Encyclopaedia Britannica, 11th Edition, "Frost" to "Fyzabad"

Chapter I: Functions of Real Variables (8)

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A second type is developed as follows: the primary hypha forms a
septum below its apex as before, and the terminal conidium, thus
abstricted, puts out a branch at its apex, which starts as a mere
point and rapidly swells to a second conidium; this repeats the
process, and so on, so that we now have a chain of conidia developed
in acropetal succession, the oldest being below, and, as in
_Penicillium_, &c., branches put forth lower down may repeat the
process (_Hormodendron_). In all these cases we may speak of simple
conidiophores. The simple sporophore does not necessarily terminate in
conidia, however. In _Mucor_, for example, the end of the primary
hypha swells into a spheroidal head (sporangium), the protoplasm of
which undergoes segmentation into more or less numerous globular
masses, each of which secretes an enveloping cell-wall and becomes a
spore (endospore), and branched systems of sporangia may arise as
before (_Thamnidium_). Such may be termed sporangiophores. In
_Sporodinia_ the branches give rise also to short branches, which meet
and fuse their contents to form zygospores. In Peronospora,
Saprolegnia, &c., the ends of the branches swell up into sporangia,
which develop zoospores in their interior (zoosporangia), or their
contents become oospheres, which may be fertilized by the contents of
other branches (antheridia) and so form egg-cases (oogonia). Since in
such cases the sporophore bears sexual cells, they may be conveniently
termed gametophores.

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 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, pores, &c.) to denote
peculiarities of distribution of the hymenium over the 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.

In such examples as the above we may regard the hymenium (_Solenia_,
_Cyphella_), zygospores, or asci as truly invested by later growth,
but in the vast majority of cases the processes which result in the
enclosure of the spores, asci, &c., in a "fructification" are much
more involved, inasmuch as the latter is developed in the interior of
hyphal tissues, which are by no means obviously homologous with a
stroma. Thus in _Penicillium_, _Eurotium_, _Erysiphe_, &c., hyphal
ends which are the initials of ascogenous branches, are invested by
closely packed branches at an early stage of development, and the asci
develop inside what has by that time become a complete investment.
Whether a true sexual process precedes these processes or not does not
affect the present question, the point being that the resulting
spheroidal "fructification" (cleistocarp, perithecium) has a definite
wall of its own not directly comparable with a stroma. In other cases
(_Hypomyces_, _Nectria_) the perithecia arise on an already mature
stroma, while yet more numerous examples can be given (_Poronia_,
_Hypoxylon_, _Claviceps_, &c.) where the perithecia originate below
the surface of a stroma formed long before. Similarly with the various
types of conidial or oidial "fructifications," termed pycnidia,
spermogonia, aecidia, &c. In the simplest of these cases--e.g.
_Fumago_--a single mycelial cell divides by septa in all three planes
until a more or less solid clump results. Then a hollow appears in the
centre owing to the more rapid extension of the outer parts, and into
this hollow the cells lining it put forth short sporogenous branches,
from the tips of which the spores (stylospores, conidia, spermatia)
are abstricted. In a similar way are developed the pycnidia of
_Cicinnobolus_, _Pleospora_, _Cucurbitaria_, _Leptosphaeria_ and
others. In other cases (_Diplodia_, _Aecidium_, &c.) conidial or
oidial "fructifications" arise by a number of hyphae interweaving
themselves into a knot, as if they were forming a Sclerotium. The
outer parts of the mass then differentiate as a wall or investment,
and the interior becomes a hollow, into which hyphal ends grow and
abstrict the spores. Much more complicated are the processes in a
large series of "fructifications," where the mycelium first develops a
densely packed mass of hyphae, all alike, in which labyrinths of
cavities subsequently form by separation of hyphae in the previously
homogeneous mass, and the hymenium covers the walls of these cavities
and passages as with a lining layer. Meanwhile differences in
consistency appear in various strata, and a dense outer protective
layer (peridium), soft gelatinous layers, and so on are formed, the
whole eventually attaining great complexity--e.g. puff-balls,
earth-stars and various _Phalloideae_.

_Spore-Distribution._--Ordinary conidia and similarly abstricted dry
spores are so minute, light and numerous that their dispersal is
ensured by any current of air or water, and we also know that rats and
other burrowing animals often carry them on their fur; similarly with
birds, insects, slugs, worms, &c., on claws, feathers, proboscides,
&c., or merely adherent to the slimy body. In addition to these
accidental modes of dispersal, however, there is a series of
interesting adaptations on the part of the fungus itself. Passing over
the locomotor activity of zoospores (_Pythium_, _Peronospora_,
_Saprolegnia_) we often find spores held under tension in sporangia
(_Pilobolus_) or in asci (_Peziza_) until ripe, and then forcibly shot
out by the sudden rupture of the sporangial wall under the pressure of
liquid behind--mechanism comparable to that of a pop-gun, if we
suppose air replaced by watery sap. Even a single conidium, held tense
to the last moment by the elastic cell-wall, may be thus shot forward
by a spurt of liquid under pressure in the hypha abstricting it (e.g.
_Empusa_), and similarly with _basidiospores_ (_Coprinus_, _Agaricus_,
&c.). A more complicated case is illustrated by _Sphaerobolus_, where
the entire mass of spores, enclosed in its own peridium, is suddenly
shot up into the air like a bomb from a mortar by the elastic
retroversion of a peculiar layer which, up to the last moment,
surrounded the bomb, and then suddenly splits above, turns inside out,
and drives the former as a projectile from a gun. Gelatinous or
mucilaginous degenerations of cell-walls are frequently employed in
the interests of spore dispersal. The mucilage surrounding endospores
of _Mucor_, conidia of _Empusa_, &c., serves to gum the spore to
animals. Such gums are formed abundantly in pycnidia, and, absorbing
water, swell and carry out the spores in long tendrils, which emerge
for days and dry as they reach the air, the glued spores gradually
being set free by rain, wind, &c. In oidial chains (_Sclerotinia_) a
minute double wedge of wall-substance arises in the middle lamella
between each pair of contiguous oidia, and by its enlargement splits
the separating lamella. These disjunctors serve as points of
application for the elastic push of the swelling spore-ends, and as
the connecting outer lamella of cell-wall suddenly gives way, the
spores are jerked asunder. In many cases the slimy masses of spermatia
(_Uredineae_), conidia (_Claviceps_), basidiospores (_Phallus_,
_Coprinus_), &c., emit more or less powerful odours, which attract
flies or other insects, and it has been shown that bees carry the
fragrant oidia of _Sclerotinia_ to the stigma of _Vaccinium_ and
infect it, and that flies carry away the foetid spores of _Phallus_,
just as pollen is dispersed by such insects. Whether the strong odour
of trimethylamine evolved by the spores of _Tilletia_ attracts insects
is not known.

The recent observations and exceedingly ingenious experiments of Falck
have shown that the sporophores of the Basidiomycetes--especially 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 II.--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, Discomycetes,
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 well-marked
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 100
species--of great importance as comprising the agents of "damping off"
disease (_Pythium_), vine-mildew (_Plasmopara_), potato disease
(Phytophthora), onion-mildew (_Peronospora_). _Pythium_ is a
semi-aquatic form attacking seedlings which are too plentifully
supplied with water; its hyphae penetrate the cell-walls and rapidly
destroy the watery tissues of the living plant; then the fungus lives
in the dead remains. When the free ends of the hyphae emerge again
into the air they swell up into spherical bodies which may either fall
off and behave as conidia, each putting out a germ-tube and infecting
the host; or the germ-tube itself swells up into a zoosporangium which
develops a number of zoospores. In the rotting tissues branches of the
older mycelium similarly swell up and form antheridia and oogonia
(fig. 4). The contents of the antheridium are not set free, but that
organ penetrates the oogonium by means of a narrow outgrowth, the
fertilizing tube, and a male nucleus then passes over into the single
oosphere, which at first multinucleate becomes uninucleate before
fertilization. _Pythium_ is of interest as illustrating the dependence
of zoospore-formation on conditions and the indeterminate nature of
conidia. The other genera are more purely parasitic; the mycelium
usually sends haustoria into the cells of the host and puts out
branched, aerial conidiophores through the stomata, the branches of
which abstrict numerous "conidia"; these either germinate directly or
their contents break up into zoospores (fig. 5). The development of
the "conidia" as true conidial spores or as zoosporangia may occur in
one and the same species (_Cystopus candidus_, _Phytophthora
infestans_) as in _Pythium_ described above; in other cases the direct
conidial germination is characteristic of genera--e.g. _Peronospora_;
while others emit zoospores--e.g. _Plasmopara_, &c. In _Cystopus_
(_Albugo_) the "conidia" are abstricted in basipetal chain-like series
from the ends of hyphae which come to the surface in tufts and break
through the epidermis as white pustules. Each "conidium" contains
numerous nuclei and is really a zoosporangium, as after dispersal it
breaks up into a number of zoospores. The Peronosporaceae reproduce
themselves sexually by means of antheridia and oogonia as described in
_Pythium_. In _Cystopus Bliti_ the oosphere contains numerous nuclei,
and all the male nuclei from the antheridium pass into it, the male
and female nuclei then fusing in pairs. We thus have a process of
"multiple fertilization"; the oosphere really represents a large
number of undifferentiated gametes and has been termed a coenogamete.
Between _Cystopus Bliti_ on the one hand and _Pythium de Baryanum_ on
the other a number of cytologically intermediate forms are known. The
oospore on germination usually gives origin to a zoosporangium, but
may form directly a germ tube which infects the host.

FIG. 4.--Fertilization of the Peronosporeae. After Wager.

1, _Peronospora parasitica_. Young multinucleate oogonium (og) and
antheridium (an).

2, _Albugo candida_. Oogonium with the central uninucleate oosphere
and the fertilizing tube (a) of the antheridium which introduces the
male nucleus.

3, The same. Fertilized egg-cell (o) surrounded by the periplasm
(p).]

A, B, Section of Leaf of Potato with sporangiophores of
_Phytophthora infestans_ passing through the stomata D, on the under
surface of the leaf.

E, Sporangia.

F, G, H, J, Further development of the sporangia.

K, Germination of the zoospores formed in the sporangia.

L, M, N, Fertilization of the oogonium and development of the
oospore in _Peronospora_.]

_Saprolegniaceae_ are aquatic forms found growing usually on dead
insects lying in water but occasionally on living fish (e.g. the
salmon disease associated with _Saprolegnia ferax_). The chief genera
are _Saprolegnia, Achlya, Pythiopsis, Dictyuchus, Aplanes._ Motile
zoospores which escape from the zoosporangium are present except in
Aplanes. The sexual reproduction shows all transitions between forms
which are normally sexual, like the Peronosporaceae, to forms in which
no antheridium is developed and the oospheres develop
parthenogenetically. The oogonia, unlike the Peronosporaceae, contain
more than one oosphere. Klebs has shown that the development of
zoosporangia or of oogonia and pollinodia respectively in
_Saprolegnia_ is dependent on the external conditions; so long as a
continued stream of suitable food-material is ensured the mycelium
grows on without forming reproductive organs, but directly the
supplies of nitrogenous and carbonaceous food fall below a certain
degree of concentration sporangia are developed. Further reduction of
the supplies of food effects the formation of oogonia. This explains
the sequence of events in the case of a _Saprolegnia_-mycelium
radiating from a dead fly in water. Those parts nearest the fly and
best supplied develop barren hyphae only; in a zone at the periphery,
where the products of putrefaction dissolved in the water form a
dilute but easily accessible supply, the zoosporangia are developed in
abundance; oogonia, however, are only formed in the depths of this
radiating mycelium, where the supplies of available food materials are
least abundant.

_Chytridineae._--These parasitic and minute, chiefly aquatic, forms
may be looked upon as degenerate Oomycetes, since a sexual process and
feeble unicellular mycelium occur in some; or they may be regarded as
series of primitive forms leading up to higher members. There is no
means of deciding the question. They are usually included in
Oomycetes, but their simple structure, minute size, usually uniciliate
zoospores, and their negative characters would justify their retention
as a separate group. It contains less than 200 species, chiefly
parasitic on or in algae and other water-plants or animals, of various
kinds, or in other fungi, seedlings, pollen and higher plants. They
are often devoid of hyphae, or put forth fine protoplasmic filaments
into the cells of their hosts. After absorbing the cell-contents of
the latter, which it does in a few hours or days, the fungus puts out
a sporangium, the contents of which break up into numerous minute
swarm-spores, usually one-ciliate, rarely two-ciliate. Any one of
these soon comes to rest on a host-cell, and either pierces it and
empties its contents into its cavity, where the further development
occurs (_Olpidium_), or merely sends in delicate protoplasmic
filaments (_Rhizophydium_) or a short hyphal tube of, at most, two or
three cells, which acts as a haustorium, the further development
taking place outside the cell-wall of the host (_Chytridium_). In some
cases resting spores are formed inside the host (_Chytridium_), and
give rise to zoosporangia on germination. In a few species a sexual
process is described, consisting in the conjugation of similar cells
(_Zygochytrium_) or the union of two dissimilar ones (_Polyphagus_).
In the development of distinct antheridial and oogonial cells the
allied Ancylistineae show close alliances to _Pythium_ and the
Oomycetes. On the other hand, the uniciliate zoospores of _Polyphagus_
have slightly amoeboid movements, and in this and the
pseudopodium-like nature of the protoplasmic processes, such forms
suggest resemblances to the Myxomycetes. Opinions differ as to whether
the Chytridineae are degraded or primitive forms, and the group still
needs critical revision. Many new forms will doubtless be discovered,
as they are rarely collected on account of their minuteness. Some
forms cause damping off of seedlings--e.g. _Olpidium Brassicae_;
others discoloured spots and even tumour-like swellings--e.g.
_Synchytium Scabiosae_, _S. Succisae_, _Urophlyctis_, &c., on higher
plants. Analogies have been pointed out between Chytridiaceae and
unicellular algae, such as Chlorosphaeraceae, Protococcaceae,
"Palmellaceae," &c., some of which are parasitic, and suggestions may
be entertained as to possible origin from such algae.

The _Zygomycetes_, of which about 200 species are described, are
especially important from a theoretical standpoint, since they
furnished the series whence Brefeld derived the vast majority of the
fungi. They are characterized especially by the zygospores, but the
asexual organs (sporangia) exhibit interesting series of changes,
beginning with the typical sporangium of _Mucor_ containing numerous
endospores, passing to cases where, as in _Thamnidium_, these are
accompanied with more numerous small sporangia (sporangioles)
containing few spores, and thence to _Chaetocladium_ and
_Piptocephalis_, where the sporangioles form but one spore and fall
and germinate as a whole; that is to say, the monosporous sporangium
has become a conidium, and Brefeld regarded these and similar series
of changes as explaining the relation of ascus to conidium in higher
fungi. According to his view, the ascus is in effect the sporangium
with several spores, the conidium the sporangiole with but one spore,
and that not loose but fused with the sporangiole wall. On this basis,
with other interesting morphological comparisons, Brefeld erected his
hypothesis, now untenable, that the Ascomycetes and Basidiomycetes
diverge from the Zygomycetes, the former having particularly
specialized the ascus (sporangial) mode of reproduction, the latter
having specialized the conidial (indehiscent one-spored sporangiole)
mode. In addition to sporangia and the conidial spores referred to,
some Mucorini show a peculiar mode of vegetative reproduction by means
of gemmae or chlamydospores--i.e. short segments of the hyphae become
stored with fatty reserves and act as spores. The gemmae formed on
submerged Mucors may bud like a yeast, and even bring about alcoholic
fermentation in a saccharine solution.

FIG. 6.--_Mucor Mucedo._ Different stages in the formation and
germination of the zygospore. (After Brefeld, 1-4. 5 from v. Tavel,
_Pilze_.)

1, Two conjugating branches in contact.

2, Septation of the conjugating cells (a) from the suspensors (b).

3, More advanced stage, the conjugating cells (a) are still distinct
from one another; the warty thickenings of their walls have
commenced to form.

4, Ripe zygospore (b) between the suspensors (a).

5, Germinating zygospore with a germ-tube bearing a sporangium.]

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 spore-formation 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 nature_ is
necessary for the formation of zygospores. These structures cannot
then be produced from the product of a single spore nor even from the
thalli derived from _any_ two spores. The two kinds of thalli
Blakeslee considers to have a differentiation 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
zygote-development 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
well-known 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_.

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 _Ascomycetes_.

FIG. 7.--Germinating resting-gonidia. A, of _Ustilago receptaculorum_; B, of _Tilletia Caries_.

sp, The gonidium.

pm, The promycelium.

d, The sporidia: in B the sporidia have coalesced in pairs at v.]

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 half-way stage in the evolution of the true Basidiomycetes from
Phycomycetes, the _Tilletia_ type leading to the true basidium
(Autobasidium), the _Ustilago_ type to the protobasidium, with lateral
spores; but this view is based on very poor evidence, so that it is
best to place these forms 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.

II. _Ascomycetes._--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 life-history. The advantage or significance
of the second (ascus) fusion is not clearly understood.

FIG. 8.--Development of the Ascus.

A-C, _Pyronema confluens_. (After Harper.)

D, Young ascus of _Boudiera_ with eight spores. (After Claussen.)]

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 _Ascoidea_) 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.

Fig. 9.--_Sphaerotheca Castagnei_. Fertilization and Development of
the Perithecium. (After Harper.)

1, Oogonium (og) with the antheridial branch (az) applied to its
surface

2, Separation of antheridium (an).

3, Passage of the antheridial nucleus towards that of the oogonium.

4, Union of the nuclei.

5, Fertilized oogonium surrounded by two layers of hyphae derived
from the stalk-cell (st).

6, The multicellular ascogonium derived by division from the
oogonium; the terminal cell with the two nuclei (as) gives rise to
the ascus.]

_Exoascaceae_ are a small group of doubtful extent here used to
include _Exoascus_, _Taphrina_, _Ascorticium_ and _Endomyces_. The
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.
10). _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
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
growth, the limits are still far too ill-defined for complete
botanical treatment of the genus. A typical yeast is able to develop
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.

FIG. 10.--_Taphrina Pruni._ Transverse section through the epidermis
of an infected plum. Four ripe asci, a1, a2, with eight spores, a3,
a4, with yeast-like conidia abstricted from the spores. After
Sadebeck.

st, Stalk-cells of the asci.

m, Filaments of the mycelium cut transversely.

cut, Cuticle.

sp, Epidermis.]

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:

Two questions of great theoretical importance have been raised over
and over again in connexion with yeasts, namely, (1) the morphological
one as to whether yeasts are merely degraded forms of higher fungi, as
would seem implied by their tendency to form elongated, hypha-like
cells in the veils, and their development of "ascospores" as well as
by the wide occurrence of yeast-like "sprouting forms" in other fungi
(e.g. _Mucor_, Exoasci, Ustilagineae, higher Ascomycetes and
Basidiomycetes); and (2) the question as to the physiological nature
and meaning of fermentation. With regard to the first question no
satisfactory proof has as yet been given that Saccharomycetes are
derivable by culture from any higher form, the recent statements to
that effect not having been confirmed. At the same time there are
strong grounds for insisting on the resemblances between _Endomyces_,
a hyphal fungus bearing yeast-like asci, and such a form as
_Saccharomyces anomalus_. Concerning the second question, the recent
investigations of Buchner 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.

_Cytology._--The study of the nucleus of yeast-cells is rendered
difficult by the presence of other deeply staining granules termed by
Guillermond _metachromatic granules_. These have often been mistaken
for nuclei and have to be carefully distinguished by differential
stains. In the process of budding the nucleus divides apparently by a
process of direct division. In the formation of spores the nucleus of
the cell divides, the protoplasm collects round the nuclei to form the
spores by free-cell formation; the protoplasm (epiplasm) not used in
this process becomes disorganized. A fusion of nuclei was originally
described by Jansens and Leblanc, but it was observed neither by Wager
nor Guillermond and is probably absent. In _Schizosaccharomyces_ and
_Zygosaccharomyces_, however, we have a fusion of nuclei in connexion
with the conjugation of cells which precedes sporangium-formation. The
theory may be put forward that the ordinary forms have been derived
from sexual forms like _Schizosaccharomyces_ and _Zygosaccharomyces_
by a loss of sexuality, the sporangium being formed
parthenogenetically without any nuclear fusion. This suggests a
possible relationship to _Eremascus_, which can only doubtfully be
placed in the Ascomycetes (_vide supra_).

_Carpoascomycetes._--The other divisions of the Ascomycetes may be
distinguished as Carpoascomycetes because they do not bear the asci
free on the mycelium but enclosed in definite fruit bodies or
ascocarps. The ascocarps can be distinguished into two portions, a
mass of sterile or vegetative hyphae forming the main mass of the
fruit body, and surrounding the fertile ascogenous hyphae which bear
at their ends the asci. When the ascogonium (female organ) is present
the ascogenous hyphae arise from it, with or without its previous
fusion with an antheridium. In other cases the ascogenous hyphae arise
directly from the vegetative hyphae. In connexion with this condition
of reduction a fusion of nuclei has been observed in _Humaria
rutilans_ and is probably of frequent occurrence. The asci may be
derived from the terminal cell of the branches of the ascogenous
hyphae, but usually they are derived from the penultimate cell, the
tip curving over to form the so-called crozier. By this means the
ascus cell is brought uppermost, and after the fusion of the two
nuclei it develops enormously and produces the ascospores. The
ascospores escape from the asci in various ways, sometimes by a
special ejaculation-mechanism. The Ascomycetes, at least the
Carpoascomycetes, exhibit a well-marked alternation of sexual and
asexual generations. The ordinary mycelium is the gametophyte since it
bears the ascogonia and antheridia when present; the ascogenous hyphae
with their asci represent the sporophyte since they are derived from
the fertilized ascogonium. The matter is complicated by the apogamous
transition from gametophyte to sporophyte in the absence of the
ascogonium; also by the fact that there are normally two fusions in
the life-history as mentioned earlier. If there are two fusions one
would expect two reductions, and Harper has suggested that the
division of the nuclei into eight in the ascus, instead of into four
spores as in most reduction processes, is associated with a _double_
reduction process in the ascus. Miss Fraser in _Humaria rutilans_
finds two reductions: a normal synaptic reduction in the first nuclear
division of the ascus, and a peculiar reduction division termed
_brachymeiosis_ in the third ascus division.

Various types of ascocarp are characteristic of the different
divisions of the Carpoascomycetes: the cleistothecium, apothecium and
perithecium.

_Perisporineae._--This includes two chief families, Erysiphaceae and
Perisporiaceae. They are characterized by an ascocarp without any
opening to the exterior, the ascospores being set free by the decay or
rupture of the ascocarp wall; such a fruit-body is termed a
_cleistothecium_ (cleistocarp). The Erysiphaceae are a sharply marked
group of forms which live as parasites. They form a superficial
mycelium on the surface of the plant, the hyphae not usually
penetrating the tissues but merely sending haustoria into the
epidermal cells. Only in rare cases is the mycelium intercellular.
Owing to their appearance they go by the popular name of mildews.
_Sphaerotheca Humuli_ is the well known hop-mildew, _Sphaerotheca
Mors-Uvae_ is the gooseberry mildew, the recent advent of which has
led to special legislation in Great Britain to prevent its spreading,
as when rampant it makes the culture of gooseberries impossible.
_Erysiphe_, _Uncinula_ and _Phyllactinia_ are other well-known genera.
The form of the fruit body, the difference and the nature of special
outgrowths upon it--the appendages--are characteristic of the various
genera. Besides peritheca the members of the Erysiphaceae possess
conidia borne in simple chains. De Bary brought forward very strong
evidence for the origin of the ascocarp in _Sphaerotheca_ and
_Erysiphe_ by a sexual process, but Harper in 1895 was the first to
prove conclusively, by the observation of the nuclear fusion, that
there was a definite fertilization in _Sphaerotheca Humuli_ by the
fusion of a male (antheridial) nucleus with a female, ascogonial
(oogonial) nucleus. Since then Harper has shown that the same process
occurs in _Erysiphe_ and _Phyllactinia_.

A, Small portion of mycelium with conidiophore (c), and archicarp
(as).

B, The spiral archicarp (as), with the antheridium (p).

D, The same, beginning to be surrounded by the hyphae forming the
perithecium wall.

D, The perithecium.

E, F, Sections of young perithecia.

w, Parietal cells.

f, Pseudo-parenchyma.

as, Ascogonium.

G, An ascus.

H, An ascospore.]

The Perisporiaceae are saprophytic forms, the two chief genera being
_Aspergillus_ and _Penicillium_. The blue-green mould _P. crustaceum_
and the green mould _A. herbariorium_ ( = _Eurotium herbariorum_) are
extraordinarily widely distributed, moulds being found on almost any
food-material which is exposed to the air. They have characteristic
conidiophores bearing numerous conidia, and also cleistothecia which
are spherical in form and yellowish in colour. The latter arise from
the crown of a spirally coiled archicarp (bearing an ascogonium at its
end) and a straight antheridium. Vegetative hyphae then grow up and
surround these and enclose them in a continuous sheath of plectenchyma
(fig. 11). It has lately been shown by Fraser and Chambers that in
_Eurotium_ both ascogonium and antheridium contain a number of nuclei
(i.e. are coenogametes), but that the antheridium disorganizes without
passing its contents into the ascogonium. There is apparently a
reduced sexual process by the fusion of the ascogonial (female) nuclei
in pairs. _Aspergillus Oryzae_ plays an important part in
saccharifying the starch of rice, maize, &c., by means of the abundant
diastase it secretes, and, in symbiosis with a yeast which ferments
the sugar formed, has long been used by the Japanese for the
preparation of the alcoholic liquor sake. The process has now been
successfully introduced into European commerce.

FIG. 12.--_Peziza aurantiaca._ (After Krombholz, nat. size.)]

m, Mycelium.

c, Archicarp.

l, Pollinodium.

s, Ascogenous filaments.

a, Asri.

r, p, The sterile tissue from which the paraphyses h spring.]

_Discomycetes._--Used in its widest sense this includes the
Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is
characterized in general by the possession of an ascocarp which,
though usually a completely closed structure during the earlier stages
of development, at maturity opens out to form a bowl or saucer-shaped
organ, thus completely exposing the layer of asci which forms the
hymenium. Such an ascocarp goes by the name of _apothecium_. Owing to
the shape of the fruit-body many of these forms are known as
cup-fungi, the cup or apothecium often attaining a large size,
sometimes several inches across (fig. 12). Functional male and female
organs have been shown to exist in _Pyronema_ and _Boudiera_; in
_Lachnea stercorea_ both ascogonia and antheridia are present, but the
antheridium is non-functional, the ascogonial (female) nuclei fusing
in pairs; this is also the case in _Humaria granulata_ and _Ascobolus
furfuraceus_, where the antheridium is entirely absent. In _H.
rutilans_, however, both sexual organs are absent and the ascogenous
hyphae arise apogamously from the ordinary hyphae of the mycelim. In
all these cases the ascogonium and antheridium contain numerous
nuclei; they are to be looked upon as gametangia in which there is no
differentiation of gametes, and since they act as single gametes they
are termed coenogametes. In some forms as in _Ascobolus_ the
ascogonium is multicellular, the various cells communicating by pores
in the transverse walls (fig. 13).

In the Helvellaceae there is no apothecium but a large irregular fruit
body which at maturity bears the asci on its surface. The development
is only slightly known, but there is some evidence for believing that
the fruit-body is closed in its very early stages.

FIG. 14.--Perithecium of Podospora fimiseda in longitudinal section
After v. Tavel.

s, Asci.

a, Paraphyses.

e, Periphyses.

m, Mycelial hyphae.]

The genus _Peziza_ (in its widest sense) may be taken as the type of
the group. Most of them grow on living plants or on dead vegetable
remains, very often on fallen wood; a number, however, are found
growing on earth which is rich in humus. The genus _Sclerotinia_ may
be mentioned here; a number of forms have been investigated by
Woronin. The conidia are fragrant and are carried by bees to the
stigma of the bilberry; here they germinate with the pollen and the
hyphae pass with the pollen tubes down the style; the former infect
the ovules and produce sclerotia, therein reducing the fruits to a
mummified condition. From the sclerotia later the apothecium develops.
One species, _S. heteroica_, is _heteroecious_; the ascospores
infecting the leaves of _Vaccinium uliginosum_, while the conidia
which then arise infect only _Ledum palustre_. This is the only case
of heteroecism known in the vegetable kingdom outside the Uredineae.

_Pyrenomycetes._--This is an extraordinarily large and varied group of
forms which mostly live parasitically or saprophytically on vegetable
tissue, but a few are parasitic on insect-larvae. The group is
characterized by a special type of ascocarp, the _perithecium_. This
is typically of a flask-shaped form opening with a small pore at the
top. The asci live at the bottom often mixed with paraphyses, while
the upper "neck" of the flask is lined with special hyphae, the
periphyses, which aid in the ejection of the spores (fig. 14). The
simpler forms bear the perithecia directly on the mycelium, but the
more highly developed forms often bear them on a special mycelial
development--the stroma, which is often of large size and special
shape and colour, and of dense consistence. The cytological details of
development of the perithecia are not well known; most of them appear
to develop their ascogenous hyphae in an apogamous way without any
connexion with an ascogonium. Besides the special ascocarps, accessory
reproductive organs are known in the majority of cases in the form of
conidia.

_Tuberineae._--These are a small group of fungi including the
well-known truffles. They are found living saprophytically (in part
parasitically) underground in forests. The asci are developed in the
large dense fruit bodies (cleistothecia) and the spores escape by the
decay of the wall. The fruit-body is of complicated structure, but its
early stages of development are not known. Many of the fruit-bodies
have a pleasant flavour and are eaten under the name of truffles
(_Tuber brumale_ and other species). The exact life-history of the
truffle is not known.

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Encyclopaedia Britannica, 11th Edition, "Frost" to "Fyzabad"Chapter I: Functions of Real Variables (8)

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