Part 10

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).=[12]

This group resembles _Vaucheria_ and the other Siphoneæ among the Algæ.

ORGANS OF NUTRITION. The mycelium is formed of a single cell, often thread-like and abundantly branched (Fig. 78). Vegetative propagation by chlamydospores and oidia. Asexual reproduction by endospores (sometimes _swarmspores_) and conidia. Sexual reproduction by conjugation of two hyphæ as in the Conjugatæ, or by fertilisation of an egg-cell in an oogonium. On this account the class of the Phycomycetes is divided into two sub-classes: ZYGOMYCETES and OOMYCETES.

Sub-Class I. =Zygomycetes.=

Sexual reproduction takes place by zygospores, which function as resting-spores, and arise in consequence of _conjugation_ (Fig. 81); in the majority of species these are rarely found, and only under special conditions. The most common method of reproduction is by endospores, by acrogenous conidia, by chlamydospores, or by oidia. _Swarmspores are wanting._ Parasites and saprophytes (order 6 and 7). The zygospores are generally produced when the formation of sporangia has ceased; _e.g._ by the suppression of the sporangial-hyphæ (_Mucor mucedo_), or by the diminution of oxygen; _Pilobolus crystallinus_ forms zygospores, when the sporangia are infected with saprophytic _Piptocephalis_ or _Pleotrachelus_.

=A.= Asexual reproduction only by sporangia.

Order 1. =Mucoraceæ.= The spherical sporangia contain many spores. The zygospore is formed between two unicellular branches (gametes).

The unicellular mycelium (Fig. 78) of the Mucoraceæ branches abundantly, and lives, generally, as a saprophyte on all sorts of dead organic remains. Some of these Fungi are known to be capable of producing _alcoholic fermentation_, in common with the Saccharomyces. This applies especially to _Chlamydomucor racemosus_ (_Mucor racemosus_), when grown in a saccharine solution, and deprived of oxygen; the mycelium, under such conditions, becomes divided by transverse walls into a large number of small cells. Many of these swell out into spherical or club-shaped cells, and when detached from one another become chlamydospores, which abstrict new cells of similar nature (Fig. 79). These chlamydospores were formerly erroneously termed “mucor-yeast,” but they must not be confounded with the yeast-conidia (page 94). They are shortened hyphæ, and are not conidia of definite size, shape, and point of budding. Oidia are also found in _Chlamydomucor_.

The Mucoraceæ, in addition to the chlamydospores and oidia, have a more normal and ordinary method of reproduction; viz., by _spores_ which are formed without any sexual act. _Mucor_ has round sporangia; from the mycelium one or more long branches, sometimes several centimetres in length, grow vertically into the air; the apex swells (Figs. 78, 80) into a sphere which soon becomes separated from its stalk by a transverse wall; in the interior of this sphere (sporangium) a number of spores are formed which eventually are set free by the rupture of the wall. The transverse wall protrudes into the sporangium and forms the well-known columella (Fig. 80 _d_, _e_). The formation of spores takes place in various ways among the different genera.

SEXUAL REPRODUCTION by conjugation takes place in the following manner. The ends of two hyphæ meet (Fig. 81) and become more or less club-shaped; the ends of each of these are cut off by a cell-wall, and two new small cells (Fig. 81 _A_) are thus formed, these coalesce and give rise to a new cell which becomes the very thick-walled zygote (zygospore), and germinates after period of rest, producing a new hypha, which bears a sporangium (Fig. 81 _E_).

_Mucor mucedo_, Pin-mould, resembles somewhat in appearance _Penicillium crustaceum_ and is found growing upon various organic materials (bread, jam, dung, etc.).

_Pilobolus_ (Figs. 83, 84) grows on manure. Its sporangium (Fig. 84 _a″_) is formed during the night and by a peculiar mechanism (page 92) is shot away from the plant in the course of the day. This generally takes place in the summer, between eight and ten a.m. The sporangium is shot away to a height which may be 300 times greater than that of the plant itself, and by its stickiness it becomes attached to portions of plants, etc., which are in the vicinity. If these are eaten by animals, the spores pass into the alimentary canal and are later on, sometimes even in a germinating condition, passed out with the excrement, in which they form new mycelia.

_Phycomyces nitens_ (“Oil-mould”) is the largest of the Mould Fungi; its sporangiophores may attain the height of 10–30 c.m.

Order 2. =Rhizopaceæ.= _Rhizopus nigricans_ (_Mucor stolonifer_) which lives on decaying fruits containing sugar, on bread, etc., has, at the base of the sporangiophores, tufts of rhizoids, _i.e._ hyphæ, which function as organs of attachment. From these, “runners” are produced which in a similar manner develope sporangiophores and rhizoids.

Order 3. =Thamnidiaceæ.= On the same sporangiophore, in addition to a large, terminal, many-spored sporangium, many smaller, lateral sporangia are formed with a few spores. Thamnidium.

=B.= Asexual reproduction by sporangia and conidia.

Order 4. =Choanephoraceæ.= _Choanephora_ with creeping endophytic mycelium, and perpendicular sporangiophores.

Order 5. =Mortierellaceæ.= _Mortierella polycephala_ produces on the same mycelium conidia and sporangiophores. _M. rostafinskii_ has a long stalked sporangiophore, which is surrounded at its base by a covering of numerous felted hyphæ.

=C.= Asexual reproduction only by conidia.

Order 6. =Chætocladiaceæ.= The conidia are abstricted singly and acrogenously. _Chætocladium_ is a parasite on the larger Mucoraceæ.

Order 7. =Piptocephalidaceæ.= The conidia are formed acrogenously and in a series, by transverse divisions. The zygospore arises at the summit of the conjugating hyphæ, which are curved so as to resemble a pair of tongs. _Piptocephalis_ and _Syncephalis_ live parasitically on the larger Mucoraceæ.

Sub-Class 2. =Oomycetes.=

Sexual reproduction is oogamous with the formation of brown, thick-walled _oospores_ which germinate after a period of rest. Asexual reproduction by conidia and _swarmspores_. Parasites, seldom saprophytes.

The oospores are large spores which are formed from the egg-cell (oosphere) of the _oogonium_ (oosporangium, Fig. 89, 95). A branch of the mycelium attaches itself to the oogonium and forms at its apex the so-called “_antheridium_” (pollinodium[13]): this sends one or more slender prolongations (fertilising tubes) through the wall of the oogonium to the egg-cell.

A fertilisation, a passage of the contents of the antheridium to the egg-cell, has as yet only been observed in _Pythium_; in _Phytophthora_ only one small mass of protoplasm passes through the fertilising tube to the egg-cell; in _Peronospora_ and the Saprolegniaceæ no protoplasm can be observed to pass through the fertilising tube, so that in these instances _parthenogenesis_ takes place; _Saprolegnia thuretii_, etc., have generally even no antheridia, but nevertheless form normal oospores. Fertilisation of the egg-cell by means of self-motile _spermatozoids_ is only found in _Monoblepharis sphærica_.

=A.= Asexual reproduction by conidia only.

Family 1. =Entomophthorales.=

The mycelium is richly branched. The family is a transitional step to the conidia-bearing Zygomycetes, since the oospores of many members of this family arise, and are formed, like zygospores.

Order 1. =Entomophthoraceæ.= Mycelium abundantly developed. This most frequently lives parasitically in living insects, causing their death. The conidiophores forming the conidial-layer project from the skin, and abstrict a proportionately large conidium which is ejected with considerable force, and by this means transferred to other insects. These become infected by the entrance of the germ-tube into their bodies. The spherical, brown resting-spores develope inside the bodies of insects and germinate by emitting a germ-tube.

GENERA: _Empusa_ has a good many species which are parasitic on flies, moths, grasshoppers, plant-lice. The conidia emit a germ-tube which pierces the skin of the insect; a number of secondary conidia are then produced inside its body, by division or by gemmation similar to that taking place in yeast, each of which grows and becomes a long unbranched hypha, and these eventually fill up the body of the animal, causing distension and death. Each of these hyphæ projects through the skin, and abstricts a conidium, which is ejected by a squirting contrivance. The best known species is _E. muscæ_ (Fig. 85), which makes its appearance epidemically towards autumn on the common house-fly, and shows itself by the dead flies which are found on the windows and walls attached by their probosces, distended wings, and legs. They have swollen abdomen, broad white belts of hyphæ between the abdominal rings, and are surrounded by a circle of whitish dust formed by the ejected conidia.--_Entomophthora_ sends out, at definite places, from the mycelium hidden in the insect’s body, bundles of hyphæ, which serve the purpose of holding fast the dead insects, the ramifications attaching themselves to the substratum: the conidiophores are branched, the conidia are ejected by the divisional walls between the hyphæ and the conidia dividing into two layers, those which terminate the hyphæ suddenly expanding and throwing the conidia into the air. _E. radicans_ makes its appearance epidemically on caterpillars.

=B.= Asexual reproduction by zoospores or conidia.

Family 2. =Chytridiales.=

In this family the mycelium is very sparsely developed or is wanting. The entire plant consists principally or entirely of a single zoosporangium whose zoospores have generally one cilium. The resting-spores arise either directly from the zoosporangium, which, instead of forming zoospores, surrounds itself by a thick cell-wall; or they are formed by the conjugation of two cells (in which case they are spoken of as oospores). Microscopic Fungi, parasitic on water plants (especially Algæ) or small aquatic animals, seldom on land plants.

Order 1. =Olpidiaceæ.= Without mycelium. Swarmspores and resting-spores.

In the _Olpidieæ_, the swarmspores, probably, most frequently form themselves into a plasmodium (naked mass of protoplasm) which may become a single zoosporangium or a resting sporangium. _Olpidium trifolii_ occurs in _Trifolium repens_.--In the _Synchytrieæ_ the plasmodium emerging from the swarmspores breaks up either at once, or after a period of rest, into smaller plasmodia, each of which will become a zoosporangium. _Synchytrium anemones_ is found on _Anemone nemorosa_; _S. mercurialis_ on _Mercurialis perennis_; _S. aureum_ on many plants, particularly _Lysimachia nummularia_.

Order 2. =Rhizidiaceæ.= Mycelium present. Zoospores and resting-spores.

_Chytridium_ (Fig. 86). _Obelidium_ (Fig. 87) is bicellular; the one cell is the mycelium, the other the zoosporangium; found on insects. The species of _Cladochytrium_ are intercellular parasites on marsh plants. _Physoderma._

Order 3. =Zygochytriaceæ.= Mycelium present. Zoospores and oospores. The latter are the product of the conjugation of two cells (Fig. 88).

_Polyphagus euglenæ_ on _Euglena viridis_. _Urophlyctis pulposa_ on species of _Chenopodium_.

Family 3. =Mycosiphonales.=

The mycelium is bladder-like or branched. Zoospores. Sexual reproduction by oospores, which are produced in oogonia. The latter are fertilised, in some forms, by the antheridium.

Order 1. =Ancylistaceæ.= The entire bladder-like mycelium is used for the construction of zoosporangia, oogonia, or antheridia. _Lagenedium_ is parasitic on _Spirogyra_, etc.

Order 2. =Peronosporaceæ.= Almost entirely _parasites_. The unicellular, often very long and abundantly branched mycelium lives in the intercellular spaces of living plants, especially in the green portions, and these are more or less destroyed and deformed in consequence. Special small branches (_suction-organs_, “_haustoria_”) are pushed into the cells in order to abstract nourishment from them. Both oospores and conidia germinate either immediately, or they develope into sporangia with swarmspores, having always two cilia. Only one oospore is formed in each oogonium; its contents (Fig. 89) divide into a centrally placed egg-cell and the “periplasm” surrounding it; this is of a paler colour and on the maturity of the oospore forms its thick, brown, external covering.

_The Potato-fungus_ (_Phytophthora infestans_) is of great interest. Its thallus winters in the Potato-tuber; other organs for passing the winter, such as oospores, are not known. When the tuber germinates, the Fungus-hyphæ penetrate the young shoot and keep pace with the aerial growth and development of the plant. The conidiophores emerge through the stomata, especially on the under side of the leaves; they branch like a tree (Fig. 90), and appear to the naked eye as a fine mould on the surface of the plant. The disease soon makes itself known by the brown colouring of those parts of the plant which are attacked, and by their withering. An ovoid conidium arises at first by the formation of a dividing wall at the apex of each branch of the conidiophore (Fig. 90 _c c_), and immediately underneath it another is formed, which pushes the first to one side, and so on. These conidia sometimes germinate directly, and form a mycelium, but most frequently their protoplasm divides into many small masses, each of which becomes a pear-shaped zoospore provided with two cilia (Fig. 91). Water is required for their germination, and when the ripe conidia are placed in a drop of water the swarm-cells are formed in the course of about five hours. They swarm about in rain and dewdrops in the Potato-fields, and are carried with the water to the Potato-plants and to the tubers in the soil. The wind also very easily conveys the conidia to healthy Potato-fields and infects them. The enormous quantity of conidia and swarm-cells that may be formed in the course of a summer explains the rapid spreading of the disease; and the preceding makes it clear why wet summers are favourable to its existence. When the swarm-cells germinate, they round off, and then surround themselves with a cell-wall which grows out into the germ-tube, and _pierces through the epidermis_ of the host-plant (Fig. 92). Having entered the host, a new mycelium is formed. The potato disease, since 1845, has been rampant in Europe; it has, no doubt, been introduced from America, which, it must be remembered, is the home of the Potato-plant.

The conidia exhibit various characters which are employed for the separation of the genera. _Pythium_ is the most simple form. The contents of the terminally-formed conidia emerge as a spherical mass and divide into swarmspores. _P. de Baryanum_ lives in the seedlings of many different Flowering-plants, which it completely destroys.--_Phytophthora_ is distinguished by the circumstance that the sparsely-branched conidiophores bear, sympodially, chains of conidia. Besides the Potato-fungus (see above), _Ph. fagi_ belongs to this group; it developes oospores very abundantly, and does great harm to seedlings of the Beech, Sycamore, and Pine trees.--_Peronospora_ generally has conidiophores which are repeatedly forked, and bear a conidium on each of the most extreme ramifications. Many do great harm to their host-plants. _P. viticola_, on Vines, and _P. nivea_, on umbelliferous plants, have swarmspores, which are absent in the following species of this genus: _P. sparsa_, on Roses; _P. gangliformis_, on composites; _P. alsinearum_, on Stitchwort; _P. parasitica_, on cruciferous plants; _P. viciæ_, on Vetches and Peas; _P. schachtii_, on Beets; _P. violacea_, on the flowers of _Scabiosa_; _P. radii_, on the ray-florets of _Matricaria_.--_Cystopus_ (_Albugo_) has the conidia developed in chains, which form a cohesive white layer underneath the epidermis of the host-plant. _Cystopus candidus_, on cruciferous plants, especially Shepherd’s Purse and _Brassica_; the germination commences on the cotyledons, and from this point the mycelium developes together with the host-plant; _C. cubicus_, on the leaves of Compositæ.