Part 9

The relation of the parasitic Fungus to the host-plant is mainly of two kinds. In the one case, the cell-contents are destroyed, the protoplasm is killed, and the cellular tissue becomes discoloured and dies (_Peronospora_, _Armillaria mellea_, _Polyporus_); in the other case, the parasite has an irritating effect on the cellular tissue, whereby the affected organ grows more rapidly and becomes larger than normal, producing _hypertrophy_. Such malformations are termed _Fungi-galls_ (Mycocecidia); in this manner “witches’ brooms” are produced by _Æcidium_, “pocket-plum” by _Taphrina_, and other deformities by _Exobasidium_ and _Cystopus candidus_. This hypertrophy may either be produced by a vigorous cell-multiplication, which is most frequently the case, or by the enlargement of the individual cells (_Synchytrium_, _Calyptospora_). The relation between host and Fungus among the Lichens is of a very peculiar nature, termed “_symbiosis_.”

=Vegetative Organs.= The vegetative parts of a Fungus are termed its _mycelium_.[10] This is formed of a mass of long, cylindrical, branched cells resembling threads (and hence termed _hyphæ_), which have a continued apical growth. The mycelium, in its early development, shows a well-marked difference between the two main groups of true Fungi: in the _Phycomycetes_, or Algal Fungi, the mycelium has no transverse walls, and is therefore unicellular, while in the _Mesomycetes_ and _Mycomycetes_ it is provided with dividing walls, which gradually arise during growth, in the youngest hyphæ; intercalary transverse walls may also be formed at a later period. In the hyphæ of some of the Higher Fungi (_Hymenomycetes_), connections may be formed between two contiguous cells of the same hypha, by a protuberance growing out from an upper cell just above the transverse wall, and forming a junction with the cell below. These are known as _clamp-connections_; they appear to be of use in affording communication between the two cells.

The hyphæ of Fungi, where they come in contact with one another, often grow together, so that =H=-formed combinations (fusions) are produced, which give rise to very compact felted tissue. When the hyphæ are not only closely interwoven, but also united and provided with many transverse walls, the mycelium assumes the appearance of a tissue with isodiametric cells, and is then termed _pseudo-parenchyma_. The hyphæ-walls are sometimes very much thickened, and composed of several layers, and the external layers, by the absorption of water, may often swell very much and become mucilaginous. In some instances the walls are colourless, in others coloured, the most frequent colour being brown. The cell-contents may also be coloured, and in that case are generally yellow; this colour is chiefly connected with the fat (oil) which may be found in abundance in the Fungi, whilst starch is invariably absent in all the true Fungi.

The mycelium assumes many different forms; sometimes it appears as a thread-like, cobwebby, loose tissue, less frequently as firm strands, thin or thick membranes, horn-like plates or tuber-like bodies. The _thread-like_ mycelium may, in the parasitic Fungi, be intercellular or intracellular, according as it only extends into the interstices between the cells or enters into the cells proper. In the first case there are generally found haustoria, or organs of suction (_e.g._ among the _Peronosporaceæ_; _Taphrina_, on the contrary, has no haustoria); but haustoria are also found among the epiphytic Fungi (_e.g._ Erysiphaceæ). Intracellular mycelia are found in the Rust-Fungi, in _Claviceps purpurea_, _Entomophthora_, etc. In spite of its delicate structure, this mycelium may live a long time, owing to the circumstance that it continues to grow peripherally, while the older parts gradually die off (“fairy rings”).

_String-like_ mycelia may be found, for example, in _Phallus_, _Coprinus_, and are formed of hyphæ, which run more or less parallel to each other. _Membrane-like_ mycelia are chiefly to be found in Fungi growing on tree-stems (Polyporaceæ and Agaricaceæ); they may have a thickness varying from that of the finest tissue-paper to that of thick leather, and may extend for several feet. The peculiar horny or leather-like strands and plates which, for instance, appear in _Armillaria mellea_, are known as _Rhizomorpha_; they may attain a length of more than fifty feet. The _tuber-like_ mycelia or _sclerotia_ play the part of resting mycelia, since a store of nourishment is accumulated in them, and after a period of rest they develope organs of reproduction. The sclerotia are hard, spherical, or irregular bodies, from the size of a cabbage seed to that of a hand, internally white or greyish, with a brown or black, pseudo-parenchymatous, external layer. Sclerotia only occur in the higher Fungi, and are found both in saprophytes, _e.g. Coprinus_, and in parasites, _e.g. Claviceps_ (Ergot), _Sclerotinia_.

=Reproduction.= SEXUAL REPRODUCTION is found only among the lower Fungi which stand near to the Algæ, the Algal-Fungi, and takes place by the same two methods as in the Algæ, namely by _conjugation_ and by the _fertilisation_ of the egg-cell in the oogonium.

The majority of Fungi have only ASEXUAL reproduction. The most important methods of this kind of reproduction are the _sporangio-fructification_ and the _conidio-fructification_.

In the SPORANGIO-FRUCTIFICATION the _spores_ (endospores) _arise inside_ a mother-cell, the sporangium (Fig. 80). Spores without a cell-wall, which move in water by means of cilia and hence are known as _swarmspores_ or _zoospores_, are found among the Oomycetes, the sporangia in which these are produced being called swarm-sporangia or zoosporangia (Figs. 86, 87, 91, 94).

In the CONIDIO-FRUCTIFICATION the _conidia_ (exospores) arise on special hyphæ (conidiophores), or directly from the mycelium. When conidiophores are present, the conidia are developed upon them terminally or laterally, either in a basipetal succession (in many Fungi, for example in _Penicillium_, Fig. 111, _Erysiphe_, _Cystopus_), or acropetally (in which method the chains of conidia are often branched; examples, _Pleospora vulgaris_, _Hormodendron cladosporioides_). All conidia are at first unicellular, sometimes at a later stage they become two-celled or multicellular through the formation of partition-walls (_Piptocephalis_). The conidia with thick, brown cell-walls, and contents rich in fats (_resting conidia_), can withstand unfavourable external conditions for a much longer period than conidia with thin walls and poor in contents.

The SPORANGIA arise either from the ordinary cells of the mycelium (_Protomyces_), or are borne on special hyphæ. They are generally spherical (_Mucor_, Fig. 80; Saprolegniaceæ), egg-, pear-, or club-shaped (Ascomycetes), more rarely they are cylindrical or spindle-shaped. While among the Phycomycetes the size, form, and number of spores are indefinite in each species, in the Ascomycetes the sporangia (_asci_) have a definite size, form, and number of spores. The spores of the Ascomycetes are known as ascospores.

The sporangio-fructification is found under three main forms.

1. FREE SPORANGIOPHORES which are either single (_Mucor_, Fig. 78), or branched (_Thamnidium_).

2. SPORANGIAL-LAYERS. These are produced by a number of sessile or shortly-stalked sporangia, being formed close together like a palisade (_Taphrina_, Fig. 105).

3. SPORANGIOCARPS. These consist usually of many sporangia enclosed in a covering, they are found only in the Carpoasci, and are also known as _ascocarps_. The parts of an ascocarp are the _covering_ (_peridium_), and the _hymenium_, which is in contact with the inner wall of the peridium, and is generally made up of asci, and sterile, slender hyphæ. The latter either penetrate between the asci and are branched and multicellular (_paraphyses_, Figs. 103 _d_, 123, 125, 129), or clothe those parts of the inner wall which bear no asci (_periphyses_; among many peronocarpic Ascomycetes, _e.g. Chætomium_, _Sordaria_, _Stictosphæra hoffmanni_). The ascocarps are produced directly from the mycelium, or from a _stroma_, that is a vegetative body of various forms, in which they may be embedded (Figs. 116 _B_, _C_).

Among the conidio-fructifications there are, in the same way, three divisions.

1. FREE CONIDIOPHORES (Fig. 109). The form of the conidiophores, the shape, and number of its spores are various. In the most highly developed Fungi, the Basidiomycetes, there are, however, special more highly developed conidiophores, the _basidia_, which have a definite form and spores of a definite shape and number. The conidia borne on basidia are called _basidiospores_.

2. CONIDIAL-LAYERS. (_a_) The SIMPLEST case of this is found when the conidiophores arise directly from the mycelium, parallel to one another, and form a flat body (_e.g. Exobasidium vaccinii_, _Hypochnus_; among the Phycomycetes, _Empusa muscæ_ and _Cystopus_). (_b_) In a HIGHER form the conidial-layers are thick, felted threads (_stroma_) inserted between the mycelium and the _hymenium_ (_i.e._ the region of the conidiophores). Examples are found in a section of the Pyrenomycetes (Fig. 122). (_c_) The HIGHEST form has the _basidial-layer_, that is a conidial-layer with more highly developed conidiophores (basidia). The basidial-layer, with stroma, and the hymenium (region of the basidia), forms the basidio-fructification, which is branched in the Clavariaceæ, and hat-shaped in other Hymenomycetes (in these groups the hymenium is confined to the lower side of the pileus).

The hymenium of the conidial-layer and basidial-layer is composed entirely of conidiophores, or of conidiophores and sterile hyphæ (_paraphyses_) which are probably always unicellular. Paraphyses are found in _Entomophthora radicans_, and in certain Basidiomycetes (_e.g. Corticium_).

3. CONIDIOCARPS (_pycnidia_). A special covering surrounds the conidia-forming elements. The inner side of this covering (_peridium_) bears the hymenium, _i.e._ those elements from which the conidia are abstricted. The conidiocarps arise either immediately from the hyphæ or from a _stroma_ in which they are generally embedded. Conidiocarps are entirely wanting in the Phycomycetes. On the other hand they are found among the Ascomycetes and Basidiomycetes, and in the latter group the conidiocarps contain more highly differentiated conidiophores (basidia) and are known as _basidiocarps_. Conidiocarps with simple conidiophores, are found only among the Basidiomycetes, in the Uredinaceæ, and in _Craterocolla cerasi_. In the Ascomycetes (Figs. 120 _d_, _e_; 117 _a_, _b_; 123 _a_; 124 _b_) the conidiocarps are visible, as points, to the naked eye, while the basidiocarps of the Basidiomycetes (Figs. 170, 171, 173–176, 178–180) vary from the size of a pea to that of a child’s head. The “spermogonia” of the Ascomycetes and Lichenes, are conidiocarps with small conidia (_microconidia_) which germinate sometimes more slowly than other conidia, and formerly were erroneously considered as male reproductive cells, and called spermatia.

The conidia of the Fungi are not primitive structures. The comparison of the sporangia and conidia among the Zygomycetes, and among the species of the genus _Peronospora_ shows, that the conidia are aberrant formations, and that they have arisen through the degeneration of the sporangium, which, by the reduction of its spores to one, has itself become a spore.

In the genera _Thamnidium_ and _Chætocladium_ the gradual diminution of the sporangia, and the reduction of the number of spores can be distinctly followed. In _Thamnidium_ the number of spores is often reduced to one, which is _free_ in the sporangium. In _Chætocladium_ however the sporangia are typically _one-spored_, the spore is always united with the sporangium, and the two become a single body, the so-called _conidium_, which is in reality a closed sporangium. How close is the connection between the sporangia and conidia of _Thamnidium_ and _Chætocladium_, is seen from the fact that, in the conidial stage of _Chætocladium_ the same whorl-form of branching appears as in the sporangial stages of _Thamnidium chætocladioides_, and also, that the conidia of _Ch. fresenianum_ throw off the former sporangium-wall (exosporium), while _Ch. jonesii_ germinates without shedding its exosporium. The Phycomycetes have doubtless sprung from Water-Algæ and inherit the sporangia from them. On this supposition, as the Phycomycetes assumed a terrestrial mode of life, the sporangia would become adapted to the distribution of the spores by means of the air, the sporangia would become small, contain dust-like spores, and would eventually become closed-sporangia, _i.e._ conidia. The conidia are a terrestrial method for the multiplication of Fungi. In the Hemiasci and the Ascomycetes the sporangia are still preserved, but in every instance they are adapted to terrestrial spore-distribution, their spores being set free on the destruction of the sporangium-wall (generally shot out) and distributed through the air. For further examples of spore-distribution see below, p. 91–93.

The reproduction of Fungi is accomplished not only by spores and conidia, but also sometimes by _chlamydospores_. These are fundaments[11] of sporangiophores and conidiophores, which have taken on a resting condition in the form of a spore, and are able to germinate and produce carpophores. In the formation of the chlamydospores the hyphæ accumulate reserve materials at the expense of the neighbouring cells; in the undivided hyphæ of the Phycomycetes transverse walls are formed, and finally the chlamydospores are set free by the decay of the empty cells connecting them with the mycelium. One must distinguish between _oidia_ and _true chlamydospores_. The former are more simple, the latter are a somewhat more differentiated form of carpophore fundaments, which serve for propagation in the same manner as spores. In _Chlamydomucor racemosus_ the chlamydospores grow out into the air and form differentiated carpophores. In the Autobasidiomycetes they only germinate vegetatively, and not with the formation of fructifications. From _Chlamydomucor_ up to the Autobasidiomycetes the successive development of the fructification, which is interrupted by the formation of the chlamydospores, degenerates more and more. Among certain Ustilagineæ the chlamydospores (brand-spores) no longer germinate with the production of fructifications. In the Uredinaceæ, only one of the three chlamydospore-forms has the property of producing fructifications on germination; the other forms only germinate vegetatively, like ordinary spores, and in the same manner as the chlamydospores of the Autobasidiomycetes. In the Hemibasidii, and the Uredinaceæ, in _Protomyces_, the chlamydospores are the chief means of reproduction. They are found also among the Ascomycetes.

The sporangia and the conidia of the Fungi have their common origin in the sporangia of the Phycomycetes. The asci (and the Ascomycetes which are characterised by these bodies) are descended from the sporangia-forming, lower Fungi; the basidia (and the Basidiomycetes) from those which bear conidia. _The sporangia of the Phycomycetes are the primitive form and the starting point for all the reproductive forms of the Fungi._ The chlamydospores appear besides in all classes of Fungi as supplementary forms of reproduction, and are of no importance in determining relationships. Although the expression “fruit” must essentially be applied to true Phanerogams, yet, through usage, the term “_fruit-forms_,” is employed to designate the forms or means of reproduction of Fungi, and the organs of reproduction are known as _organs of fructification_, the sporangiophores and conidiophores as _fruit-bearers_ (_carpophores_), and the sporangiocarps, conidiocarps, and basidiocarps as “_fruit-bodies_.”

The majority of Fungi have more than one method of reproduction, often on various hosts (Uredinaceæ). Species with one, two, or more than two methods of reproduction are spoken of as having monomorphic, dimorphic, or pleomorphic fructification. Monomorphic, _e.g._ the Tuberaceæ; dimorphic, _Mucor_, _Piptocephalis_, Saprolegniaceæ, _Penicillium crustaceum_; pleomorphic, _Puccinia graminis_, _Capnodium salicinum_ (in the last species there are five methods of reproduction: yeast-like conidia, free conidiophores, conidiocarps with small and large conidia, and ascocarps).

=The liberation and distribution of the spores and conidia.= The spores and conidia, on account of their small size and lightness, are spread far and wide by currents in the air, but in addition to this method, insects and other animals frequently assist in disseminating them. The liberation of the conidia is occasionally effected by the complete shrinking away of the conidiophore, but more frequently by abstriction from the conidiophores, either by their gradually tapering to a point, or by the dissolution of a cross-wall (generally of a mucilaginous nature). The individual links of conidia-chains are detached from one another in the same way, or often by means of small, intercalary cells, which are formed at the base of the individual links, and becoming slimy, dissolve upon the maturity of the spores. Special contrivances for ejecting the spores and conidia may often be found. In _Peronospora_ the cylindrical fruit-hyphæ in the dry condition become strap-shaped and also twisted. These are very hygroscopic, and the changes of form take place so suddenly, that the spores are violently detached and shot away. In _Empusa_ a peculiar squirting mechanism may be found (Fig. 85). Each club-shaped hypha which projects from the body of the fly, bears a conidium at its apex; a vacuole, which grows gradually larger, is formed in the slimy contents of the hypha, and the pressure thereby eventually becomes so great that the hypha bursts at its apex, and the conidium is shot into the air. By a similar mechanism, the spores of many of the Agaricaceæ are cast away from the parent-plants. In the case of _Pilobolus_ (Fig. 84) the entire sporangium is thrown for some distance into the air by a similar contrivance, the basal region of the sporangium having, by the absorption of water, been transformed into a slimy layer which is readily detached. _Sphærobolus_, a Gasteromycete, has a small, spherical fruit-body (basidiocarp), the covering of which, when ripe, suddenly bursts, and the basidiospores contained in it are forcibly ejected.

_The spores which are enclosed in asci_ are, in some instances, set free from the mother-cell (ascus) prior to their complete development (_Elaphomyces_, _Eurotium_). In the case of the majority of the Pyrenomycetes and Truffles, the asci swell by the absorption of water into a slimy mass, which gradually disappears, so that the spores lie free in the fruit-body; they either remain there till the fruit-body decays, as in those which have no aperture (Perisporiaceæ, Tuberaceæ), or the slimy mass, by its growth, is forced out through the aperture of the sporocarp, taking the spores with it (_Nectria_). The ejection of the spores by mechanical means takes place in a number of Ascomycetes, and should many spores be simultaneously ejected, a dust-cloud may be seen with the naked eye to arise in the air from the fruit-body. This is the case in the larger species of _Peziza_, _Helvella_, _Rhytisma_, when suddenly exposed to a damp current of air. A distinction is drawn between a simultaneous ejection of all the spores contained in the ascus, and an ejection at intervals (successive), when only one spore at a time is thrown out. The first of these methods is the most frequent, and is brought about by the ascus being lined with a layer of protoplasm, which absorbs water to such a degree that the elastic walls are extended at times to double their original size. The spores are forced up against the free end of the ascus, a circular rupture is made at this point, and the elastic walls contract, so that the fluid with the spores is ejected. Special means may in some instances be found to keep the spores together, and compel their simultaneous ejection. Thus, a tough slime may surround all the spores (_Saccobolus_), or a chain-apparatus, similarly formed of tough slime; or there may be a hooked appendage from each end of the spores which hooks into the appendage of the next spore (_Sordaria_). The paraphyses occurring between the asci in many Ascomycetes, also play a part in the distribution of the spores, by reason of the pressure they exercise. The asci in some of the Pyrenomycetes, which are provided with jar-shaped fruit-bodies, elongate to such an extent that, without becoming detached from their bases, they reach the mouth of the fruit-body one at a time, burst and disperse their spores, and so make room for those succeeding. An ejection of the spores at intervals from the ascus is rarer. It takes place, for instance, in _Pleospora_, whose asci have a double wall. The external wall, by absorption of water, at last becomes ruptured, and the internal and more elastic membrane forces itself out in the course of a few seconds to one of two or three times greater length and thickness, so that one spore after another is forcibly ejected from a narrow aperture at the end of the ascus.

=Germination of spores= (conidia and chlamydospores). In many spores may be found one or more _germ-pores_, _i.e._ thinner places, either in the inner membrane (uredospores, _Sordaria_) or in the external membrane (teleutospores in Rust-Fungi), through which the germination takes place. Generally this does not occur till the spores have been set free: in some Ascomycetes germination commences inside the ascus (_Taphrina_, _Sclerotinia_). The different ways in which the spores germinate may be classified into three groups.

I. THE ORDINARY GERMINATION occurs by the spore emitting a germ-tube, which immediately developes into a mycelium. In spores with a double wall it is only the inner membrane which forms the germ-tube. In swarmspores a single wall is formed after the withdrawal of the cilia, and this, by direct elongation, becomes the germ-tube. The protoplasm accumulated in the spore enters the hypha, which, in pure water, can only grow as long as the reserve nourishment lasts.

=2.= GERMINATION WITH PROMYCELIUM differs only by the circumstance that the hypha developed from the germ-tube has a very limited growth, and hence it does not immediately develope into a mycelium, but produces conidia (Rust-and Brand-Fungi). This promycelium must only be regarded as an advanced development of a conidiophore or basidium.

=3.= THE YEAST-FORMATION of conidia consists in the production of outgrowths, very much constricted at their bases, from one or more places. Each of the conidia formed in this manner may again germinate in the same way. When sufficient nourishment is present, a branched chain of such conidia is formed, and these are finally detached from one another. Yeast-like buddings from the conidia are produced in various Fungi, _e.g. Ascoidea_, _Protomyces_, Ustilagineæ, Ascomycetes, Tremellaceæ, etc. In the Ustilagineæ these conidia are an important element in the development. The budding conidia of _Exobasidium_ forms a “mould” on the nutritive solution. The yeast-like conidia are not to be confounded with the “Mucor-yeast” (comp. Mucoraceæ). For _Saccharomyces_ see Appendix to the Fungi, page 176.

In a compound spore (_i.e._ when a mass of spores are associated together) each spore germinates on its own account. There are sometimes, however, certain among them which do not germinate, but yield their contents to those which do.