Chapter 18

A. Phycomycetes.Alga-like fungi with unicellular thallus and well-marked sexual organs.Class I.—Oomycetes. Mycelium usually well developed, but sometimes poor or absent. Sexual reproduction by oogonia and antheridia; asexual reproduction by zoospores or conidia.1. Monoblepharidineae. Mycelium present, antheridia with antherozoids, oogonium with single oosphere: Monoblepharidaceae.2. Peronosporineae. Mycelium present; antheridia but no antherozoids; oogonia with one or more oospheres: Peronosporaceae, Saprolegniaceae.3. Chytridineae. Mycelium poorly developed or absent; oogonia and antheridia (without antherozoids) known in some cases; zoospores common: Chytridiaceae. Ancylistaceae.Class II.—Zygomycetes. Mycelium well developed; sexual reproduction by zygospores; asexual reproduction by sporangia and conidia.1. Mucorineae. Sexual reproduction as above, asexual by sporangia or conidia or both: Mucoraceae. Mortierellaceae, Chaetocladiaceae, Piptocephalidaceae.2. Entomophthorineae. Sexual reproduction typical but with sometimes inequality of the fusing gametes (gametangia ?): Entomophthoraceae.B.Higher Fungi.Fungi with segmental thallus; sexual reproduction sometimes with typical antheridia and oogonia (ascogonia) but usually much reduced.Class I.—Ustilaginales. Forms with septate thallus, and reproduction by chlamydospores which on germination produce sporidia; sexuality doubtful.Class II.—Ascomycetes. Thallus septate; spores developed in special type of sporangium, the ascus, the number of spores being usually eight. Sexual reproduction sometimes typical, usually reduced.Exoascineae, Saccharomycetineae, Perisporinea, Discomycetes, Pyrenomycetes, Tuberineae, Laboulbeniineae.Class III.—Basidiales. Thallus septate. Conidia (basidiospores) borne in fours on a special conidiophore, the basidium. Sexual reproduction always much reduced.1. Uredineae. Life-history in some cases very complex and with well-marked sexual process and alternation of generations, in others much reduced; basidium (promycelium) derived usually from a thick-walled spore (teleutospore).2. Basidiomycetes. Life-history always very simple, no well-marked alternation of generations; basidium borne directly on the mycelium.(A) Protobasidiomycetes. Basidia septate. Auriculariaceae, Pilacreaceae, Tremellinaceae.(B) Autobasidiomycetes. Basidia non-septate. Hymenomycetes, Gasteromycetes.

A. Phycomycetes.Alga-like fungi with unicellular thallus and well-marked sexual organs.

Class I.—Oomycetes. Mycelium usually well developed, but sometimes poor or absent. Sexual reproduction by oogonia and antheridia; asexual reproduction by zoospores or conidia.

1. Monoblepharidineae. Mycelium present, antheridia with antherozoids, oogonium with single oosphere: Monoblepharidaceae.2. Peronosporineae. Mycelium present; antheridia but no antherozoids; oogonia with one or more oospheres: Peronosporaceae, Saprolegniaceae.3. Chytridineae. Mycelium poorly developed or absent; oogonia and antheridia (without antherozoids) known in some cases; zoospores common: Chytridiaceae. Ancylistaceae.

1. Monoblepharidineae. Mycelium present, antheridia with antherozoids, oogonium with single oosphere: Monoblepharidaceae.

2. Peronosporineae. Mycelium present; antheridia but no antherozoids; oogonia with one or more oospheres: Peronosporaceae, Saprolegniaceae.

3. Chytridineae. Mycelium poorly developed or absent; oogonia and antheridia (without antherozoids) known in some cases; zoospores common: Chytridiaceae. Ancylistaceae.

Class II.—Zygomycetes. Mycelium well developed; sexual reproduction by zygospores; asexual reproduction by sporangia and conidia.

1. Mucorineae. Sexual reproduction as above, asexual by sporangia or conidia or both: Mucoraceae. Mortierellaceae, Chaetocladiaceae, Piptocephalidaceae.2. Entomophthorineae. Sexual reproduction typical but with sometimes inequality of the fusing gametes (gametangia ?): Entomophthoraceae.

1. Mucorineae. Sexual reproduction as above, asexual by sporangia or conidia or both: Mucoraceae. Mortierellaceae, Chaetocladiaceae, Piptocephalidaceae.

2. Entomophthorineae. Sexual reproduction typical but with sometimes inequality of the fusing gametes (gametangia ?): Entomophthoraceae.

B.Higher Fungi.Fungi with segmental thallus; sexual reproduction sometimes with typical antheridia and oogonia (ascogonia) but usually much reduced.

Class I.—Ustilaginales. Forms with septate thallus, and reproduction by chlamydospores which on germination produce sporidia; sexuality doubtful.Class II.—Ascomycetes. Thallus septate; spores developed in special type of sporangium, the ascus, the number of spores being usually eight. Sexual reproduction sometimes typical, usually reduced.

Class I.—Ustilaginales. Forms with septate thallus, and reproduction by chlamydospores which on germination produce sporidia; sexuality doubtful.

Class II.—Ascomycetes. Thallus septate; spores developed in special type of sporangium, the ascus, the number of spores being usually eight. Sexual reproduction sometimes typical, usually reduced.

Exoascineae, Saccharomycetineae, Perisporinea, Discomycetes, Pyrenomycetes, Tuberineae, Laboulbeniineae.

Class III.—Basidiales. Thallus septate. Conidia (basidiospores) borne in fours on a special conidiophore, the basidium. Sexual reproduction always much reduced.

1. Uredineae. Life-history in some cases very complex and with well-marked sexual process and alternation of generations, in others much reduced; basidium (promycelium) derived usually from a thick-walled spore (teleutospore).2. Basidiomycetes. Life-history always very simple, no well-marked alternation of generations; basidium borne directly on the mycelium.

2. Basidiomycetes. Life-history always very simple, no well-marked alternation of generations; basidium borne directly on the mycelium.

(A) Protobasidiomycetes. Basidia septate. Auriculariaceae, Pilacreaceae, Tremellinaceae.(B) Autobasidiomycetes. Basidia non-septate. Hymenomycetes, Gasteromycetes.

(A) Protobasidiomycetes. Basidia septate. Auriculariaceae, Pilacreaceae, Tremellinaceae.

(B) Autobasidiomycetes. Basidia non-septate. Hymenomycetes, Gasteromycetes.

A.Phycomycetes.—Most of the recent work of importance in this group deals with the cytology of sexual reproduction and of spore-formation, and the effect of external conditions on the production of reproductive organs.

Monoblepharidaceaeconsists of a very small group of aquatic forms living on fallen twigs in ponds and ditches. Only one genus,Monoblepharis, can certainly be placed here, though a somewhat similar genus,Myrioblepharis, with a peculiar multiciliate zoospore like that of Vaucheria, is provisionally placed in the same group.Monoblephariswas first described by Cornu in 1871, but from that time until 1895 when Roland Thaxter described several species from America the genus was completely lost sight of.Monoblepharishas oogonia with single oospheres and antheridia developing a few amoeboid uniciliate antherozoids; these creep to the opening of the oogonium and then swim in. The resemblance between this genus andOedogoniumamong the algae is very striking, as is also that ofMyrioblepharisandVaucheria.Peronosporaceaeare a group of endophytic parasites—about 100 species—of great importance as comprising the agents of “damping off” disease (Pythium), vine-mildew (Plasmopara), potato disease (Phytophthora), onion-mildew (Peronospora).Pythiumis a semi-aquatic form attacking seedlings which are too plentifully supplied with water; its hyphae penetrate the cell-walls and rapidly destroy the watery tissues of the living plant; then the fungus lives in the dead remains. When the free ends of the hyphae emerge again into the air they swell up into spherical bodies which may either fall off and behave as conidia, each putting out a germ-tube and infecting the host; or the germ-tube itself swells up into a zoosporangium which develops a number of zoospores. In the rotting tissues branches of the older mycelium similarly swell up and form antheridia and oogonia (fig. 4). The contents of the antheridium are not set free, but that organ penetrates the oogonium by means of a narrow outgrowth, the fertilizing tube, and a male nucleus then passes over into the single oosphere, which at first multinucleate becomes uninucleate before fertilization.Pythiumis of interest as illustrating the dependence of zoospore-formation on conditions and the indeterminate nature of conidia. The other genera are more purely parasitic; the mycelium usually sends haustoria into the cells of the host and puts out branched, aerial conidiophores through the stomata, the branches of which abstrict numerous “conidia”; these either germinate directly or their contents break up into zoospores (fig. 5). The development of the “conidia” as true conidial spores or as zoosporangia may occur in one and the same species (Cystopus candidus,Phytophthora infestans) as inPythiumdescribed above; in other cases the direct conidial germination is characteristic of genera—e.g.Peronospora; while others emit zoospores—e.g.Plasmopara, &c. InCystopus(Albugo) the “conidia” are abstricted in basipetal chain-like series from the ends of hyphae which come to the surface in tufts and break through the epidermis as white pustules. Each “conidium” contains numerous nuclei and is really a zoosporangium, as after dispersal it breaks up into a number of zoospores. The Peronosporaceae reproduce themselves sexually by means of antheridia and oogonia as described inPythium. InCystopus Blitithe oosphere contains numerous nuclei, and all the male nuclei from the antheridium pass into it, the male and female nuclei then fusing in pairs. We thus have a process of “multiple fertilization”; the oosphere really represents a large number of undifferentiated gametes and has been termed a coenogamete. BetweenCystopus Blition the one hand andPythium de Baryanumon the other a number of cytologically intermediate forms are known. The oospore on germination usually gives origin to a zoosporangium, but may form directly a germ tube which infects the host.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 4.—Fertilization of the Peronosporeae. After Wager.1,Peronospora parasitica. Young multinucleate oogonium (og) and antheridium (an).2,Albugo candida. Oogonium with the central uninucleate oosphere and the fertilizing tube (a) of the antheridium which introduces the male nucleus.3, The same. Fertilized egg-cell (o) surrounded by the periplasm (p).Fig. 5.—Phytophthora infestans. Fungus of Potato Disease.A, B, Section of Leaf of Potato with sporangiophores ofPhytophthora infestanspassing through the stomata D, on the under surface of the leaf.E, Sporangia.F, G, H, J, Further development of the sporangia.K, Germination of the zoospores formed in the sporangia.L, M, N, Fertilization of the oogonium and development of the oospore inPeronospora.Saprolegniaceaeare aquatic forms found growing usually on dead insects lying in water but occasionally on living fish (e.g.the salmon disease associated withSaprolegnia ferax). The chief genera areSaprolegnia, Achlya, Pythiopsis, Dictyuchus, Aplanes.Motile zoospores which escape from the zoosporangium are present except in Aplanes. The sexual reproduction shows all transitions between forms which are normally sexual, like the Peronosporaceae, to forms in which no antheridium is developed and the oospheres develop parthenogenetically. The oogonia, unlike the Peronosporaceae, contain more than one oosphere. Klebs has shown that the development of zoosporangia or of oogonia and pollinodia respectively inSaprolegniais dependent on the external conditions; so long as a continued stream of suitable food-material is ensured the mycelium grows on without forming reproductive organs, but directly the supplies of nitrogenous and carbonaceous food fall below a certain degree of concentration sporangia are developed. Further reduction of the supplies of food effects the formation of oogonia. This explains the sequence of events in the case of aSaprolegnia-mycelium radiating from a dead fly in water. Those parts nearest the fly and best supplied develop barren hyphae only; in a zone at the periphery, where the products of putrefaction dissolved in the water form a dilute but easily accessible supply, the zoosporangia are developed in abundance; oogonia, however, are only formed in the depths of this radiating mycelium, where the supplies of available food materials are least abundant.Chytridineae.—These parasitic and minute, chiefly aquatic, forms may be looked upon as degenerate Oomycetes, since a sexual process and feeble unicellular mycelium occur in some; or they may be regarded as series of primitive forms leading up to higher members. There is no means of deciding the question. They are usually included in Oomycetes, but their simple structure, minute size, usually uniciliate zoospores, and their negative characters would justify their retention as a separate group. It contains less than 200 species, chiefly parasitic on or in algae and other water-plants or animals, of various kinds, or in other fungi, seedlings, pollen and higher plants. They are often devoid of hyphae, or put forth fine protoplasmic filaments into the cells of their hosts. After absorbing the cell-contents of the latter, which it does in a few hours or days, the fungus puts out a sporangium, the contents of which break up into numerous minute swarm-spores, usually one-ciliate, rarely two-ciliate. Any one of these soon comes to rest on a host-cell, and either pierces it and empties its contents into its cavity, where the further development occurs (Olpidium), or merely sends in delicate protoplasmic filaments (Rhizophydium) or a short hyphal tube of, at most, two or three cells, which acts as a haustorium, the further development taking place outside the cell-wall of the host (Chytridium). In some cases resting spores are formed inside the host (Chytridium), and give rise to zoosporangia on germination. In a few species a sexual process is described, consisting in the conjugation of similar cells (Zygochytrium) or the union of two dissimilar ones (Polyphagus). In the development of distinct antheridial and oogonial cells the allied Ancylistineae show close alliances toPythiumand the Oomycetes. On the other hand, the uniciliate zoospores ofPolyphagushave slightly amoeboid movements, and in this and the pseudopodium-like nature of the protoplasmic processes, such forms suggest resemblances to the Myxomycetes. Opinions differ as to whether the Chytridineae are degraded or primitive forms, and the group still needs critical revision. Many new forms will doubtless be discovered, as they are rarely collected on account of their minuteness. Some forms cause damping off of seedlings—e.g.Olpidium Brassicae; others discoloured spots and even tumour-like swellings—e.g.Synchytium Scabiosae,S. Succisae,Urophlyctis, &c., on higher plants. Analogies have been pointed out between Chytridiaceae and unicellular algae, such as Chlorosphaeraceae, Protococcaceae, “Palmellaceae,” &c., some of which are parasitic, and suggestions may be entertained as to possible origin from such algae.TheZygomycetes, of which about 200 species are described, are especially important from a theoretical standpoint, since they furnished the series whence Brefeld derived the vast majority of the fungi. They are characterized especially by the zygospores, but the asexual organs (sporangia) exhibit interesting series of changes, beginning with the typical sporangium ofMucorcontaining numerous endospores, passing to cases where, as inThamnidium, these are accompanied with more numerous small sporangia (sporangioles) containing few spores, and thence toChaetocladiumandPiptocephalis, where the sporangioles form but one spore and fall and germinate as a whole; that is to say, the monosporous sporangium has become a conidium, and Brefeld regarded these and similar series of changes as explaining the relation of ascus to conidium in higher fungi. According to his view, the ascus is in effect the sporangium with several spores, the conidium the sporangiole with but one spore, and that not loose but fused with the sporangiole wall. On this basis, with other interesting morphological comparisons, Brefeld erected his hypothesis, now untenable, that the Ascomycetes and Basidiomycetes diverge from the Zygomycetes, the former having particularly specialized the ascus (sporangial) mode of reproduction, the latter having specialized the conidial (indehiscent one-spored sporangiole) mode. In addition to sporangia and the conidial spores referred to, some Mucorini show a peculiar mode of vegetative reproduction by means of gemmae or chlamydospores—i.e.short segments of the hyphae become stored with fatty reserves and act as spores. The gemmae formed on submerged Mucors may bud like a yeast, and even bring about alcoholic fermentation in a saccharine solution.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 6.—Mucor Mucedo.Different stages in the formation and germination of the zygospore. (After Brefeld, 1-4. 5 from v. Tavel,Pilze.)1, Two conjugating branches in contact.2, Septation of the conjugating cells (a) from the suspensors (b).3, More advanced stage, the conjugating cells (a) are still distinct from one another; the warty thickenings of their walls have commenced to form.4, Ripe zygospore (b) between the suspensors (a).5, Germinating zygospore with a germ-tube bearing a sporangium.The segments of the hyphae in this group usually contain several nuclei. At the time of sporangial formation the protoplasm with numerous nuclei streams into the swollen end of the sporangiophore and there becomes cut off by a cell-wall to form the sporangium. The protoplasm then becomes cut up by a series of clefts into a number of smaller and smaller pieces which are unicellular inPilobolus, multicellular inSporodinia. These then become surrounded by a cell-wall and form the spores. This mode of spore-formation is totally different from that in the ascus; hence one of the difficulties of the acceptance of Brefeld’s view of the homology of ascus and sporangium. The cytology of zygospore-formation is not known in detail; the so-called gametes which fuse are multinucleate and are no doubt of the nature of gametangia. The fate of these nuclei is doubtful, probably they fuse in pairs (fig. 6).Blakeslee has lately made some very important observations of the Zygomycetes. It is well known that while in some forms,e.g.Spordinia, zygospores are easily obtained, in others,e.g.most species ofMucor, they are very erratic in their appearance. This has now been explained by Blakeslee, who finds that the Mucorinae can be divided into two groups, termed homothallic and heterothallic respectively. In the first group zygospores can arise by the union of branches from thesamemycelium and so can be produced by the growth from a single spore; this group includesSpordinia grandis,Spinellus fusiger, some species ofMucor, &c. The majority of forms, however, fall into the heterothallic group, in which the association of branches from two myceliadifferent in natureis necessary for the formation of zygospores. These structures cannot then be produced from the product of a single spore nor even from the thalli derived fromanytwo spores. The two kinds of thalli Blakeslee considers to have a differentiation of the nature of sex and he distinguishes them as (+) and (−) forms; the former being usually distinguished by a somewhat greater luxuriance of growth.The classification of the Mucorini depends on the prevalence and characters of the conidia, and of the sporangia and zygospores—e.g.the presence or absence of a columella in the former, the formation of an investment round the latter. Most genera are saprophytes, but some—Chaetocladium,Piptocephalis—are parasites on other Mucorini, and one or two are associated casually with the rotting of tomatoes and other fruits, bulbs, &c., the fleshy parts of which are rapidly destroyed if once the hyphae gain entrance. Even more important is the question of mycosis in man and other animals, referred to species ofMucor, and investigated by Lucet and Costantin. Klebs has concluded that transpiration is the important factor in determining the formation of sporangia, while zygote-development depends on totally different conditions; these results have been called in question by Falck.TheEntomophthoraceaecontain three genera,Empusa,EntomophthoraandBasidiobolus. The two first genera consist of forms which are parasitic on insects.Empusa Muscaecauses the well-known epidemic in house-flies during the autumn; the dead, affected flies are often found attached to the window surrounded by a white halo of conidia.B. ranarumis found in the alimentary canal of the frog and growing on its excrement. In these three genera the conidia are cast off with a jerk somewhat in the same way as the sporangium ofPilobolus.

Peronosporaceaeare a group of endophytic parasites—about 100 species—of great importance as comprising the agents of “damping off” disease (Pythium), vine-mildew (Plasmopara), potato disease (Phytophthora), onion-mildew (Peronospora).Pythiumis a semi-aquatic form attacking seedlings which are too plentifully supplied with water; its hyphae penetrate the cell-walls and rapidly destroy the watery tissues of the living plant; then the fungus lives in the dead remains. When the free ends of the hyphae emerge again into the air they swell up into spherical bodies which may either fall off and behave as conidia, each putting out a germ-tube and infecting the host; or the germ-tube itself swells up into a zoosporangium which develops a number of zoospores. In the rotting tissues branches of the older mycelium similarly swell up and form antheridia and oogonia (fig. 4). The contents of the antheridium are not set free, but that organ penetrates the oogonium by means of a narrow outgrowth, the fertilizing tube, and a male nucleus then passes over into the single oosphere, which at first multinucleate becomes uninucleate before fertilization.Pythiumis of interest as illustrating the dependence of zoospore-formation on conditions and the indeterminate nature of conidia. The other genera are more purely parasitic; the mycelium usually sends haustoria into the cells of the host and puts out branched, aerial conidiophores through the stomata, the branches of which abstrict numerous “conidia”; these either germinate directly or their contents break up into zoospores (fig. 5). The development of the “conidia” as true conidial spores or as zoosporangia may occur in one and the same species (Cystopus candidus,Phytophthora infestans) as inPythiumdescribed above; in other cases the direct conidial germination is characteristic of genera—e.g.Peronospora; while others emit zoospores—e.g.Plasmopara, &c. InCystopus(Albugo) the “conidia” are abstricted in basipetal chain-like series from the ends of hyphae which come to the surface in tufts and break through the epidermis as white pustules. Each “conidium” contains numerous nuclei and is really a zoosporangium, as after dispersal it breaks up into a number of zoospores. The Peronosporaceae reproduce themselves sexually by means of antheridia and oogonia as described inPythium. InCystopus Blitithe oosphere contains numerous nuclei, and all the male nuclei from the antheridium pass into it, the male and female nuclei then fusing in pairs. We thus have a process of “multiple fertilization”; the oosphere really represents a large number of undifferentiated gametes and has been termed a coenogamete. BetweenCystopus Blition the one hand andPythium de Baryanumon the other a number of cytologically intermediate forms are known. The oospore on germination usually gives origin to a zoosporangium, but may form directly a germ tube which infects the host.

1,Peronospora parasitica. Young multinucleate oogonium (og) and antheridium (an).

2,Albugo candida. Oogonium with the central uninucleate oosphere and the fertilizing tube (a) of the antheridium which introduces the male nucleus.

3, The same. Fertilized egg-cell (o) surrounded by the periplasm (p).

A, B, Section of Leaf of Potato with sporangiophores ofPhytophthora infestanspassing through the stomata D, on the under surface of the leaf.

E, Sporangia.

F, G, H, J, Further development of the sporangia.

K, Germination of the zoospores formed in the sporangia.

L, M, N, Fertilization of the oogonium and development of the oospore inPeronospora.

Saprolegniaceaeare aquatic forms found growing usually on dead insects lying in water but occasionally on living fish (e.g.the salmon disease associated withSaprolegnia ferax). The chief genera areSaprolegnia, Achlya, Pythiopsis, Dictyuchus, Aplanes.Motile zoospores which escape from the zoosporangium are present except in Aplanes. The sexual reproduction shows all transitions between forms which are normally sexual, like the Peronosporaceae, to forms in which no antheridium is developed and the oospheres develop parthenogenetically. The oogonia, unlike the Peronosporaceae, contain more than one oosphere. Klebs has shown that the development of zoosporangia or of oogonia and pollinodia respectively inSaprolegniais dependent on the external conditions; so long as a continued stream of suitable food-material is ensured the mycelium grows on without forming reproductive organs, but directly the supplies of nitrogenous and carbonaceous food fall below a certain degree of concentration sporangia are developed. Further reduction of the supplies of food effects the formation of oogonia. This explains the sequence of events in the case of aSaprolegnia-mycelium radiating from a dead fly in water. Those parts nearest the fly and best supplied develop barren hyphae only; in a zone at the periphery, where the products of putrefaction dissolved in the water form a dilute but easily accessible supply, the zoosporangia are developed in abundance; oogonia, however, are only formed in the depths of this radiating mycelium, where the supplies of available food materials are least abundant.

Chytridineae.—These parasitic and minute, chiefly aquatic, forms may be looked upon as degenerate Oomycetes, since a sexual process and feeble unicellular mycelium occur in some; or they may be regarded as series of primitive forms leading up to higher members. There is no means of deciding the question. They are usually included in Oomycetes, but their simple structure, minute size, usually uniciliate zoospores, and their negative characters would justify their retention as a separate group. It contains less than 200 species, chiefly parasitic on or in algae and other water-plants or animals, of various kinds, or in other fungi, seedlings, pollen and higher plants. They are often devoid of hyphae, or put forth fine protoplasmic filaments into the cells of their hosts. After absorbing the cell-contents of the latter, which it does in a few hours or days, the fungus puts out a sporangium, the contents of which break up into numerous minute swarm-spores, usually one-ciliate, rarely two-ciliate. Any one of these soon comes to rest on a host-cell, and either pierces it and empties its contents into its cavity, where the further development occurs (Olpidium), or merely sends in delicate protoplasmic filaments (Rhizophydium) or a short hyphal tube of, at most, two or three cells, which acts as a haustorium, the further development taking place outside the cell-wall of the host (Chytridium). In some cases resting spores are formed inside the host (Chytridium), and give rise to zoosporangia on germination. In a few species a sexual process is described, consisting in the conjugation of similar cells (Zygochytrium) or the union of two dissimilar ones (Polyphagus). In the development of distinct antheridial and oogonial cells the allied Ancylistineae show close alliances toPythiumand the Oomycetes. On the other hand, the uniciliate zoospores ofPolyphagushave slightly amoeboid movements, and in this and the pseudopodium-like nature of the protoplasmic processes, such forms suggest resemblances to the Myxomycetes. Opinions differ as to whether the Chytridineae are degraded or primitive forms, and the group still needs critical revision. Many new forms will doubtless be discovered, as they are rarely collected on account of their minuteness. Some forms cause damping off of seedlings—e.g.Olpidium Brassicae; others discoloured spots and even tumour-like swellings—e.g.Synchytium Scabiosae,S. Succisae,Urophlyctis, &c., on higher plants. Analogies have been pointed out between Chytridiaceae and unicellular algae, such as Chlorosphaeraceae, Protococcaceae, “Palmellaceae,” &c., some of which are parasitic, and suggestions may be entertained as to possible origin from such algae.

TheZygomycetes, of which about 200 species are described, are especially important from a theoretical standpoint, since they furnished the series whence Brefeld derived the vast majority of the fungi. They are characterized especially by the zygospores, but the asexual organs (sporangia) exhibit interesting series of changes, beginning with the typical sporangium ofMucorcontaining numerous endospores, passing to cases where, as inThamnidium, these are accompanied with more numerous small sporangia (sporangioles) containing few spores, and thence toChaetocladiumandPiptocephalis, where the sporangioles form but one spore and fall and germinate as a whole; that is to say, the monosporous sporangium has become a conidium, and Brefeld regarded these and similar series of changes as explaining the relation of ascus to conidium in higher fungi. According to his view, the ascus is in effect the sporangium with several spores, the conidium the sporangiole with but one spore, and that not loose but fused with the sporangiole wall. On this basis, with other interesting morphological comparisons, Brefeld erected his hypothesis, now untenable, that the Ascomycetes and Basidiomycetes diverge from the Zygomycetes, the former having particularly specialized the ascus (sporangial) mode of reproduction, the latter having specialized the conidial (indehiscent one-spored sporangiole) mode. In addition to sporangia and the conidial spores referred to, some Mucorini show a peculiar mode of vegetative reproduction by means of gemmae or chlamydospores—i.e.short segments of the hyphae become stored with fatty reserves and act as spores. The gemmae formed on submerged Mucors may bud like a yeast, and even bring about alcoholic fermentation in a saccharine solution.

1, Two conjugating branches in contact.

2, Septation of the conjugating cells (a) from the suspensors (b).

3, More advanced stage, the conjugating cells (a) are still distinct from one another; the warty thickenings of their walls have commenced to form.

4, Ripe zygospore (b) between the suspensors (a).

5, Germinating zygospore with a germ-tube bearing a sporangium.

The segments of the hyphae in this group usually contain several nuclei. At the time of sporangial formation the protoplasm with numerous nuclei streams into the swollen end of the sporangiophore and there becomes cut off by a cell-wall to form the sporangium. The protoplasm then becomes cut up by a series of clefts into a number of smaller and smaller pieces which are unicellular inPilobolus, multicellular inSporodinia. These then become surrounded by a cell-wall and form the spores. This mode of spore-formation is totally different from that in the ascus; hence one of the difficulties of the acceptance of Brefeld’s view of the homology of ascus and sporangium. The cytology of zygospore-formation is not known in detail; the so-called gametes which fuse are multinucleate and are no doubt of the nature of gametangia. The fate of these nuclei is doubtful, probably they fuse in pairs (fig. 6).

Blakeslee has lately made some very important observations of the Zygomycetes. It is well known that while in some forms,e.g.Spordinia, zygospores are easily obtained, in others,e.g.most species ofMucor, they are very erratic in their appearance. This has now been explained by Blakeslee, who finds that the Mucorinae can be divided into two groups, termed homothallic and heterothallic respectively. In the first group zygospores can arise by the union of branches from thesamemycelium and so can be produced by the growth from a single spore; this group includesSpordinia grandis,Spinellus fusiger, some species ofMucor, &c. The majority of forms, however, fall into the heterothallic group, in which the association of branches from two myceliadifferent in natureis necessary for the formation of zygospores. These structures cannot then be produced from the product of a single spore nor even from the thalli derived fromanytwo spores. The two kinds of thalli Blakeslee considers to have a differentiation of the nature of sex and he distinguishes them as (+) and (−) forms; the former being usually distinguished by a somewhat greater luxuriance of growth.

The classification of the Mucorini depends on the prevalence and characters of the conidia, and of the sporangia and zygospores—e.g.the presence or absence of a columella in the former, the formation of an investment round the latter. Most genera are saprophytes, but some—Chaetocladium,Piptocephalis—are parasites on other Mucorini, and one or two are associated casually with the rotting of tomatoes and other fruits, bulbs, &c., the fleshy parts of which are rapidly destroyed if once the hyphae gain entrance. Even more important is the question of mycosis in man and other animals, referred to species ofMucor, and investigated by Lucet and Costantin. Klebs has concluded that transpiration is the important factor in determining the formation of sporangia, while zygote-development depends on totally different conditions; these results have been called in question by Falck.

TheEntomophthoraceaecontain three genera,Empusa,EntomophthoraandBasidiobolus. The two first genera consist of forms which are parasitic on insects.Empusa Muscaecauses the well-known epidemic in house-flies during the autumn; the dead, affected flies are often found attached to the window surrounded by a white halo of conidia.B. ranarumis found in the alimentary canal of the frog and growing on its excrement. In these three genera the conidia are cast off with a jerk somewhat in the same way as the sporangium ofPilobolus.

B.Higher Fungi.—Now that Brefeld’s view of the origin of these forms from the Zygomycetes has been overthrown, the relationship of the higher and lower forms of fungi is left in obscurity. The termEumycetesis sometimes applied to this group to distinguish them from the Phycomycetes, but as the same name is also applied to the fungi as a whole to differentiate them from the Mycetozoa and Bacteria, the term had best be dropped. The Higher Fungi fall into three groups: theUstilaginales, of doubtful position, and the two very sharply marked groupsBasidialesandAscomycetes.

sp, The gonidium.

pm, The promycelium.

d, The sporidia: in B the sporidia have coalesced in pairs atv.

I.Ustilaginales.—This includes two families Ustilaginaceae (smuts) and Tilletiaceae (bunts). The bunts and smuts which damage our grain and fodder plants comprise about 400 species of internal parasites, found in all countries on herbaceous plants, and especially on Monocotyledons. They are remarkable for their dark spores developed in gall-like excrescences on the leaves, stems, &c., or in the fruits of the host. The discovery of the yeast-conidia of these fungi, and their thorough investigation by Brefeld, have thrown new lights on the group, as also have the results elucidating the nature of the ordinary dark spores—smuts, bunt, &c.—which by their mode of origin and development are chlamydospores. When the latter germinate a slender “promycelium” is put out; inUstilagoand its allies this is transversely septate, and bears lateral conidia (sporidia); inTilletiaand its allies non-septate, and bears a terminal tuft of conidia (sporidia) (fig. 7). Brefeld regarded the promycelium as a kind ofbasidium, bearing lateral or terminal conidia (comparable tobasidiospores), but since the number of basidiospores is not fixed, and the basidium has not yet assumed very definite morphological characters, Brefeld termed the groupHemibasidii, and regarded them as a half-way stage in the evolution of the true Basidiomycetes from Phycomycetes, theTilletiatype leading to the true basidium (Autobasidium), theUstilagotype to the protobasidium, with lateral spores; but this view is based on very poor evidence, so that it is best to place these forms as a separate group, theUstilaginales. The yeast-conidia, which bud off from the conidia or their resulting mycelium when sown in nutrient solutions, are developed in successive crops by budding exactly as in the yeast plant, but they cannot ferment sugar solutions. It is the rapid spread of these yeast-conidia in manure and soil waters which makes it so difficult to get rid of smuts, &c., in the fields, and they, like the ordinary conidia, readily infect the seedling wheat, oats, barley or other cereals. Infection in these cases occurs in the seedling at the place where root and shoot meet, and the infecting hypha having entered the plant goes on living in it and growing up with it as if it had no parasitic action at all. When the flowers form, however, the mycelium sends hyphae into the young ovaries and rapidly replaces the stores of sugar and starch, &c., which would have gone to make the grain, by the soot-like mass of spores so well known as smut, &c. These spores adhere to the grain, and unless destroyed, by “steeping” or other treatment, are sown with it, and again produce sporidia and yeast-conidia which infect the seedlings. In other species the infection occurs through the style of the flower, but the fungus after reaching the ovule develops no further during that year but remains dormant in the embryo of the seed. On germination, however, the fungus behaves in the same way as one which has entered in the seedling stage. The cytology of these forms is very little known; Dangeard states that there is a fusion of two nuclei in the chlamydospore, but this requires confirmation. Apart from this observation there is no other trace of sexuality in the group.II.Ascomycetes.—This, except in the case of a few of the simpler forms, is a very sharply marked group characterized by a special type of sporangium, the ascus. In the development of the ascus we find two nuclei at the base which fuse together to form the single nucleus of the youngascus. The single nucleus divides by three successive divisions to form eight nuclei lying free in the protoplasm of the ascus. Then by a special method, described first by Harper, a mass of protoplasm is cut out round each nucleus; thus eight uninucleate ascospores are formed by free-cell formation. The protoplasm remaining over is termedepiplasmand often contains glycogen (fig. 8). In some cases nuclear division is carried further before spore-formation occurs, and the number of spores is then 16, 32 and 64, &c.; in a few cases the number of spores is less than eight by abortion of some of the eight nuclei. The ascus is thus one of the most sharply characterized structures among the fungi.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 8.—Development of the Ascus.A-C,Pyronema confluens. (After Harper.)D, Young ascus ofBoudierawith eight spores. (After Claussen.)In some forms we find definite male and female sexual organs (Sphaerotheca,Pyronema, &c.), in others the antheridium is abortive or absent, but the ascogonium (oogonium) is still present and the female nuclei fuse in pairs (Lachnea stercorea,Humaria granulata,Ascobolus furfuraceus); while in other forms ascogonium and antheridium are both absent and fusion occurs between vegetative nuclei (Humaria rutilans, and probably the majority of other forms). In other cases the sexual fusion is apparently absent altogether, as inExoascus. In the first case (fig. 9) we have a true sexual process, while in the second and third cases we have areducedsexual process in which the fusion of other nuclei has replaced the fusion of the normal male and female nuclei. It is to be noted that all the forms exhibit the fusion of nuclei in the ascus, so that those with the normal or reduced sexual process described above have two nuclear fusions in their life-history. The advantage or significance of the second (ascus) fusion is not clearly understood.The group of the Hemiasci was founded by Brefeld to include forms which were supposed to be a connecting link between Phycomycetes and Ascomycetes. As mentioned before, the connexion between these two groups is very doubtful, and the derivation of the ascus from an ordinary sporangium of the Zygomycetes cannot be accepted. The majority of the forms which were formerly included in this group have been shown to be either true Phycomycetes (likeAscoidea) or true Ascomycetes (likeThelebolus).EremascusandDipodascus, which are often placed among the Hemiasci, possibly do not belong to the Ascomycetes series at all.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 9.—Sphaerotheca Castagnei. Fertilization and Development of the Perithecium. (After Harper.)1, Oogonium (og) with the antheridial branch (az) applied to its surface2, Separation of antheridium (an).3, Passage of the antheridial nucleus towards that of the oogonium.4, Union of the nuclei.5, Fertilized oogonium surrounded by two layers of hyphae derived from the stalk-cell (st).6, The multicellular ascogonium derived by division from the oogonium; the terminal cell with the two nuclei (as) gives rise to the ascus.Exoascaceaeare a small group of doubtful extent here used to includeExoascus,Taphrina,AscorticiumandEndomyces. The mycelium is very much reduced in extent. The asci are borne directly on the mycelium and are therefore fully exposed, being devoid from the beginning of any investment. TheTaphrineae, which includeExoascusandTaphrina, are important parasites—e.g.pocket-plums and witches’ brooms on birches, &c., are due to their action (fig. 10).ExoascusandAscorticiumpresent interesting parallels toExobasidiumandCorticiumamong the Basidiomycetes.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 10.—Taphrina Pruni.Transverse section through the epidermis of an infected plum. Four ripe asci,a1,a2, with eight spores,a3,a4, with yeast-like conidia abstricted from the spores. After Sadebeck.st, Stalk-cells of the asci.m, Filaments of the mycelium cut transversely.cut, Cuticle.sp, Epidermis.Saccharomycetaceaeinclude the well-known yeasts which belong mainly to the genusSaccharomyces. They are characterized by their unicellular nature, their power of rapid budding, their capacity for fermenting various sugars, and their power of forming endogenousspores. The sporangium with its endogenous spores has been compared with an ascus, and on these grounds the group is placed among the Ascomycetes—a very doubtful association. The group has attained an importance of late even beyond that to which it was brought by Pasteur’s researches on alcoholic fermentation, chiefly owing to the exact results of the investigations of Hansen, who first applied the methods of pure cultures to the study of these organisms, and showed that many of the inconsistencies hitherto existing in the literature were due to the coexistence in the cultures of several species or races of yeasts morphologically almost indistinguishable, but physiologically very different. About fifty species ofSaccharomycesare described more or less completely, but since many of these cannot be distinguished by the microscope, and some have been found to develop physiological races or varieties under special conditions of growth, the limits are still far too ill-defined for complete botanical treatment of the genus. A typical yeast is able to develop new cells by budding when submerged in a saccharine solution, and to ferment the sugar—i.e.so to break up its molecules that, apart from small quantities used for its own substance, masses of it out of all proportion to the mass of yeast used become resolved into other bodies, such as carbon dioxide and alcohol, the process requiring little or no oxygen. Brefeld regards the budding process as the formation of conidia. Under other conditions, of which the temperature is an important one, the nucleus in the yeast-cell divides, and each daughter-nucleus again, and four spores are formed in the mother cell, a process obviously comparable to the typical development of ascospores in an ascus. Under yet other conditions the quiescent yeast-cells floating on the surface of the fermented liquor grow out into elongated sausage-shaped or cylindrical cells and branching cell-series, which mat together into mycelium-like veils. At the bottom of the fermented liquor the cells often obtain fatty contents and thick walls, and behave as resting cells (chlamydospores). The characters employed by experts for determining a species of yeast are the sum of its peculiarities as regards form and size: the shapes, colours, consistency, &c., of the colonies grown on certain definite media; the optimum temperature for spore-formation, and for the development of the “veils”; and the behaviour as regards the various sugars.The following summary of some of the principal characteristics of half-a-dozen species will serve to show how such peculiarities can be utilized for systematic purposes:Species.Optimum Temperature forCharacters ofSugars Fermented andProducts, &c.Spores.Veils.Fermentation.Cells.Spores.S. cereviseae I.30°20°-28°HighRoundedGloboidInverts maltose and saccharoseand form alcohol 4-6 vol. %.S. Pastorianus I27°-5°26°-28°LowRoundedGloboidS. ellipsoideus25°33°-34°LowRoundedGloboidS. anomalus28°-31°?HighEllipticalHat-shapedDitto, and evolves a fragrant ether.S. Ludwigii30°-31°??ElongatedGloboidWill not invert maltose.S. membranaefaciens30°?HighElongatedGloboidInverts neither maltose nor saccharose.Two questions of great theoretical importance have been raised over and over again in connexion with yeasts, namely, (1) the morphological one as to whether yeasts are merely degraded forms of higher fungi, as would seem implied by their tendency to form elongated, hypha-like cells in the veils, and their development of “ascospores” as well as by the wide occurrence of yeast-like “sprouting forms” in other fungi (e.g.Mucor, Exoasci, Ustilagineae, higher Ascomycetes and Basidiomycetes); and (2) the question as to the physiological nature and meaning of fermentation. With regard to the first question no satisfactory proof has as yet been given that Saccharomycetes are derivable by culture from any higher form, the recent statements to that effect not having been confirmed. At the same time there are strong grounds for insisting on the resemblances betweenEndomyces, a hyphal fungus bearing yeast-like asci, and such a form asSaccharomyces anomalus. Concerning the second question, the recent investigations of Buchner and others have shown that a ferment (zymase) can be extracted from yeast-cells which causes sugar to break up into carbon dioxide and alcohol. It has since been shown by Buchner and Albert that yeast-cells which have been killed by alcohol and ether, or with acetone, still retain the enzyme. Such material is far more active than the zymase obtained originally by Buchner from the expressed juice of yeast-cells. Thus alcoholic fermentation is brought into line with the other fermentations.Schizosaccharomycesincludes a few species in which the cells do not “bud” but become elongated and then divide transversely. In the formation of sporangia two cells fuse together by means of outgrowths, in a manner very similar to that ofSpirogyra; sometimes, however, the wall between two cells merely breaks down. The fused cell becomes a sporangium, and in it eight spores are developed. In certain cases single cells develop parthenogenetically, without fusion, each cell producing, however, only four spores. InZygosaccharomycesdescribed by Barker (1901) we have a form of the usual sprouting type, but here again there is a fusion of two cells to form a sporangium.Cytology.—The study of the nucleus of yeast-cells is rendered difficult by the presence of other deeply staining granules termed by Guillermondmetachromatic granules. These have often been mistaken for nuclei and have to be carefully distinguished by differential stains. In the process of budding the nucleus divides apparently by a process of direct division. In the formation of spores the nucleus of the cell divides, the protoplasm collects round the nuclei to form the spores by free-cell formation; the protoplasm (epiplasm) not used in this process becomes disorganized. A fusion of nuclei was originally described by Jansens and Leblanc, but it was observed neither by Wager nor Guillermond and is probably absent. InSchizosaccharomycesandZygosaccharomyces, however, we have a fusion of nuclei in connexion with the conjugation of cells which precedes sporangium-formation. The theory may be put forward that the ordinary forms have been derived from sexual forms likeSchizosaccharomycesandZygosaccharomycesby a loss of sexuality, the sporangium being formed parthenogenetically without any nuclear fusion. This suggests a possible relationship toEremascus, which can only doubtfully be placed in the Ascomycetes (vide supra).Carpoascomycetes.—The other divisions of the Ascomycetes may be distinguished as Carpoascomycetes because they do not bear the asci free on the mycelium but enclosed in definite fruit bodies or ascocarps. The ascocarps can be distinguished into two portions, a mass of sterile or vegetative hyphae forming the main mass of the fruit body, and surrounding the fertile ascogenous hyphae which bear at their ends the asci. When the ascogonium (female organ) is present the ascogenous hyphae arise from it, with or without its previous fusion with an antheridium. In other cases the ascogenous hyphae arise directly from the vegetative hyphae. In connexion with this condition of reduction a fusion of nuclei has been observed inHumaria rutilansand is probably of frequent occurrence. The asci may be derived from the terminal cell of the branches of the ascogenous hyphae, but usually they are derived from the penultimate cell, the tip curving over to form the so-called crozier. By this means the ascus cell is brought uppermost, and after the fusion of the two nuclei it develops enormously and produces the ascospores. The ascospores escape from the asci in various ways, sometimes by a special ejaculation-mechanism. The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations. The ordinary mycelium is the gametophyte since it bears the ascogonia and antheridia when present; the ascogenous hyphae with their asci represent the sporophyte since they are derived from the fertilized ascogonium. The matter is complicated by the apogamous transition from gametophyte to sporophyte in the absence of the ascogonium; also by the fact that there are normally two fusions in the life-history as mentioned earlier. If there are two fusions one would expect two reductions, and Harper has suggested that the division of the nuclei into eight in the ascus, instead of into four spores as in most reduction processes, is associated with adoublereduction process in the ascus. Miss Fraser inHumaria rutilansfinds two reductions: a normal synaptic reduction in the first nuclear division of the ascus, and a peculiar reduction division termedbrachymeiosisin the third ascus division.Various types of ascocarp are characteristic of the different divisions of the Carpoascomycetes: the cleistothecium, apothecium and perithecium.Perisporineae.—This includes two chief families, Erysiphaceae and Perisporiaceae. They are characterized by an ascocarp without any opening to the exterior, the ascospores being set free by the decay or rupture of the ascocarp wall; such a fruit-body is termed acleistothecium(cleistocarp). The Erysiphaceae are a sharply marked group of forms which live as parasites. They form a superficial mycelium on the surface of the plant, the hyphae not usually penetrating the tissues but merely sending haustoria into the epidermal cells. Only in rare cases is the mycelium intercellular. Owing to their appearance they go by the popular name of mildews.Sphaerotheca Humuliis the well known hop-mildew,Sphaerotheca Mors-Uvaeis the gooseberry mildew, the recent advent of which has led to special legislation in Great Britain to prevent its spreading, as when rampant it makes the culture of gooseberries impossible.Erysiphe,UncinulaandPhyllactiniaare other well-known genera. The form of the fruit body, the difference and the nature of special outgrowths upon it—the appendages—are characteristic of the various genera. Besides peritheca the members of the Erysiphaceae possess conidia borne in simple chains. De Bary brought forward very strong evidence for the origin of the ascocarp inSphaerothecaandErysipheby a sexual process, but Harper in 1895 was the first to prove conclusively, by the observation of the nuclear fusion, that there was a definite fertilization inSphaerotheca Humuliby the fusion of a male (antheridial) nucleus with a female, ascogonial (oogonial) nucleus. Since then Harper has shown that the same process occurs inErysipheandPhyllactinia.Fig. 11.—Development ofEurotium repens. (After De Bary.)A, Small portion of mycelium with conidiophore (c), and archicarp (as).B, The spiral archicarp (as), with the antheridium (p).D, The same, beginning to be surrounded by the hyphae forming the perithecium wall.D, The perithecium.E, F, Sections of young perithecia.w, Parietal cells.f, Pseudo-parenchyma.as, Ascogonium.G, An ascus.H, An ascospore.The Perisporiaceae are saprophytic forms, the two chief genera beingAspergillusandPenicillium. The blue-green mouldP. crustaceumand the green mouldA. herbariorium(=Eurotium herbariorum) are extraordinarily widely distributed, moulds being found on almost any food-material which is exposed to the air. They have characteristic conidiophores bearing numerous conidia, and also cleistothecia which are spherical in form and yellowish in colour. The latter arise from the crown of a spirally coiled archicarp (bearing an ascogonium at its end) and a straight antheridium. Vegetative hyphae then grow up and surround these and enclose them in a continuous sheath of plectenchyma (fig. 11). It has lately been shown by Fraser and Chambers that inEurotiumboth ascogonium and antheridium contain a number of nuclei (i.e.are coenogametes), but that the antheridium disorganizes without passing its contents into the ascogonium. There is apparently a reduced sexual process by the fusion of the ascogonial (female) nuclei in pairs.Aspergillus Oryzaeplays an important part in saccharifying the starch of rice, maize, &c., by means of the abundant diastase it secretes, and, in symbiosis with a yeast which ferments the sugar formed, has long been used by the Japanese for the preparation of the alcoholic liquor saké. The process has now been successfully introduced into European commerce.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 13.—Ascobolus furfuraceus.Diagrammatic section of the fructification. (After Janczewski.)Fig. 12.—Peziza aurantiaca.(After Krombholz, nat. size.)m, Mycelium.c, Archicarp.l, Pollinodium.s, Ascogenous filaments.a, Asri.r,p, The sterile tissue from which the paraphyseshspring.Discomycetes.—Used in its widest sense this includes the Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is characterized in general by the possession of an ascocarp which, though usually a completely closed structure during the earlier stages of development, at maturity opens out to form a bowl or saucer-shaped organ, thus completely exposing the layer of asci which forms the hymenium. Such an ascocarp goes by the name ofapothecium. Owing to the shape of the fruit-body many of these forms are known as cup-fungi, the cup or apothecium often attaining a large size, sometimes several inches across (fig. 12). Functional male and female organs have been shown to exist inPyronemaandBoudiera; inLachnea stercoreaboth ascogonia and antheridia are present, but the antheridium is non-functional, the ascogonial (female) nuclei fusing in pairs; this is also the case inHumaria granulataandAscobolus furfuraceus, where the antheridium is entirely absent. InH. rutilans, however, both sexual organs are absent and the ascogenous hyphae arise apogamously from the ordinary hyphae of the mycelim. In all these cases the ascogonium and antheridium contain numerous nuclei; they are to be looked upon as gametangia in which there is no differentiation of gametes, and since they act as single gametes they are termed coenogametes. In some forms as inAscobolusthe ascogonium is multicellular, the various cells communicating by pores in the transverse walls (fig. 13).From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 14.—Perithecium of Podospora fimiseda in longitudinal section. After v. Tavel.s, Asci.a, Paraphyses.e, Periphyses.m, Mycelial hyphae.In the Helvellaceae there is no apothecium but a large irregular fruit body which at maturity bears the asci on its surface. The development is only slightly known, but there is some evidence for believing that the fruit-body is closed in its very early stages.The genusPeziza(in its widest sense) may be taken as the type of the group. Most of them grow on living plants or on dead vegetable remains, very often on fallen wood; a number, however, are found growing on earth which is rich in humus. The genusSclerotiniamay be mentioned here; a number of forms have been investigated by Woronin. The conidia are fragrant and are carried by bees to the stigma of the bilberry; here they germinate with the pollen and the hyphae pass with the pollen tubes down the style; the former infect the ovules and produce sclerotia, therein reducing the fruits to a mummified condition. From the sclerotia later the apothecium develops. One species,S. heteroica, isheteroecious; the ascospores infecting the leaves ofVaccinium uliginosum, while the conidia which then arise infect onlyLedum palustre. This is the only case of heteroecism known in the vegetable kingdom outside the Uredineae.Pyrenomycetes.—This is an extraordinarily large and varied group of forms which mostly live parasitically or saprophytically on vegetable tissue, but a few are parasitic on insect-larvae. The groupis characterized by a special type of ascocarp, theperithecium. This is typically of a flask-shaped form opening with a small pore at the top. The asci live at the bottom often mixed with paraphyses, while the upper “neck” of the flask is lined with special hyphae, the periphyses, which aid in the ejection of the spores (fig. 14). The simpler forms bear the perithecia directly on the mycelium, but the more highly developed forms often bear them on a special mycelial development—the stroma, which is often of large size and special shape and colour, and of dense consistence. The cytological details of development of the perithecia are not well known; most of them appear to develop their ascogenous hyphae in an apogamous way without any connexion with an ascogonium. Besides the special ascocarps, accessory reproductive organs are known in the majority of cases in the form of conidia.Tuberineae.—These are a small group of fungi including the well-known truffles. They are found living saprophytically (in part parasitically) underground in forests. The asci are developed in the large dense fruit bodies (cleistothecia) and the spores escape by the decay of the wall. The fruit-body is of complicated structure, but its early stages of development are not known. Many of the fruit-bodies have a pleasant flavour and are eaten under the name of truffles (Tuber brumaleand other species). The exact life-history of the truffle is not known.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 15.—Armillaria mellea.(After Ruhland.)A, Young basidium with the two primary nuclei.B, After fusion of the two nuclei.Hypholoma appendiculatum.C, A basidium before the four nuclei derived from the secondary nucleus of the basidium have passed into the four basidiospores.D, Passage of a nucleus through the sterigma into the basidiospore.Laboulbeniineaeare a group of about 150 species of fungi found on insects, especially beetles, and principally known from the researches of Thaxter in America. The plant is a small, dark brown, erect structure (receptacle) of a few cells, and 1-10 mm. high, attached to the insect by the lowermost end (foot), and easily mistaken for a hair or similar appendage of the insect. The receptacle ends above in appendages, each consisting of one or a few cells, some of which are the male organs, others the female organs, and others again may be barren hairs. The male organ (antheridium) consists of a few cells, the terminal one of which either abstricts from its end, or emits from its interior the non-motile spermatia, reminding us of those of the Florideae. The female organ is essentially a flask-shaped structure; the neck of the flask growing out as the trichogyne, and the belly composed of an axial carpogenic cell surrounded by investing cells, and with one cell (trichophoric) between it and the trichogyne. These three elements—trichogyne, trichophoric cell, and carpogenic cell—are regarded as the procarp. The spermatia have been shown by Thaxter to fuse with the trichogyne, after which the axial cell below (carpogenic cell) undergoes divisions, and ultimately forms asci containing ascospores, while cells investing this form a perithecium, the whole structure reminding us essentially of the fructification of a Pyrenomycete. Many modifications in details occur, and the plants may be dioecious. No injury is done to the infested insects. It has lately been shown that there is a fusion of nuclei in connexion with ascus formation, so that there can be no doubt of the position of this extraordinary group of plants among the Ascomycetes. The various cells of these organisms are connected by large pits which are traversed by thick protoplasmic threads connecting one cell with the next. In this point and in their method of fertilization the Laboulbeniineae suggest a possible relationship of Ascomycetes and the Red Algae.Basidiales.—This very large group of plants is characterized by the possession of a special type of conidiophore—the basidium, which gives its name to the group. The basidium is a unicellular or multicellular structure from which four basidiospores arise as outgrowths; it starts as a binucleate structure, but soon, like the ascus, becomes uninucleate by the fusion of the two nuclei. Then two successive nuclear divisions occur resulting in the formation of four nuclei which later migrate respectively into the four basidiospores (fig. 15). The Basidiales are further characterized by the complete loss of normal sexuality, but at some time or other in the life-history there takes place an association of two nuclei in a cell; the two nuclei are derived from separate cells or possibly in some cases are sister nuclei of the same cell. The two nuclei when once associated are termed “conjugate” nuclei, and they always divide at the same time, a half of each passing into each cell. This conjugate condition is finally brought to a close by the nuclear fusion in the basidium. Between the nuclear association and the nuclear fusion in the basidium many thousands of cell generations may be intercalated. This nuclear association of equivalent nuclei apparently represents a reduced sexual process (like the fusion of female nuclei inHumaria granulataand of vegetative nuclei inH. rutilans, among the Ascomycetes) in which, however, the actual fusion (normally, in a sexual process, occurring immediately after association) is delayed until the formation of the basidium. During the tetrad division in the basidium nuclear reduction occurs. There is thus in all the Basidiales an alternation of generations, obscured, however, by the apogamous transition from the gametophyte to sporophyte. The sporophyte may be considered to begin at the stage of nuclear association and end with the nuclear reduction in the basidium.Fig. 16.—Puccinia graminis.A, Mass of teleutospores (t) on a leaf of couch-grass.e, Epidermis ruptured.b, Sub-epidermal fibres. (After De Bary.)B, Part of vertical section through leaf of Berberis vulgaris, witha, aecidium fruits,p, peridium, andsp, spermogonia. (After Sachs.)C, Mass of uredospores (ur), with one teleutospore (t).sh, Sub-hymenial hyphae. (After De Bary.)Uredineae.—This is a large group of about 2000 forms. They are all intercellular parasites living mostly on the leaves of higher plants. Owing to the presence of oily globules of an orange-yellow or rusty-red colour in their hyphae and spores they are termed Rust-Fungi. They are distinguished from the other fungi and the rest of the Basidiales by the great variety of the spores and the great elaboration of the life-history to be found in many cases. Five different kinds of spores may be present—teleutospores, sporidia (= basidiospores), aecidiospores, spermatia and uredospores (fig. 16). The teleutospore, with the sporidia which arise from it, is always present, and the division into genera is based chiefly on its characters. The teleutospore puts forth on germination a four-celled structure, the promycelium or basidium, and this bears later four sporidia or basidiospores, one on each cell. When the sporidia infect a plant the mycelium so produced gives origin to aecidiospores and spermatia; the aecidiospores on infection produce a mycelium which bears uredospores and later teleutospores. This is the life-history of the most complicated forms, of the so-calledeuforms. In theopsisforms the uredospores are absent, the mycelium from the aecidiospores producing directly the teleutospores. Inbrachyandhemithe aecidiospores are absent, the mycelium from the sporidia giving origin directly to the uredospores; the former possess spermatia, in the latter they are absent. Inleptoandmicroforms both aecidiospores and uredospores are absent, the sporidia producing a mycelium which gives rise directly to teleutospores; in theleptoforms the teleutospores can germinate directly, in themicroforms only after a period of rest. We have thus a series showing a progressive reduction in the complexity of the life-history, theleptoandmicroforms having a life-history like that of the Basidiomycetes. Theeuandopsisforms may exhibit the remarkable phenomenon of heteroecism,i.e.the dependence of the fungus on two distinct host-plants for the completion of the life-history. Heteroecism is very common in this group and is now known in over one hundred and fifty species. In all cases of heteroecism the sporidia infect one host leading to the production of aecidiospores and spermatia (if present), while the aecidiospores are only able to infect anotherhost on which the uredospores (if present) and the teleutospores are developed. A few examples are appended:Species.Teleutospores onAecidiospores onColeosporium SenecionisPinusSenecioMelampsora RostrupiPopulusMecurialisPucciniastrum GoeppertianaVacciniumAbiesGymnosporangium SabinaeJuniperusPyrusUromyces PisiPisum, &c.EuphorbiaPuccinia graminisTriticum, &c.BerberisP. dispersaSecale, &c.AnchusaP. coronataAgrostisRhamnusP. Ari-PhalaridisPhalarisArumP. CaricisCarexUrticaCronartium RibicolaRibesPinusChrysomyxa RhododendriRhododendronPiceaSome of the Uredineae also exhibit the peculiarity of the development of biologic forms within a single morphological species, sometimes termed specialization of parasitism; this will be dealt with later under the section Physiology.From Strasburger’sLehrbuch der Botanik, by permission of Gustav Fischer.Fig. 17.—Phragmidium Violaceum.(After Blackman.)A, Portion of a young aecidium.st, Sterile cell.a, Fertile cells; ata2the passage of a nucleus from the adjoining cell is seen.B, Formation of the first spore-mother-cell (sm), from the basal cell (a) of one of the rows of spores.C, A further stage in which from sm1the first aecidiospore (a) and the intercalary cell (z) have arisen.sm2, The second spore-mother-cell.D, Ripe aecidiosporeCytology of Uredineae.—The study of the nuclear behaviour of the cells of the Uredineae has thrown great light on the question of sexuality. This group like the rest of the Basidiales exhibits an association of nuclei at some point in its life-history, but unlike the case of the Basidiomycetes the point of association in the Uredineae is very well defined in all those forms which possess aecidiospores. We find thus that in theeuandopsisforms the association of nuclei takes place at the base of the aecidium which produces the aecidiospores. There we find an association of nuclei either by the fusion of two similar cells as described by Christmann or by the migration of the nucleus of a vegetative cell into a special cell of the aecidium. After this association the nuclei continue in the conjugate condition so that the aecidiospores, the uredospore-bearing mycelium, the uredospores and the young teleutospores all contain two paired nuclei in their cells (fig. 17). Before the teleutospore reaches maturity the nuclei fuse, and the uninucleate condition then continues again until aecidium formation. In thehemi,brachy,microandleptoforms, which possess no aecidium, we find that the association takes place at various points in the ordinary mycelium but always before the formation of the uredospores in thehemiandbrachyforms, and before the formation of teleutospores inmicroandleptoform. Whether the association of nuclei in the ordinary mycelium takes place by the migration of a nucleus from one cell to another or whether two daughter nuclei become conjugate in one cell, is not yet clear. The most reasonable interpretation of the spermatia is that they are abortive male cells. They have never been found to cause infection, and they have not the characters of conidia; the large size of their nuclei, the reduction of their cytoplasm and the absence of reserve material and their thin cell wall all point to their being male gametes. Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte. The subjoined diagram will indicate the relationship of the forms.Basidiomycetes.—This group is characterized by its greatly reduced life-history as compared with that of theeuforms among the Uredineae. All the forms have the same life-history as theleptoforms of that group, so that there is no longer any trace of sexual organs. There is also a further reduction in that the basidium is not derived from a teleutospore but is borne directly on the mycelium. Formerly, before the relationship of promycelium and basidium were understood, the Uredineae were considered as quite independent of the Basidiomycetes. Later, however, these Uredineae were placed as a mere subdivision of the Basidiomycetes. Although the Uredineae clearly lead on to the Basidiomycetes, yet owing to their retaining in many cases definite traces of sexual organs they are clearly a more primitive group. Their marked parasitic habit also separates them off, so that they are best included with the Basidiomycetes in a larger cohort which may be called Basidiales. Most of Basidiomycetes are characterized by the large sporophore on which the basidia with its basidiospores are borne.FromAnnals of Botany, by permission of the Clarendon Press.Fig. 18.It must be clearly borne in mind that though the Basidiomycetes show no traces of differentiated sexual organs yet, like themicroandleptoforms of the Uredineae, they still show (in the association of nuclei and later fusion of nuclei in the basidium), a reduced fertilization which denotes their derivation, through the Uredineae, from more typically sexual forms. No one has yet made out in any form the exact way in which the association of nuclei takes place in the group. The mycelium is always found to contain conjugate nuclei before the formation of basidia, but the point at which the conjugate condition arises seems very variable. Miss Nichols finds that it occurs very soon after the germination of the spore inCoprinus, but no fusion of cells or migration of nuclei was to be observed.Fig. 19.—Amanita muscaria.A, The young plant.B, The mature plant.C, Longitudinal section of mature plant.p, Thepileus.g, The gills.a, Theannulus, or remnant ofvelum partiale,v, Remains ofvolvaorvelum universale.s, The stalk.Protobasidiomycetes.—This, by far the smaller division of Basidiomycetes, includes those forms which have a septate basidium. There are three families—Auriculariaceae, Pilacreaceae and Tremellinaceae. The first named contains a small number of forms with the basidium divided like the promycelium of the Uredineae. They are characterized by their gelatinous consistence and large size of their sporophore.Hirneola(Auricularia)Auricula-Judaeis the well-known Jew’s Ear, so named from the resemblance of the sporophore to a human ear.The Pilacreaceae are a family found by Brefeld to contain the genusPilacre.P. Petersiihas a transversely divided basidium as inAuriculariaceae, but the basidia are surrounded with a peridium-like sheath. TheTremellinaceaeare characterized by the possession of basidia which are divided by twoverticalwalls at right angles to one another. From each of the four segments in the case ofTremellaa long outgrowth arises which reaches to the surface of the hymeniumand bears the basidiospores. InDacryomycesonly two outgrowths and two spores are produced.Autobasidiomycetes.—In this by far the larger division of the Basidiomycetes the basidia are undivided and the four basidiospores are borne on short sterigmata nearly always at the apex of the basidium. The group may be divided into two main divisions,HymenomycetesandGasteromycetes.Fig.20.—Agaricus mucidus. Portion of hymenium.s, Sporidia;st, sterigmata;g, sterile cells;c, cystidium, with operculumo.Hymenomycetesare 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 melleusandPolyporus annosusare examples of parasitic forms which attack and destroy living trees, whileMerulius lacrymansis the well-known “dry rot” fungus.Gasteromycetesare 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 outerperidiumand an inner spore-bearing mass, thegleba. 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 areBovista, 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 thePhalloideae, 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.

II.Ascomycetes.—This, except in the case of a few of the simpler forms, is a very sharply marked group characterized by a special type of sporangium, the ascus. In the development of the ascus we find two nuclei at the base which fuse together to form the single nucleus of the youngascus. The single nucleus divides by three successive divisions to form eight nuclei lying free in the protoplasm of the ascus. Then by a special method, described first by Harper, a mass of protoplasm is cut out round each nucleus; thus eight uninucleate ascospores are formed by free-cell formation. The protoplasm remaining over is termedepiplasmand often contains glycogen (fig. 8). In some cases nuclear division is carried further before spore-formation occurs, and the number of spores is then 16, 32 and 64, &c.; in a few cases the number of spores is less than eight by abortion of some of the eight nuclei. The ascus is thus one of the most sharply characterized structures among the fungi.

A-C,Pyronema confluens. (After Harper.)

D, Young ascus ofBoudierawith eight spores. (After Claussen.)

In some forms we find definite male and female sexual organs (Sphaerotheca,Pyronema, &c.), in others the antheridium is abortive or absent, but the ascogonium (oogonium) is still present and the female nuclei fuse in pairs (Lachnea stercorea,Humaria granulata,Ascobolus furfuraceus); while in other forms ascogonium and antheridium are both absent and fusion occurs between vegetative nuclei (Humaria rutilans, and probably the majority of other forms). In other cases the sexual fusion is apparently absent altogether, as inExoascus. In the first case (fig. 9) we have a true sexual process, while in the second and third cases we have areducedsexual process in which the fusion of other nuclei has replaced the fusion of the normal male and female nuclei. It is to be noted that all the forms exhibit the fusion of nuclei in the ascus, so that those with the normal or reduced sexual process described above have two nuclear fusions in their life-history. The advantage or significance of the second (ascus) fusion is not clearly understood.

The group of the Hemiasci was founded by Brefeld to include forms which were supposed to be a connecting link between Phycomycetes and Ascomycetes. As mentioned before, the connexion between these two groups is very doubtful, and the derivation of the ascus from an ordinary sporangium of the Zygomycetes cannot be accepted. The majority of the forms which were formerly included in this group have been shown to be either true Phycomycetes (likeAscoidea) or true Ascomycetes (likeThelebolus).EremascusandDipodascus, which are often placed among the Hemiasci, possibly do not belong to the Ascomycetes series at all.

1, Oogonium (og) with the antheridial branch (az) applied to its surface

2, Separation of antheridium (an).

3, Passage of the antheridial nucleus towards that of the oogonium.

4, Union of the nuclei.

5, Fertilized oogonium surrounded by two layers of hyphae derived from the stalk-cell (st).

6, The multicellular ascogonium derived by division from the oogonium; the terminal cell with the two nuclei (as) gives rise to the ascus.

Exoascaceaeare a small group of doubtful extent here used to includeExoascus,Taphrina,AscorticiumandEndomyces. The mycelium is very much reduced in extent. The asci are borne directly on the mycelium and are therefore fully exposed, being devoid from the beginning of any investment. TheTaphrineae, which includeExoascusandTaphrina, are important parasites—e.g.pocket-plums and witches’ brooms on birches, &c., are due to their action (fig. 10).ExoascusandAscorticiumpresent interesting parallels toExobasidiumandCorticiumamong the Basidiomycetes.

st, Stalk-cells of the asci.

m, Filaments of the mycelium cut transversely.

cut, Cuticle.

sp, Epidermis.

Saccharomycetaceaeinclude the well-known yeasts which belong mainly to the genusSaccharomyces. They are characterized by their unicellular nature, their power of rapid budding, their capacity for fermenting various sugars, and their power of forming endogenousspores. The sporangium with its endogenous spores has been compared with an ascus, and on these grounds the group is placed among the Ascomycetes—a very doubtful association. The group has attained an importance of late even beyond that to which it was brought by Pasteur’s researches on alcoholic fermentation, chiefly owing to the exact results of the investigations of Hansen, who first applied the methods of pure cultures to the study of these organisms, and showed that many of the inconsistencies hitherto existing in the literature were due to the coexistence in the cultures of several species or races of yeasts morphologically almost indistinguishable, but physiologically very different. About fifty species ofSaccharomycesare described more or less completely, but since many of these cannot be distinguished by the microscope, and some have been found to develop physiological races or varieties under special conditions of growth, the limits are still far too ill-defined for complete botanical treatment of the genus. A typical yeast is able to develop new cells by budding when submerged in a saccharine solution, and to ferment the sugar—i.e.so to break up its molecules that, apart from small quantities used for its own substance, masses of it out of all proportion to the mass of yeast used become resolved into other bodies, such as carbon dioxide and alcohol, the process requiring little or no oxygen. Brefeld regards the budding process as the formation of conidia. Under other conditions, of which the temperature is an important one, the nucleus in the yeast-cell divides, and each daughter-nucleus again, and four spores are formed in the mother cell, a process obviously comparable to the typical development of ascospores in an ascus. Under yet other conditions the quiescent yeast-cells floating on the surface of the fermented liquor grow out into elongated sausage-shaped or cylindrical cells and branching cell-series, which mat together into mycelium-like veils. At the bottom of the fermented liquor the cells often obtain fatty contents and thick walls, and behave as resting cells (chlamydospores). The characters employed by experts for determining a species of yeast are the sum of its peculiarities as regards form and size: the shapes, colours, consistency, &c., of the colonies grown on certain definite media; the optimum temperature for spore-formation, and for the development of the “veils”; and the behaviour as regards the various sugars.

The following summary of some of the principal characteristics of half-a-dozen species will serve to show how such peculiarities can be utilized for systematic purposes:

Two questions of great theoretical importance have been raised over and over again in connexion with yeasts, namely, (1) the morphological one as to whether yeasts are merely degraded forms of higher fungi, as would seem implied by their tendency to form elongated, hypha-like cells in the veils, and their development of “ascospores” as well as by the wide occurrence of yeast-like “sprouting forms” in other fungi (e.g.Mucor, Exoasci, Ustilagineae, higher Ascomycetes and Basidiomycetes); and (2) the question as to the physiological nature and meaning of fermentation. With regard to the first question no satisfactory proof has as yet been given that Saccharomycetes are derivable by culture from any higher form, the recent statements to that effect not having been confirmed. At the same time there are strong grounds for insisting on the resemblances betweenEndomyces, a hyphal fungus bearing yeast-like asci, and such a form asSaccharomyces anomalus. Concerning the second question, the recent investigations of Buchner and others have shown that a ferment (zymase) can be extracted from yeast-cells which causes sugar to break up into carbon dioxide and alcohol. It has since been shown by Buchner and Albert that yeast-cells which have been killed by alcohol and ether, or with acetone, still retain the enzyme. Such material is far more active than the zymase obtained originally by Buchner from the expressed juice of yeast-cells. Thus alcoholic fermentation is brought into line with the other fermentations.

Schizosaccharomycesincludes a few species in which the cells do not “bud” but become elongated and then divide transversely. In the formation of sporangia two cells fuse together by means of outgrowths, in a manner very similar to that ofSpirogyra; sometimes, however, the wall between two cells merely breaks down. The fused cell becomes a sporangium, and in it eight spores are developed. In certain cases single cells develop parthenogenetically, without fusion, each cell producing, however, only four spores. InZygosaccharomycesdescribed by Barker (1901) we have a form of the usual sprouting type, but here again there is a fusion of two cells to form a sporangium.

Cytology.—The study of the nucleus of yeast-cells is rendered difficult by the presence of other deeply staining granules termed by Guillermondmetachromatic granules. These have often been mistaken for nuclei and have to be carefully distinguished by differential stains. In the process of budding the nucleus divides apparently by a process of direct division. In the formation of spores the nucleus of the cell divides, the protoplasm collects round the nuclei to form the spores by free-cell formation; the protoplasm (epiplasm) not used in this process becomes disorganized. A fusion of nuclei was originally described by Jansens and Leblanc, but it was observed neither by Wager nor Guillermond and is probably absent. InSchizosaccharomycesandZygosaccharomyces, however, we have a fusion of nuclei in connexion with the conjugation of cells which precedes sporangium-formation. The theory may be put forward that the ordinary forms have been derived from sexual forms likeSchizosaccharomycesandZygosaccharomycesby a loss of sexuality, the sporangium being formed parthenogenetically without any nuclear fusion. This suggests a possible relationship toEremascus, which can only doubtfully be placed in the Ascomycetes (vide supra).

Carpoascomycetes.—The other divisions of the Ascomycetes may be distinguished as Carpoascomycetes because they do not bear the asci free on the mycelium but enclosed in definite fruit bodies or ascocarps. The ascocarps can be distinguished into two portions, a mass of sterile or vegetative hyphae forming the main mass of the fruit body, and surrounding the fertile ascogenous hyphae which bear at their ends the asci. When the ascogonium (female organ) is present the ascogenous hyphae arise from it, with or without its previous fusion with an antheridium. In other cases the ascogenous hyphae arise directly from the vegetative hyphae. In connexion with this condition of reduction a fusion of nuclei has been observed inHumaria rutilansand is probably of frequent occurrence. The asci may be derived from the terminal cell of the branches of the ascogenous hyphae, but usually they are derived from the penultimate cell, the tip curving over to form the so-called crozier. By this means the ascus cell is brought uppermost, and after the fusion of the two nuclei it develops enormously and produces the ascospores. The ascospores escape from the asci in various ways, sometimes by a special ejaculation-mechanism. The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations. The ordinary mycelium is the gametophyte since it bears the ascogonia and antheridia when present; the ascogenous hyphae with their asci represent the sporophyte since they are derived from the fertilized ascogonium. The matter is complicated by the apogamous transition from gametophyte to sporophyte in the absence of the ascogonium; also by the fact that there are normally two fusions in the life-history as mentioned earlier. If there are two fusions one would expect two reductions, and Harper has suggested that the division of the nuclei into eight in the ascus, instead of into four spores as in most reduction processes, is associated with adoublereduction process in the ascus. Miss Fraser inHumaria rutilansfinds two reductions: a normal synaptic reduction in the first nuclear division of the ascus, and a peculiar reduction division termedbrachymeiosisin the third ascus division.

Various types of ascocarp are characteristic of the different divisions of the Carpoascomycetes: the cleistothecium, apothecium and perithecium.

Perisporineae.—This includes two chief families, Erysiphaceae and Perisporiaceae. They are characterized by an ascocarp without any opening to the exterior, the ascospores being set free by the decay or rupture of the ascocarp wall; such a fruit-body is termed acleistothecium(cleistocarp). The Erysiphaceae are a sharply marked group of forms which live as parasites. They form a superficial mycelium on the surface of the plant, the hyphae not usually penetrating the tissues but merely sending haustoria into the epidermal cells. Only in rare cases is the mycelium intercellular. Owing to their appearance they go by the popular name of mildews.Sphaerotheca Humuliis the well known hop-mildew,Sphaerotheca Mors-Uvaeis the gooseberry mildew, the recent advent of which has led to special legislation in Great Britain to prevent its spreading, as when rampant it makes the culture of gooseberries impossible.Erysiphe,UncinulaandPhyllactiniaare other well-known genera. The form of the fruit body, the difference and the nature of special outgrowths upon it—the appendages—are characteristic of the various genera. Besides peritheca the members of the Erysiphaceae possess conidia borne in simple chains. De Bary brought forward very strong evidence for the origin of the ascocarp inSphaerothecaandErysipheby a sexual process, but Harper in 1895 was the first to prove conclusively, by the observation of the nuclear fusion, that there was a definite fertilization inSphaerotheca Humuliby the fusion of a male (antheridial) nucleus with a female, ascogonial (oogonial) nucleus. Since then Harper has shown that the same process occurs inErysipheandPhyllactinia.

A, Small portion of mycelium with conidiophore (c), and archicarp (as).

B, The spiral archicarp (as), with the antheridium (p).

D, The same, beginning to be surrounded by the hyphae forming the perithecium wall.

D, The perithecium.

E, F, Sections of young perithecia.

w, Parietal cells.

f, Pseudo-parenchyma.

as, Ascogonium.

G, An ascus.

H, An ascospore.

The Perisporiaceae are saprophytic forms, the two chief genera beingAspergillusandPenicillium. The blue-green mouldP. crustaceumand the green mouldA. herbariorium(=Eurotium herbariorum) are extraordinarily widely distributed, moulds being found on almost any food-material which is exposed to the air. They have characteristic conidiophores bearing numerous conidia, and also cleistothecia which are spherical in form and yellowish in colour. The latter arise from the crown of a spirally coiled archicarp (bearing an ascogonium at its end) and a straight antheridium. Vegetative hyphae then grow up and surround these and enclose them in a continuous sheath of plectenchyma (fig. 11). It has lately been shown by Fraser and Chambers that inEurotiumboth ascogonium and antheridium contain a number of nuclei (i.e.are coenogametes), but that the antheridium disorganizes without passing its contents into the ascogonium. There is apparently a reduced sexual process by the fusion of the ascogonial (female) nuclei in pairs.Aspergillus Oryzaeplays an important part in saccharifying the starch of rice, maize, &c., by means of the abundant diastase it secretes, and, in symbiosis with a yeast which ferments the sugar formed, has long been used by the Japanese for the preparation of the alcoholic liquor saké. The process has now been successfully introduced into European commerce.

m, Mycelium.

c, Archicarp.

l, Pollinodium.

s, Ascogenous filaments.

a, Asri.

r,p, The sterile tissue from which the paraphyseshspring.

Discomycetes.—Used in its widest sense this includes the Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is characterized in general by the possession of an ascocarp which, though usually a completely closed structure during the earlier stages of development, at maturity opens out to form a bowl or saucer-shaped organ, thus completely exposing the layer of asci which forms the hymenium. Such an ascocarp goes by the name ofapothecium. Owing to the shape of the fruit-body many of these forms are known as cup-fungi, the cup or apothecium often attaining a large size, sometimes several inches across (fig. 12). Functional male and female organs have been shown to exist inPyronemaandBoudiera; inLachnea stercoreaboth ascogonia and antheridia are present, but the antheridium is non-functional, the ascogonial (female) nuclei fusing in pairs; this is also the case inHumaria granulataandAscobolus furfuraceus, where the antheridium is entirely absent. InH. rutilans, however, both sexual organs are absent and the ascogenous hyphae arise apogamously from the ordinary hyphae of the mycelim. In all these cases the ascogonium and antheridium contain numerous nuclei; they are to be looked upon as gametangia in which there is no differentiation of gametes, and since they act as single gametes they are termed coenogametes. In some forms as inAscobolusthe ascogonium is multicellular, the various cells communicating by pores in the transverse walls (fig. 13).

s, Asci.

a, Paraphyses.

e, Periphyses.

m, Mycelial hyphae.

In the Helvellaceae there is no apothecium but a large irregular fruit body which at maturity bears the asci on its surface. The development is only slightly known, but there is some evidence for believing that the fruit-body is closed in its very early stages.

The genusPeziza(in its widest sense) may be taken as the type of the group. Most of them grow on living plants or on dead vegetable remains, very often on fallen wood; a number, however, are found growing on earth which is rich in humus. The genusSclerotiniamay be mentioned here; a number of forms have been investigated by Woronin. The conidia are fragrant and are carried by bees to the stigma of the bilberry; here they germinate with the pollen and the hyphae pass with the pollen tubes down the style; the former infect the ovules and produce sclerotia, therein reducing the fruits to a mummified condition. From the sclerotia later the apothecium develops. One species,S. heteroica, isheteroecious; the ascospores infecting the leaves ofVaccinium uliginosum, while the conidia which then arise infect onlyLedum palustre. This is the only case of heteroecism known in the vegetable kingdom outside the Uredineae.

Pyrenomycetes.—This is an extraordinarily large and varied group of forms which mostly live parasitically or saprophytically on vegetable tissue, but a few are parasitic on insect-larvae. The groupis characterized by a special type of ascocarp, theperithecium. This is typically of a flask-shaped form opening with a small pore at the top. The asci live at the bottom often mixed with paraphyses, while the upper “neck” of the flask is lined with special hyphae, the periphyses, which aid in the ejection of the spores (fig. 14). The simpler forms bear the perithecia directly on the mycelium, but the more highly developed forms often bear them on a special mycelial development—the stroma, which is often of large size and special shape and colour, and of dense consistence. The cytological details of development of the perithecia are not well known; most of them appear to develop their ascogenous hyphae in an apogamous way without any connexion with an ascogonium. Besides the special ascocarps, accessory reproductive organs are known in the majority of cases in the form of conidia.

Tuberineae.—These are a small group of fungi including the well-known truffles. They are found living saprophytically (in part parasitically) underground in forests. The asci are developed in the large dense fruit bodies (cleistothecia) and the spores escape by the decay of the wall. The fruit-body is of complicated structure, but its early stages of development are not known. Many of the fruit-bodies have a pleasant flavour and are eaten under the name of truffles (Tuber brumaleand other species). The exact life-history of the truffle is not known.

A, Young basidium with the two primary nuclei.

B, After fusion of the two nuclei.Hypholoma appendiculatum.

C, A basidium before the four nuclei derived from the secondary nucleus of the basidium have passed into the four basidiospores.

D, Passage of a nucleus through the sterigma into the basidiospore.

Laboulbeniineaeare a group of about 150 species of fungi found on insects, especially beetles, and principally known from the researches of Thaxter in America. The plant is a small, dark brown, erect structure (receptacle) of a few cells, and 1-10 mm. high, attached to the insect by the lowermost end (foot), and easily mistaken for a hair or similar appendage of the insect. The receptacle ends above in appendages, each consisting of one or a few cells, some of which are the male organs, others the female organs, and others again may be barren hairs. The male organ (antheridium) consists of a few cells, the terminal one of which either abstricts from its end, or emits from its interior the non-motile spermatia, reminding us of those of the Florideae. The female organ is essentially a flask-shaped structure; the neck of the flask growing out as the trichogyne, and the belly composed of an axial carpogenic cell surrounded by investing cells, and with one cell (trichophoric) between it and the trichogyne. These three elements—trichogyne, trichophoric cell, and carpogenic cell—are regarded as the procarp. The spermatia have been shown by Thaxter to fuse with the trichogyne, after which the axial cell below (carpogenic cell) undergoes divisions, and ultimately forms asci containing ascospores, while cells investing this form a perithecium, the whole structure reminding us essentially of the fructification of a Pyrenomycete. Many modifications in details occur, and the plants may be dioecious. No injury is done to the infested insects. It has lately been shown that there is a fusion of nuclei in connexion with ascus formation, so that there can be no doubt of the position of this extraordinary group of plants among the Ascomycetes. The various cells of these organisms are connected by large pits which are traversed by thick protoplasmic threads connecting one cell with the next. In this point and in their method of fertilization the Laboulbeniineae suggest a possible relationship of Ascomycetes and the Red Algae.

Basidiales.—This very large group of plants is characterized by the possession of a special type of conidiophore—the basidium, which gives its name to the group. The basidium is a unicellular or multicellular structure from which four basidiospores arise as outgrowths; it starts as a binucleate structure, but soon, like the ascus, becomes uninucleate by the fusion of the two nuclei. Then two successive nuclear divisions occur resulting in the formation of four nuclei which later migrate respectively into the four basidiospores (fig. 15). The Basidiales are further characterized by the complete loss of normal sexuality, but at some time or other in the life-history there takes place an association of two nuclei in a cell; the two nuclei are derived from separate cells or possibly in some cases are sister nuclei of the same cell. The two nuclei when once associated are termed “conjugate” nuclei, and they always divide at the same time, a half of each passing into each cell. This conjugate condition is finally brought to a close by the nuclear fusion in the basidium. Between the nuclear association and the nuclear fusion in the basidium many thousands of cell generations may be intercalated. This nuclear association of equivalent nuclei apparently represents a reduced sexual process (like the fusion of female nuclei inHumaria granulataand of vegetative nuclei inH. rutilans, among the Ascomycetes) in which, however, the actual fusion (normally, in a sexual process, occurring immediately after association) is delayed until the formation of the basidium. During the tetrad division in the basidium nuclear reduction occurs. There is thus in all the Basidiales an alternation of generations, obscured, however, by the apogamous transition from the gametophyte to sporophyte. The sporophyte may be considered to begin at the stage of nuclear association and end with the nuclear reduction in the basidium.

A, Mass of teleutospores (t) on a leaf of couch-grass.

e, Epidermis ruptured.

b, Sub-epidermal fibres. (After De Bary.)

B, Part of vertical section through leaf of Berberis vulgaris, witha, aecidium fruits,p, peridium, andsp, spermogonia. (After Sachs.)

C, Mass of uredospores (ur), with one teleutospore (t).

sh, Sub-hymenial hyphae. (After De Bary.)

Uredineae.—This is a large group of about 2000 forms. They are all intercellular parasites living mostly on the leaves of higher plants. Owing to the presence of oily globules of an orange-yellow or rusty-red colour in their hyphae and spores they are termed Rust-Fungi. They are distinguished from the other fungi and the rest of the Basidiales by the great variety of the spores and the great elaboration of the life-history to be found in many cases. Five different kinds of spores may be present—teleutospores, sporidia (= basidiospores), aecidiospores, spermatia and uredospores (fig. 16). The teleutospore, with the sporidia which arise from it, is always present, and the division into genera is based chiefly on its characters. The teleutospore puts forth on germination a four-celled structure, the promycelium or basidium, and this bears later four sporidia or basidiospores, one on each cell. When the sporidia infect a plant the mycelium so produced gives origin to aecidiospores and spermatia; the aecidiospores on infection produce a mycelium which bears uredospores and later teleutospores. This is the life-history of the most complicated forms, of the so-calledeuforms. In theopsisforms the uredospores are absent, the mycelium from the aecidiospores producing directly the teleutospores. Inbrachyandhemithe aecidiospores are absent, the mycelium from the sporidia giving origin directly to the uredospores; the former possess spermatia, in the latter they are absent. Inleptoandmicroforms both aecidiospores and uredospores are absent, the sporidia producing a mycelium which gives rise directly to teleutospores; in theleptoforms the teleutospores can germinate directly, in themicroforms only after a period of rest. We have thus a series showing a progressive reduction in the complexity of the life-history, theleptoandmicroforms having a life-history like that of the Basidiomycetes. Theeuandopsisforms may exhibit the remarkable phenomenon of heteroecism,i.e.the dependence of the fungus on two distinct host-plants for the completion of the life-history. Heteroecism is very common in this group and is now known in over one hundred and fifty species. In all cases of heteroecism the sporidia infect one host leading to the production of aecidiospores and spermatia (if present), while the aecidiospores are only able to infect anotherhost on which the uredospores (if present) and the teleutospores are developed. A few examples are appended:

Some of the Uredineae also exhibit the peculiarity of the development of biologic forms within a single morphological species, sometimes termed specialization of parasitism; this will be dealt with later under the section Physiology.

A, Portion of a young aecidium.

st, Sterile cell.

a, Fertile cells; ata2the passage of a nucleus from the adjoining cell is seen.

B, Formation of the first spore-mother-cell (sm), from the basal cell (a) of one of the rows of spores.

C, A further stage in which from sm1the first aecidiospore (a) and the intercalary cell (z) have arisen.

sm2, The second spore-mother-cell.

D, Ripe aecidiospore

Cytology of Uredineae.—The study of the nuclear behaviour of the cells of the Uredineae has thrown great light on the question of sexuality. This group like the rest of the Basidiales exhibits an association of nuclei at some point in its life-history, but unlike the case of the Basidiomycetes the point of association in the Uredineae is very well defined in all those forms which possess aecidiospores. We find thus that in theeuandopsisforms the association of nuclei takes place at the base of the aecidium which produces the aecidiospores. There we find an association of nuclei either by the fusion of two similar cells as described by Christmann or by the migration of the nucleus of a vegetative cell into a special cell of the aecidium. After this association the nuclei continue in the conjugate condition so that the aecidiospores, the uredospore-bearing mycelium, the uredospores and the young teleutospores all contain two paired nuclei in their cells (fig. 17). Before the teleutospore reaches maturity the nuclei fuse, and the uninucleate condition then continues again until aecidium formation. In thehemi,brachy,microandleptoforms, which possess no aecidium, we find that the association takes place at various points in the ordinary mycelium but always before the formation of the uredospores in thehemiandbrachyforms, and before the formation of teleutospores inmicroandleptoform. Whether the association of nuclei in the ordinary mycelium takes place by the migration of a nucleus from one cell to another or whether two daughter nuclei become conjugate in one cell, is not yet clear. The most reasonable interpretation of the spermatia is that they are abortive male cells. They have never been found to cause infection, and they have not the characters of conidia; the large size of their nuclei, the reduction of their cytoplasm and the absence of reserve material and their thin cell wall all point to their being male gametes. Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte. The subjoined diagram will indicate the relationship of the forms.

Basidiomycetes.—This group is characterized by its greatly reduced life-history as compared with that of theeuforms among the Uredineae. All the forms have the same life-history as theleptoforms of that group, so that there is no longer any trace of sexual organs. There is also a further reduction in that the basidium is not derived from a teleutospore but is borne directly on the mycelium. Formerly, before the relationship of promycelium and basidium were understood, the Uredineae were considered as quite independent of the Basidiomycetes. Later, however, these Uredineae were placed as a mere subdivision of the Basidiomycetes. Although the Uredineae clearly lead on to the Basidiomycetes, yet owing to their retaining in many cases definite traces of sexual organs they are clearly a more primitive group. Their marked parasitic habit also separates them off, so that they are best included with the Basidiomycetes in a larger cohort which may be called Basidiales. Most of Basidiomycetes are characterized by the large sporophore on which the basidia with its basidiospores are borne.

It must be clearly borne in mind that though the Basidiomycetes show no traces of differentiated sexual organs yet, like themicroandleptoforms of the Uredineae, they still show (in the association of nuclei and later fusion of nuclei in the basidium), a reduced fertilization which denotes their derivation, through the Uredineae, from more typically sexual forms. No one has yet made out in any form the exact way in which the association of nuclei takes place in the group. The mycelium is always found to contain conjugate nuclei before the formation of basidia, but the point at which the conjugate condition arises seems very variable. Miss Nichols finds that it occurs very soon after the germination of the spore inCoprinus, but no fusion of cells or migration of nuclei was to be observed.

A, The young plant.

B, The mature plant.

C, Longitudinal section of mature plant.

p, Thepileus.

g, The gills.

a, Theannulus, or remnant ofvelum partiale,

v, Remains ofvolvaorvelum universale.

s, The stalk.

Protobasidiomycetes.—This, by far the smaller division of Basidiomycetes, includes those forms which have a septate basidium. There are three families—Auriculariaceae, Pilacreaceae and Tremellinaceae. The first named contains a small number of forms with the basidium divided like the promycelium of the Uredineae. They are characterized by their gelatinous consistence and large size of their sporophore.Hirneola(Auricularia)Auricula-Judaeis the well-known Jew’s Ear, so named from the resemblance of the sporophore to a human ear.

The Pilacreaceae are a family found by Brefeld to contain the genusPilacre.P. Petersiihas a transversely divided basidium as inAuriculariaceae, but the basidia are surrounded with a peridium-like sheath. TheTremellinaceaeare characterized by the possession of basidia which are divided by twoverticalwalls at right angles to one another. From each of the four segments in the case ofTremellaa long outgrowth arises which reaches to the surface of the hymeniumand bears the basidiospores. InDacryomycesonly two outgrowths and two spores are produced.

Autobasidiomycetes.—In this by far the larger division of the Basidiomycetes the basidia are undivided and the four basidiospores are borne on short sterigmata nearly always at the apex of the basidium. The group may be divided into two main divisions,HymenomycetesandGasteromycetes.

Hymenomycetesare 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 melleusandPolyporus annosusare examples of parasitic forms which attack and destroy living trees, whileMerulius lacrymansis the well-known “dry rot” fungus.

Gasteromycetesare 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 outerperidiumand an inner spore-bearing mass, thegleba. 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 areBovista, 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 thePhalloideae, 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.

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