are not really homogeneous but consist of a number of forms which are sharply distinguishable by their infecting power. Eriksson found, for example, that the well-known species Puccinia graminis could be split up into a number of forms which though morphologically similar were physiologically distinct. He found that the species really consisted of six distinct races, each having a more or less narrow range of grasses on which it can live. The six races he named P. graminis Secalis, Tritici, Avenae, Airae, Agrostis, Poae. The first named will grow on rye and barley but not on wheat or oat. The form Tritici is the least sharply marked and will grow on wheat, barley, rye and oat but not on the other grasses. The form Avenae will grow on oat and many grasses but not on the other three cereals mentioned. The last three forms grow only on the genera Aira, Agrostis and Poa respectively. All these forms have of course their aecidium-stage on the barberry. The terms biologic forms, biological species, physiological species, physiological races, specialized forms have all been applied to these; perhaps the term biologic forms is the most satisfactory. A similar specialization has been observed by Marshall Ward in the Puccinia parasitic on species of Bromus, and by Neger, Marchal and especially Salmon in the Erysiphaceae. In the last-named family the single morphological species Erysiphe graminis is found growing on the cereals, barley, oat, wheat, rye and a number of wild grasses (such as Poa, Bromus, Dactylis). On each of these host-plants the fungus has become specialized so that the form on barley cannot infect the other three cereals or the wild grasses and so on. Just as the uredospores and aecidiospores both show these specialized characters in the case of Puccinia graminis so we find that both the conidia and ascospores of E. graminis show this phenomenon. Salmon has further shown in investigating the relation of E. graminis to various species of the genus, Bromus, that certain species may act as “bridging species,” enabling the transfer of a biologic form to a host-plant which it cannot normally infect. Thus the biologic form on B. racemosus cannot infect B. commutatus. If, however, conidia from B. racemosus are sown on B. hordaceus, the conidia which develop on that plant are now able to infect B. commutatus; thus B. hordaceus acts as a bridging species. Salmon also found that injury of a leaf by mechanical means, by heat, by anaesthetics, &c., would affect the immunity of the plant and allow infection by conidia which was not able to enter a normal leaf. The effect of the abnormal conditions is probably to stop the production of, or weaken or destroy the protective enzymes or antitoxins, the presence of which normally confers immunity on the leaf.
Symbiosis.—The remarkable case of life in common first observed in lichens, where a fungus and an alga unite to form a compound organism—the lichen—totally different from either, has now been proved to be universal in these plants, and lichens are in all cases merely algae enmeshed in the interwoven hyphae of fungi (see LICHENS). This dualism, where the one constituent (alga) furnishes carbohydrates, and the other (fungus) ensures a supply of mineral matters, shade and moisture, has been termed symbiosis. Since then numerous other cases of symbiosis have been demonstrated. Many trees are found to have their smaller roots invaded by fungi and deformed by their action, but so far from these being injurious, experiments go to show that this mycorhiza (fungus-root) is necessary for the well-being of the tree. This is also the case with numerous other plants of moors and woodlands—e.g. Ericaceae, Pyrolaceae, Gentianaceae, Orchidaceae, ferns, &c. Recent experiments have shown that the difficulties of getting orchid seeds to germinate are due to the absence of the necessary fungus, which must be in readiness to infect the young seedling immediately after it emerges from the seed. The well-known failures with rhododendrons, heaths, &c., in ordinary garden soils are also explained by the need of the fungus-infected peat for their roots. The rôle of the fungus appears to be to supply materials from the leaf-mould around, in forms which ordinary root-hairs are incapable of providing for the plant; in return the latter supports the fungus at slight expense from its abundant stores of reserve materials. Numerous other cases of symbiosis have been discovered among the fungi of fermentation, of which those between Aspergillus and yeast in saké manufacture, and between yeasts and bacteria in kephir and in the ginger-beer plant are best worked out. For cases of symbiosis see [Bacteriology].
Authorities.—General: Engler and Prantl, Die natürlichen Pflanzenfamilien, i. Teil (1892 onwards); Zopf, Die Pilze (Breslau, 1890); De Bary, Comparative Morphology of Fungi, &c. (Oxford, 1887); von Tafel, Vergleichende Morphologie der Pilze (Jena, 1892); Brefeld, Unters. aus dem Gesamtgebiete der Mykologie, Heft i. 13 (1872-1905); Lotsy, Vorträge über botanische Stammesgeschichte (Jena, 1907). Distribution, &c.: Cooke, Introduction to the Study of Fungi (London, 1895); Felix in Zeitschr. d. deutsch. geologisch. Gesellsch. (1894-1896); Staub, Sitzungsber. d. bot. Sec. d. Kgl. ungarischen naturwiss. Gesellsch. zu Budapest (1897). Anatomy, &c.: Bommer, “Sclerotes et cordons mycéliens,” Mém. de l’Acad. Roy. de Belg. (1894); Mangin, “Observ. sur la membrane des mucorinées,” Journ. de Bot. (1899); Zimmermann, Die Morph. und Physiologie des Pflanzenzellkernes (Jena, 1896); Wisselingh, “Microchem. Unters. über die Zellwände d. Fungi,” Pringsh. Jahrb. B. 31, p. 619 (1898); Istvanffvi, “Unters. über die phys. Anat. der Pilze,” Prings. Jahrb. (1896). Spore Distribution: Fulton, “Dispersal of the Spores of Fungi by Insects,” Ann. Bot. (1889); Falck, “Die Sporenverbreitung bei den Basidiomyceten,” Beitr. zur Biol. d. Pflanzen, ix. (1904). Spores and Sporophores: Zopf, Die Pilze; also the works of von Tafel and Brefeld. Classification: van Tieghem, Journ. de bot. p. 77 (1893), and the works of Brefeld, Engler and Prantl, von Tafel, Saccardo and Lotsy already cited, Oomycetes: Wager, “On the Fertilization of Peronospora parasitica,” Ann. Bot. vol. xiv. (1900); Stevens, “The Compound Oosphere of Albugo Bliti,” Bot. Gaz. vol. 28 (1899); “Gametogenesis and Fertilization in Albugo,” ibid. vol. 32 (1901); Miyake, “The Fertilization of Pythium de Baryanum,” Ann. of Bot. vol. xv. (1901); Trow, “On Fertilization in the Saprolegnieae,” Ann. of Bot. vol. xviii. (1904); Thaxter, “New and Peculiar Aquatic Fungi,” Bot. Gaz. vol. 20 (1895); Lagerheim, “Unters. über die Monoblepharideae,” Bih. Svenska Vet. Acad. Handlingar, 25. Afd. iii. (1900); Woronin, “Beitrag zur Kenntnis der Monoblepharideen,” Mém. de l’Acad. Imp. d. Sc. de St-Pétersbourg, 8 sér. vol. 16 (1902). Zygomycetes: Harper, “Cell-division in Sporangia and Asci,” Ann. Bot. vol. xiii. (1899); Klebs, Die Bedingungen der Fortpflanzung, &c. (Jena, 1896), and “Zur Physiologie der Fortpflanzung” Prings. Jahr. (1898 and 1899), “Über Sporodinia grandis,” Bot. Zeit. (1902); Falck, “Die Bedingungen der Zygotenbildung bei Sporodinia grandis,” Cohn’s Beitr. z. Biol. d. Pflanzen, Bd. 8 (1902); Gruber “Verhalten der Zellkerne in den Zygosporen von Sporodinia grandis,” Ber. d. deutschen bot. Ges. Bd. 19 (1901); Blakeslee, “Sexual Reproduction in the Mucorineae,” Proc. Am. Acad. (1904); “Zygospore germination in the Mucorineae,” Annales mycologici (1906). Ustilagineae: Plowright, British Uredineae and Ustilagineae (London, 1889); Massee, British Fungi (Phycomycetes and Ustilagineae) (London, 1891); Brefeld, Unters. aus dem Gesamtgeb. der Mykol. Hefte xi. and xii.; and Falck, “Die Bluteninfektion bei den Brandpilzen,” ibid. Heft xiii. 1905; Dangeard, “La Reproduction sexuelle des Ustilaginées,” C.R., Oct. 9, 1893; Maire, “Recherches cytologiques et taxonomiques sur les Basidiomyceten,” Annexé au Bull. de la Soc. Mycol. de France (1902). Saccharomycetaceae: Jorgensen, The Micro-organisms of Fermentation (1899); Barker, Ann. of Bot. vol. xiv. (1901); “On Spore-formation among the Saccharomycetes,” Journ. of the Fed. Institute of Brewing, vol. 8 (1902); Guillermond, Recherches cytologiques sur lés levures (Paris, 1902); Hansen, Centralbl. f. Bakt. u. Parasitenp. Abt. ii. Bd. 12 (1904). Exoascaceae: Giesenhagen, “Taphrina, Exoascus, Magnusiella” (complete literature given), Bot. Zeit. Bd. 7 (1901). Erysiphaceae: Harper, “Die Entwicklung des Perithecium bei Sphaerotheca castagnei,” Ber. d. deut bot Ges. (1896); “Sexual Reproduction and the Organization of the Nucleus in certain Mildews,” Publ. Carnegie Institution (Washington, 1906); Blackman & Fraser, “Fertilization in Sphaerotheca,” Ann. of Bot. (1905). Perisporiaceae: Brefeld, Untersuchungen aus dem Gesamtgeb. der Mykol. Heft 10 (1891); Fraser and Chamber, Annales mycologici (1907). Discomycetes: Harper, “Über das Verhalten der Kerne bei Ascomyceten,” Jahr. f. wiss. Bot. Bd. 29 (1890); “Sexual Reproduction in Pyronema confluens,” Ann. of Bot. 14 (1900); Claussen, “Zur Entw. der Ascomyceten,” Boudiera, Bot. Zeit. Bd. 63 (1905); Dangeard, “Sur le Pyronema confluens,” Le Botaniste, 9 série (1903) (and numerous papers in same journal earlier and later); Ramlow, “Zur Entwick. von Thelebolus stercoren,” Bot. Zeit. (1906); Woronin, “Über die Sclerotienkrankheit der Vaccineen Beeren,” Mem. de l’Acad. Imp. des Sciences de St-Pétersbourg, 7 série, 36 (1888); Dittrich, “Zur Entwickelungsgeschichte der Helvellineen,” Cohn’s Beitr. z. Biol. d. Pflanzen (1892). Pyrenomycetes: Fisch, “Beitr. z. Entwickelungsgeschichte einiger Ascomyceten,” Bot. Zeit. (1882); Frank, “Über einige neue u. weniger bekannte Pflanzkrankh.,” Landw. Jahrb. Bd. 12 (1883); Ward, “Onygena equina, a horn-destroying fungus,” Phil. Trans., vol. 191 (1899); Dawson, “On the Biology of Poroniapunctata,” Ann. of Bot. 14 (1900). Tuberineae: Buchholtz, “Zur Morphologie u. Systematik der Fungi hypogaei,” Ann. Mycol. Bd. 1 (1903); Fischer in Engler and Prantl, Die natürlichen Pflanzenfamilien (1896). Laboulbeniineae: Thaxter, “Monograph of the Laboulbeniaceae,” Mem. Amer. Acad. of Arts and Sciences, vol. 12 (1895). Uredineae: Eriksson and Henning, Die Getreideroste (Stockholm, 1896); Eriksson, Botan. Gaz. vol. 25 (1896); “On the Vegetative Life of some Uredineae,” Ann. of Bot. (1905); Klebahn, Die wirtwechselnden Rostpilze (Berlin, 1904); Sapin-Trouffy, “Recherches histologiques sur la famille des Urédinées,” Le Botaniste (1896-1897); Blackman, “On the Fertilization, Alternation of Generations and General Cytology of the Uredineae,” Ann. of Bot. vol. 18 (1904); Blackman and Fraser, “Further Studies on the Sexuality of Uredineae,” Ann. of Bot. vol. 20 (1906); Christman, “Sexual Reproduction of Rusts,” Ann. of Bot. vol. 20 (1906); Ward, “The Brooms and their Rust Fungus,” Ann. of Bot. vol. 15 (1901). Basidiomycetes: Dangeard, “La Reprod. sexuelle des Basidiomycètes,” Le Botaniste (1894 and 1900); Maire, “Recherches cytologiques et taxonomiques sur les Basidiomycètes,” Annexe du Bull. de la Soc. Mycol. de France (1902); Möller, “Protobasidiomyceten,” Schimper’s Mitt. aus den Tropen, Heft 8 (Jena, 1895); Nichols, “The Nature and Origin of the Binucleated Cells in certain Basidiomycetes,” Trans. Wisconsin Acad. of Sciences, vol. 15 (1905); Wager, “The Sexuality of the Fungi,” Ann. of Bot. 13 (1899); Woronin, “Exobasidium Vaccinii,” Verh. Naturf. Ges. zu Freiburg, Bd. 4 (1867). Fermentation: Buchner, “Gährung ohne Hefezellen,” Bot. Zeit. Bd. 18 (1898); Albert, Cent. f. Bakt. Bd. 17 (1901); Green, The Soluble Ferments and Fermentation (Cambridge, 1899). Parasitism: “On some Relations between Host and Parasite,” Proc. Roy. Soc. vol. 47 (1890); “A Lily Disease,” Ann. of Botany, vol. 2 (1888); Eriksson & Hennings, Die Getreideroste (vide supra); Ward, “On the Question of Predisposition and Immunity in Plants,” Proc. Cambridge Phil. Soc. vol. 11 (1902); also Annals of Bot. vol. 16 (1902) and vol. 19 (1905); Neger, “Beitr. z. Biol. d. Erysipheen” Flora, Bde. 88 and 90 (1901-1902); Salmon, “Cultural Experiments with ‘Biologic Forms’ of the Erysiphaceae,” Phil. Trans. (1904); “On Erysiphe graminis and its adaptative parasitism within the genus, Bromus,” Ann. Mycol. vol. 11 (1904), also Ann. of Bot. vol. 19 (1905). Symbiosis: Ward, “The Ginger-Beer Plant,” Phil. Trans. Roy. Soc. (1892); “Symbiosis,” Ann. of Bot. 13 (1899); Shalk, “Der Sinn der Mykorrhizenbildung,” Jahrb. f. wiss. Bot. Bd. 34 (1900); Bernard, “On some Different Cases of Germination,” Gardener’s Chronicle (1900); Pierce, Publ. Univ. California (1900).