Compound sporophores arise when any of the branched or unbranched types of spore-bearing hyphae described above ascend into the air in consort, and are more or less crowded into definite layers, cushions, columns or other complex masses. The same laws apply to the individual hyphae and their branches as to simple sporophores, and as long as the conidia, sporangia, gametes, &c., are borne on their external surfaces, it is quite consistent to speak of these as compound sporophores, &c., in the sense described, however complex they may become. Among the simplest cases are the sheet-like aggregates of sporogenous hyphae in Puccinia, Uromyces, &c., or of basidia in Exobasidium, Corticium, &c., or of asci in Exoascus, Ascocorticium, &c. In the former, where the layer is small, it is often termed a sorus, but where, as in the latter, the sporogenous layer is extensive, and spread out more or less sheet-like on the supporting tissues, it is more frequently termed a hymenium. Another simple case is that of the columnar aggregates of sporogenous hyphae in forms like Stilbum, Coremium, &c. These lead FIG. 2. - Peronospora parasilica (De Bary). Conidiophore with conidia.
us to cases where the main mass of the sporophore forms a supporting tissue of closely crowded or interwoven hyphae, the sporogenous terminal parts of the hyphae being found at the periphery or apical regions only. Here we have the cushion-like type (stroma) of Nectria and many Pyrenomycetes, the clavate "receptacle" of Clavaria, &c., passing into the complex forms met with in Sparassis, Xylaria, Polyporei, and Agaricini, &c. In these cases the compound sporophore is often termed the hymenophore, and its various parts demand special names (pileus, stipes, gills, po--es, &c.) to denote peculiarities of distribution of the hymenium owlthe surface.
Other series of modifications arise in which the tissues corresponding to the stroma invest the sporogenous hyphal ends, and thus enclose the spores, asci, basidia, &c., in a cavity. In the simplest case the stroma, after bearing its crop of conidia or oidia, develops ascogenous branches in the loosened meshes of its interior (e.g. Onygena). Another simple case is where the plane or slightly convex surface of the stroma rises at its margins and overgrows the sporogenous hyphal ends, so that the spores, asci, &c., come to lie in the depression of a cavity - e.g. Solenia, Cyphella - and even simpler cases are met with in Mortierella, where the zygospore is invested by the overgrowth of a dense mat of closely branching hyphae, and in Gymnoascus, where a loose mat of similarly barren hyphae covers in the tufts of asci as they develop.
The recent observations and exceedingly ingenious experiments of Falck have shown that the sporophores of the Basidiomycetesespecially the large sporophores of such forms as Boletus, Polyporus- contain quantities of reserve combustible material which are burnt up by the active metabolism occurring when the fruit-body is ripe. By this means the temperature of the sporophore is raised and the difference between it and the surrounding air may be one of several degrees. As a result convection currents are produced in the air which are sufficient to catch the basidiospores in their fall and carry them, away from the regions of comparative atmospheric stillness near the ground, to the upper air where more powerful air-currents can bring about their wide distribution.
Classification
It has been accepted for some time now that the majority of the fungi proper fall into three main groups, the Phycomycetes, Ascomycetes and Basidiomycetes, the Schizomycetes and Myxomycetes (Mycetozoa) being considered as independent groups not coming under the true fungi.
The chief schemes of classification put forward in detail have been those of P. A. Saccardo (1882-1892), of Oskar Brefeld and Von Tavel (1892), of P. E. L. Van Tieghem (1893) and of J. Schroeter (1892). The scheme of Brefeld, which was based on the view that the Ascomycetes and Basidiomycetes were completely asexual and that these two groups had been derived from one division (Zygomycetes) of the Phycomycetes, has been very widely accepted. The recent work of the last twelve years has shown, however, that the two higher groups of fungi exhibit distinct sexuality, of either a normal or reduced type, and has also rendered very doubtful the view of the origin of these two groups from the Phycomycetes. The real difficulty of classification of the fungi lies in the polyphyletic nature of the group. There is very little doubt that the primitive fungi have been derived by degradation from the lower algae. It appears,. however, that such a degradation has occurred not only once in evolution but on several occasions, so that we have in the Phycomycetes not a series of naturally related forms, but groups. which have arisen perfectly independently of one another from various groups of the algae. It is also possible in the absence of satisfactory intermediate forms that the Ascomycetes and Basidiomycetes have also been derived from the algae independently of the Phycomycetes, and perhaps of one another.
A natural classification on these lines would obviously be very complicated, so that in the present state of our knowledge it will be best to retain the three main groups mentioned above,. bearing in mind that the Phycomycetes especially are far from being a natural group. The following gives a tabular survey of the scheme adopted in the present article: 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 I I. - 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, Disco mycetes, 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 wellmarked 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. - 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 ioo 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 semiaquatic 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 From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 4. - Fertilization of the Peronosporeae. (After Wager, X 666.) I, Peronospora parasitica. Young tube (a) of the antheridium multinucleate oogonium (og) which introduces the male and antheridium (an). nucleus.
2, Albugo candida. Oogonium 3, The same. Fertilized egg with the central uninucleate cell (o) surrounded by the oosphere and the fertilizing periplasm (p).
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 A ?°. k FIG. 5. - Phytophthora infestans. Fungus of Potato Disease.
A, B, Section of Leaf of Potato F, G, H, J, Further development with sporangiophores of Phy- of the sporangia.
tophthora infestans passing K, Germination of the zoospores through the stomata D, on formed in the sporangia. the under surface of the leaf. L, M, N, Fertilization of the E, Sporangia. oogonium and development of the oospore in Peronospora. to a zoosporangium, but may form directly a germ tube which infects the host.
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, A planes. 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.
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 in Pilobolus, multicellular in Sporodinia. These then become surrounded by a cell-wall and form the spores. This mode of sporeformation 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 of Mucor, 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 the same mycelium and so can be produced by the growth from a single spore; this group includes Spordinia grandis, Spinellus fusiger, some species of Mucor, &c. The majority of forms, however, fall into the heterothallic group, in which the association of branches from two mycelia different in I nature is necessary for the 2, formation of zygospores.
These structures cannot 3, then be produced from the product of a single spore nor even from the thalli derived from any two spores. The two kinds of 4, thalli Blakeslee considers to have a differentiation 5, 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 of Mucor, and investigated by Lucet and Costantin. Klebs has concluded that transpiration is the important factor in determining the formation of sporangia, while zygotedevelopment depends on totally different conditions; these results have been called in question by Falck.
The Entomophthoraceae contain three genera, Empusa, Entomophthora and Basidiobolus. The two first genera consist of forms which are parasitic on insects. Empusa Muscae causes the wellknown 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. ranarum is 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 of Pilobolus. 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, I-4 X225, 5 X circa 60, from v. Tavel, Pilze.) Two conjugating branches in contact. Septation of the conjugating cells (a) from the suspensors (b).
More advanced stage, the conjugating cells (a) are still distinct from one another; the warty thickenings of their walls have commenced to form.
Ripe zygospore (b) between the suspensors (a).
Germinating zygospore with a germtube bearing a sporangium.
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 term Eumycetes is 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: the Ustilaginales, of doubtful position, and the two very sharply marked groups Basidiales and A scomycetes. 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; in Ustilago and its allies this is transversely septate, and bears lateral conidia (sporidia); in Tilletia and its allies non-septate, and bears a terminal tuft of conidia (sporidia) (fig. 7). Brefeld regarded the promycelium as a kind of basidium, bearing lateral or terminal conidia (comparable to basidiospores), but since the number of basidiospores is not fixed, and the basidium has not yet assumed very definite morphological characters, Brefeld termed the group Hemibasidii, and regarded them as a halfway stage in the evolution of the true Basidiomycetes from Ph co Y Y mycetes, the Tilletia type leading to the true basidium (Autobasidium), the Ustilago type to the proto pm basidium, with lateral spores; but this p m view is based on very poor evidence, so that it is best to place these forms ?p ,c;,::, as a separate group, the Ustilaginales. 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.
A scomycetes. - 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 young ascus. 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 termed epiplasm and 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.
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 in Exoascus. In the first case (fig. 9) we have a true sexual process, while in the second and third cases we have a reduced sexual 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 lifehistory. 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 (like A scoidea) or true Ascomycetes (like Thelebolus). Eremascus and Dipodascus, which are often placed among the Hemiasci, possibly do not belong to the Ascomycetes series at all.
Exoascaceae are a small group of doubtful extent here used to include Exoascus, Taphrina, Ascorticium and Endomyces. The I, Oogonium (og) with the an5, Fertilized oogonium sur theridial branch (az) applied rounded by two layers of to its surface. hyphae derived from the 2, Separation of antheridium stalk-cell (st).
(an). 6, The multicellular ascogonium 3, Passage of the antheridial derived by division from the nucleus towards that of the oogonium; the terminal cell oogonium. with the two nuclei (as) 4, Union of the nuclei. gives rise to the ascus.
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. The Taphrineae, which include Exoascus and Taphrina, are important parasites - e.g. pocket-plums and witches' brooms on birches, &c., are due to their action (fig. io). Exoascus and Ascorticium present interesting parallels to Exobasidium and Corticium among the Basidiomycetes.
Saccharomycetaceae include the well-known yeasts which belong mainly to the genus Saccharomyces. They are characterized by their unicellular nature, their power of rapid budding, their capacity for fermenting various sugars, and their power of forming endogenous From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 9. - Sphaerotheca Castagnei. Fertilization and Development of the Perithecium. (After Harper.) From Vine's Students' Text Book of Botany, by permission of Swan Sonnenschein & Co.
FIG. 7. - Germinating resting-gonidia. A, of Ustilago receptaculorum; of Tilletia Caries (X 460).
sp, The gonidium.
pm, The promycelium.
d, The sporidia: in B the sporidia have coalesced in pairs at v. From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 8. - Development of the Ascus.
A - C, Pyronema confluens. (After Harper.) Young ascus of Boudiera with eight spores. (After Claussen.) D, spores. 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 of Saccharomyces are 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 - ?u growth, the limits are still far too ill-defined for complete ep botanical treatment of the genus.
A typical yeast is able to develop b 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: 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.
Schizosaccharomyces includes 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 of Spirogyra; 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. In Zygosaccharomyces described 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.
Parasatasm
Some fungi, though able to live as saprophytes, occasionally enter the body of living plants, and are thus termed facultative parasites. The occasion may be a wound (e.g. Nectria, Dasyscypha, &c.), or the enfeeblement of the tissues of the host, or invigoration of the fungus, the mycelium of which then becomes strong enough to overcome the host's resistance (Botrytis). Many fungi, however, cannot complete their life-history apart from the host-plant. Such obligate parasites may be epiphytic (Erysipheae), the mycelium remaining on the outside and at most merely sending haustoria into the epidermal cells, or endophytic (Uredineae, Ustilagineae, &c.), when the mycelium is entirely inside the organs of the host. An epiphytic fungus is not necessarily a parasite, however, as many saprophytes (moulds, &c.) germinate and develop a loose mycelium on living leaves, but only enter and destroy the tissues after the leaf has fallen; in some cases, however, these saprophytic epiphytes can do harm by intercepting light and air from the leaf (Fumago, &c.), and such cases make it difficult to draw the line between saprophytism and parasitism. Endophytic parasites may be intracellular, when the fungus or its mycelium plunges into the cells and destroys their contents directly (Olpidium, Lagenidium, Sclerotinia, &c.), but they are far more frequently intercellular, at any rate while young, the mycelium growing in the lacunae between the cells (Peronospora, Uredineae) into which it may send short (Cystopus), or long and branched (Peronospora Calotheca) haustoria, or it extends in the middle lamella (Ustilago), or even in the solid substance of the cell-wall (Botrytis). No sharp lines can be drawn, however, since many mycelia are intercellular at first and subsequently become intracellular (Ustilagineae), and the various stages doubtless depend on the degrees of resistance which the host tissues are able to offer. Similar gradations are observed in the direct effect of the parasite on the host, which may be local (Hemileia) when the mycelium never extends far from the point of infection, or general (Phytophthora) when it runs throughout the plant. Destructive parasites rapidly ruin the whole plant-body (Pythium), whereas restrained parasites only tax the host slightly, and ill effects may not be visible for a long time, or only when the fungus is epidemic (Rhytisma). A parasite may be restricted during a long incubation-period, however, and rampant and destructive later (Ustilago). The latter fact, as well as the extraordinary fastidiousness, so to speak, of parasites in their choice of hosts or of organs for attack, point to reactions on the part of the host-plant, as well as capacities on that of the parasite, which may be partly explained in the light of what we 'now know regarding enzymes and chemotropism. Some parasites attack many hosts and almost any tissue or organ (Botrytis cinerea), others are restricted to one family (Cystopus candidus) or genus (Phytophthora infestans) or even species (Pucciniastrum Padi), and it is customary to speak of rootparasites, leaf-parasites, &c., in expression of the fact that a given parasite occurs only on such organs - e.g. Dematophora necatrix on roots, Calyptospora Goeppertiana on stems, Ustilago Scabiosae in anthers, Claviceps purpurea in ovaries, &c. Associated with these relations are the specializations which parasites show in regard to the age of the host. Many parasites can enter a seedling, but are unable to attack the same host when older - e.g. Pythium, Phytophthora omnivores. Chemotropism. - Taken in conjunction with Pfeffer's beautiful discovery that certain chemicals exert a distinct attractive influence on fungus hyphae (chemotropism), and the results of Miyoshi's experimental application of it, the phenomena of enzyme-secretion throw considerable light on the processes of infection and parasitism of fungi. Pfeffer showed that certain substances in definite concentrations cause the tips of hyphae to turn towards them; other substances, though not innutritious, repel them, as also do nutritious bodies if too highly concentrated. Marshall Ward showed that the hyphae of Botrytis pierce the cell-walls of a lily by secreting a cytase and dissolving a hole through the membrane. Miyoshi then demonstrated that if Botrytis is sown in a lamella of gelatine, and this lamella is superposed on another similar one to which a chemotropic substance is added, the tips of the hyphae at once turn from the former and enter the latter. If a thin cellulose membrane is interposed between the lamellae, the hyphae nevertheless turn chemotropically from the one lamella to the other and pierce the cellulose membrane in the process. The hyphae will also dissolve their way through a lamella of collodion, paraffin, parchment paper, elder-pith, or even cork or the wing of a fly, to do which it must excrete very different enzymes. If the membrane is of some impermeable substance, like gold leaf, the hyphae cannot dissolve its way through, but the tip finds the most minute pore and traverses the barrier by means of it, as it does a stoma on a leaf, We may hence conclude that a parasitic hyphae pierces some plants or their stomata and refuses to enter others, because in the former case there are chemotropically attractive substances present which are absent from the latter, or are there replaced by repellent poisonous or protective substances such as enzymes or antitoxins.
Symbiosis
The remarkable case of life in common first observed in lichens, where a fungus and an alga unite to form a compound organism - the lichen - totally different from either, has now been proved to be universal in these plants, and lichens are in all cases merely algae enmeshed in the interwoven hyphae of fungi (see Lichens). This dualism, where the one constituent (alga) furnishes carbohydrates, and the other (fungus) ensures a supply of mineral matters, shade and moisture, has been termed symbiosis. Since then numerous other cases of symbiosis have been demonstrated. Many trees are found to have their smaller roots invaded by fungi and deformed by their action, but so far from these being injurious, experiments go to show that this mycorhiza (fungus-root) is necessary for the well-being of the tree. This is also the case with numerous other plants of moors and woodlands - e.g. Ericaceae, Pyrolaceae, Gentianaceae, Orchidaceae, ferns, &c. Recent experiments have shown that the difficulties of getting orchid seeds to germinate are due to the absence of the necessary fungus, which must be in readiness to infect the young seedling immediately it emerges from the seed. The well-known failures with rhododendrons, heaths, &c., in ordinary garden soils are also explained by the need of the fungus-infected. peat for their roots. The role of the fungus appears to be to supply materials from the leaf-mould around, in forms which ordinary root-hairs are incapable of providing for the plant; in return the latter supports the fungus at slight expense from its abundant stores of reserve materials. Numerous other cases of symbiosis have been discovered among the fungi of fermentation, of which those between Aspergillus and yeast in sake manufacture, and between yeasts and bacteria in kephir and in the ginger-beer plant are best worked out. For cases of symbiosis see Bacteriology.
Authorities. - General: Engler and Prantl, Die natiirlichen Pflanzenfamilien, i. Teil (1892 onwards); Zopf, Die Pilze (Breslau, 1890); De Bary, Comparative Morphology of Fungi, &c. (Oxford, 1887); von Tafel, Vergleichende Morphologie der Pilze (Jena, 1892); Brefeld, Ureters. aus dem Gesamtgebiete der Mykologie, Heft i. 13 (1872-1905); Lotsy, Vortrdge fiber botanische Stammesgeschichte (Jena, 1907). Distribution, &c.: Cooke, Introduction to the Study of Fungi (London, 1895); Felix in Zeitschr. d. deutsch. geologisch. Gesellsch. (1894-1896); Staub, Sitzungsber. d. bot. Sec. d. Kgl. ungarischen naturwiss. Gesellsch. zu Budapest (1897). Anatomy, &c.: Bommer, "Sclerotes et cordons myceliens," Mem. de l'Acad. Roy. de Belg. (1894); Mangin, "Observ. sur la membrane des mucorinees," Journ. de Bot. (1899); Zimmermann, Die Morph. and Physiologie des Pflanzenzellkernes (Jena, 1896); Wisselingh, "Microchem. Unters. uber die Zellwande d. Fungi," Pringsh. Jahrb. B. 31, p. 619 (1898); Istvanffvi, "Unters. uber die phys. Anat. der Pilze," Prings. Jahrb. (1896). Spore Distribution: Fulton, "Dispersal of the Spores of Fungi by Insects," Ann. Bot. (1889); Falck, "Die Sporenverbreitung bei den Basidiomyceten," Beitr. zur Biol. d. Pflanzen, ix. (1904). Spores and Sporophores: Zopf, Die Pilze; also the works of von Tafel and Brefeld. Classification: van Tieghem, Journ. de bot. p. 77 (1893), and the works of Brefeld, Engler and Prantl, von Tafel, Saccardo and Lotsy already cited. Oomycetes : Wager, "On the Fertilization of Peronospora parasi.tica," Ann. Bot. vol. xiv. (1900); Stevens, "The Compound Oosphere of Albugo Bliti," Bot. Gaz. vol. 28 (1899); "Gametogenesis and Fertilization in Albugo," ibid. vol. 32 (1901); Miyake, "The Fertilization of Pythium de Baryanum," Ann. of Bot. vol. xv. (1901); Trow, "On Fertilization in the Saprolegnieae," Ann. of Bot. vol. xviii. (1904); Thaxter, "New and Peculiar Aquatic Fungi," Bot. Gaz. vol. 20 (1895); Lagerheim, "Unters. fiber die Monoblepharideae," Bih. Svenska Vet. Akad. Handlingar, 25. Afd. iii. (1900); Woronin, "Beitrag zur Kenntnis der Monoblepharideen," Mem. de l'Acad. Imp. d. Sc. de St-Petersbourg, 8 ser. vol. 16 (1902). Zygomycetes: Harper, "Cell-division in Sporangia and Asci," Ann. Bot. vol. xiii. (1899); Klebs, Die Bedingungen der Fortpflanzung, &c. (Jena, 1896), and "Zur Physiologie der Fortpflanzung" Prings. Jahr. (1898 and 1899), "Ober Sporodinia grandis," Bot. Zeit. (1902); Falck, "Die Bedingungen der Zygotenbildung bei Sporodinia grandis," Cohn's Beitr. z. Biol. d. Pflanzen, Bd. 8 (1902); Gruber "Verhalten der Zellkerne in den Zygosporen von Sporodinia grandis," Ber. d. deutschen bot. Ges. Bd. 19 (1901); Blakeslee, "Sexual Reproduction in the Mucorineae," Proc. Am. Acad. (1904); "Zygospore germination in the Mucorineae," Annales mycologici (1906). Ustilagineae: Plowright, British Uredineae and Ustilagineae (London, 1889); Massee, British Fungi (Phycomycetes and Ustilagineae) (London, 1891); Brefeld, Unters. aus dem Gesamtgeb. der Mykol. Hefte xi. and xii.; and Falck, "Die Bluteninfektion bei den Brandpilzen," ibid. Heft xiii. 1905; Dangeard, "La Reproduction sexuelle des Ustilaginees," C.R., Oct. 9, 1893 Maire, "Recherches cytologiques et taxonomiques sur les Basidiomyceten," Annexe au Bull. de la Soc. Mycol. de France (1902). Saccharomycetaceae: Jorgensen, The Micro-organisms of Fermentation (1899); Barker, Ann. of Bot. vol. xiv. (1901); "On Sporeformation among the Saccharomycetes," Journ. of the Fed. Institute of Brewing, vol. 8 (1902); Guillermond, Recherches cytologiques sur les levures (Paris, 1902); Hansen, Centralbl. f. Bakt. u. Parasitenp. Abt. ii. Bd. 12 (1904). Exoascaceae: Giesenhagen, "Taphrina, Exoascus, Magnusiella" (complete literature given), Bot. Zeit. Bd. 7 (1901). Erysiphaceae: Harper, "Die Entwicklung des Perithecium bei Sphaerotheca castagnei," Ber. d. deut. bot. Ges. (1896); "Sexual Reproduction and the Organization of the Nucleus in certain Mildews," Publ. Carnegie Institution (Washington, 1906); Blackman & Fraser, "Fertilization in Sphaerotheca," Ann. of Bot. (1905). Perisporiaceae: Brefeld, Untersuchungen aus dem Gesamtgeb. der Mykol. Heft pp (1891); Fraser and Chamber, Annales mycologici (1907). Discomycetes: Harper, "- fiber das Verhalten der Kerne bei Ascomyceten," Jahr. f. wiss. Bot. Bd. 29 (1890); "Sexual Reproduction in Pyronema confluens," Ann. of Bot. 14 (1900); Claussen, "Zur Entw. der Ascomyceten," Boudiera, Bot. Zeit. Bd. 63 (1905); Dangeard, "Sur le Pyronema confluens," Le Botaniste, 9 serie (1903) (and numerous papers in same journal earlier and later); Ramlow, "Zur Entwick. von Thelebolus stercoren," Bot. Zeit. (1906); Woronin, "Ober die Sclerotienkrankheit der Vaccineen Beeren," Mem. de l'Acad. Imp. des Sciences de St-Petersbourg, 7 serie, 36 (1888); Dittrich, "Zur Entwickelungsgeschichte der Helvellineen," Cohn's Beitr. z. Biol. d. Pflanzen (1892). Pyrenomycetes: Fisch, "Beitr. z. Entwickelungsgeschichte einiger Ascomyceten," Bot. Zeit. (1882); Frank, "tTber einige neue u. weniger bekannte Pflanzkrankh.," Landw. Jahrb. Bd. 12 (1883); Ward, "Onygena equina, a horn-destroying fungus," Phil. Trans. vol. 191 (1899); Dawson, "On the Biology of Poroniapunctata," Ann. of Bot. 14 (1900). Tuberineae: Buchholtz, "Zur Morphologie u. Systematik der Fungi hypogaei," Ann. Mycol. Bd. i (1903); Fischer in Engler and Prantl, Die natiirlichen Pflanzenfamilien (1896). Laboulbeniineae: Thaxter, "Monograph of the Laboulbeniaceae," Mem. Amer. Acad. of Arts and Sciences, vol. 12 (1895). Uredineae: Eriksson and Henning, Die Getreideroste (Stockholm, 1896); Eriksson, Botan. Gaz. vol. 25 (1896); "On the Vegetative Life of some Uredineae," Ann. of Bot. (1905); Klebahn, Die wirtwechselnden Rostpilze (Berlin, 1904); Sapin-Trouffy, "Recherches histologiques sur la famine des Uredinees," Le Botaniste (1896-1897); Blackman, "On the Fertilization, Alternation of Generations and General Cytology of the Uredineae," Ann. of Bot. vol. 18 (1904); Blackman and Fraser, "Further Studies on the Sexuality of Uredineae," Ann. of Bot. vol. 20 (1906); Christman, "Sexual Reproduction of Rusts," Ann. of Bot. vol. 20 (1906); Ward, "The Brooms and their Rust Fungus," Ann. of Bot. vol. 15 (1901). Basidiomycetes: Dangeard, "La Reprod. sexuelle des Basidiomycetes," Le Botaniste (1894 and 1900); Maire, "Recherches cytologiques et taxonomiques sur les Basidiomycetes," Annexe du Bull. de la Soc. Mycol. de France (1902); Moller, "Protobasidiomyceten," Schimper's Mitt. aus den Tropen, Heft 8 (Jena, 1895) Nichols, "The Nature and Origin of the Binucleated Cells in certain Basidiomycetes," Trans. Wisconsin Acad. of Sciences, vol. 15 (1905); Wager, "The Sexuality of the Fungi," Ann. of Bot. 13 (1899); Woronin, "Exobasidium Vaccinii," Verh. Naturf. Ges. zu Freiburg, Bd. 4 (1867). Fermentation: Buchner, "Gahrung ohne Hefezellen," Bot. Zeit. Bd. 18 (1898); Albert, Cent. f. Bakt. Bd. 17 (1901); Green, The Soluble Ferments and Fermentation (Cambridge, 1899). Parasitism: " On some Relations between Host and Parasite," Proc. Roy. Soc. vol. 47 (1890); "A Lily Disease," Ann. of Botany, vol. 2 (1888); Eriksson & Hennings, Die Getreideroste (vide supra); Ward, "On the Question of Predisposition and Immunity in Plants," Proc. Cambridge Phil. Soc. vol. I 1 (1902); also Annals of Bot. vol. 16 (1902) and vol. 19 (1905); Neger, "Beitr. z. Biol. d. Erysipheen" Flora, Bde. 88 and 90 (1901-1902); Salmon, "Cultural Experiments with ` Biologic Forms ' of the Erysiphaceae," Phil. Trans. (1904); "On Erysiphe graminis and its adaptative parasitism within the genus, Bromus," Ann. Mycol. vol. it (1904), also Ann. of Bot. vol. 19 (1905). Symbiosis: Ward, "The Ginger-Beer Plant," Phil. Trans. Roy. Soc. (1892); "Symbiosis," Ann. of Bot. 13 (1899); Shalk, "Der Sinn der Mykorrhizenbildung," Jahrb. f. wiss. Bot. Bd. 34 (1900); Bernard, "On some Different Cases of Germination," Gardener's Chronicle (1900); Pierce, Publ. Univ. California (1900). (H. M. W.; V. H. B.)
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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Fungi'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/f/fungi.html. 1910.
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