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 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 ). 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 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 .

—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);