Part 20

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 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.

The length of time for which conidia can retain their power of germination is shortest (being only a few weeks) in those having thin walls and containing a large supply of water (Peronosporaceæ, Uredinaceæ). In many spores a resting period is absolutely necessary before they are able to germinate (resting spores). It has been observed in some spores and conidia, that the faculty of germinating may be preserved for several years if the conditions necessary for germination remain absent (Ustilagineæ, Eurotium, Penicillium).

The optimum, minimum and maximum temperatures required for the germination of the spores has been decided in the case of a good many Fungi. A large portion of the most common Fungi have their optimum at 20°C., minimum at 1–2°C, maximum at 40°C. In the case of pathogenic Fungi the optimum is adapted to the temperature of the blood. Fungi living in manure, whose spores are often adapted to germinate in the alimentary canals of warm-blooded animals, have an optimum corresponding to the temperature of these animals, but with a little margin.

=Systematic Division.=--The lowest class of the Fungi is that of the PHYCOMYCETES, which have an unicellular mycelium, sexual and asexual reproduction, and have doubtless sprung from sporangia-bearing, lower Green Algæ. From the Phycomycetes (and certainly from the Zygomycetes) spring two well defined branches, each with numerous distinct species; to the one branch belong the HEMIASCI and the ASCOMYCETES, to the other the HEMIBASIDII and the BASIDIOMYCETES. Ascomycetes and Basidiomycetes may be united under the title of MYCOMYCETES or HIGHER FUNGI. The Hemiasci and the Hemibasidii constitute the class of MESOMYCETES. The Hemiasci are an intermediate form between Zygomycetes and Ascomycetes; the Hemibasidii a similar group between the Zygomycetes and Basidiomycetes. Mesomycetes and Mycomycetes have only asexual reproduction; sexual reproduction is wanting. Their mycelium is multicellular.

Up to the present time about 39,000 species have been described.

Review of the divisions of the Fungi:--

Class I.--=Phycomycetes (Algal-Fungi).=

Sub-Class 1. =Zygomycetes.= Sub-Class 2. =Oomycetes.= Family 1. ENTOMOPHTHORALES. Family 2. CHYTRIDIALES. Family 3. MYCOSIPHONALES.

Class II. =Mesomycetes.=

Sub-Class 1. =Hemiasci.= Sub-Class 2. =Hemibasidii (Brand-Fungi).=

Class III.--=Mycomycetes (Higher Fungi).=

Sub-Class 1. =Ascomycetes.= Series 1. =Exoasci.= Series 2. =Carpoasci.= Family 1. GYMNOASCALES. } Family 2. PERISPORIALES. } Angiocarpic Exoasci. Family 3. PYRENOMYCETES. } Family 4. HYSTERIALES. } Hemiangiocarpic Exoasci. Family 5. DISCOMYCETES.} Family 6. HELVELLALES. Gymnocarpic (?) Exoasci.

Additional: ASCOLICHENES. Lichen-forming Ascomycetes.

Sub-Class 2. =Basidiomycetes.= Series 1.--Protobasidiomycetes. Partly gymnocarpic, partly angiocarpic. Series 2. Autobasidiomycetes. Family 1. DACRYOMYCETES. Gymnocarpic. Family 2. HYMENOMYCETES. Partly gymnocarpic, partly hemiangiocarpic. Family 3. PHALLOIDEÆ. Hemiangiocarpic. Family 4. GASTEROMYCETES. Angiocarpic.

Additional: BASIDIOLICHENES. Lichen-forming Basidiomycetes.

Additional to the Fungi: FUNGI IMPERFECTI. Incompletely known (Saccharomyces, Oidium-forms, etc.).

Class 1. =Phycomycetes (Algal-Fungi).=