(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.

Monoblepharidaceae consists 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. Monoblepharis was 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. Monoblepharis has 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 and Oedogonium among the algae is very striking, as is also that of Myrioblepharis and Vaucheria.

Peronosporaceae are 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). Pythium is 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. Pythium is 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 in Pythium described above; in other cases the direct conidial germination is characteristic of genera—e.g. Peronospora; while others emit zoospores—e.g. Plasmopara, &c. In Cystopus (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 in Pythium. In Cystopus Bliti the 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. Between Cystopus Bliti on the one hand and Pythium de Baryanum on 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’s Lehrbuch der Botanik, by permission of Gustav Fischer.
Fig. 4.—Fertilization of the Peronosporeae. After Wager.

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

2, Albugo candida. Oogoniumwith the central uninucleateoosphere and the fertilizingtube (a) of the antheridiumwhich introduces the malenucleus.

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

Fig. 5.—Phytophthora infestans. Fungus of Potato Disease.

A, B, Section of Leaf of Potatowith sporangiophores of Phytophthorainfestans passingthrough the stomata D, onthe under surface of the leaf.

E, Sporangia.

F, G, H, J, Further developmentof the sporangia.

K, Germination of the zoosporesformed in the sporangia.

L, M, N, Fertilization of theoogonium and development ofthe oospore in Peronospora.

Saprolegniaceae are aquatic forms found growing usually on dead insects lying in water but occasionally on living fish (e.g. the salmon disease associated with Saprolegnia ferax). The chief genera are Saprolegnia, 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 in Saprolegnia is 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 a Saprolegnia-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 to Pythium and the Oomycetes. On the other hand, the uniciliate zoospores of Polyphagus have 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.

The Zygomycetes, 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 of Mucor containing numerous endospores, passing to cases where, as in Thamnidium, these are accompanied with more numerous small sporangia (sporangioles) containing few spores, and thence to Chaetocladium and Piptocephalis, 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’s Lehrbuch 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.