=Mode of Life.= The Fungi have no chlorophyll, and are thus unable in
any stage of their existence to assimilate carbon; they must therefore
live as _saprophytes_ or _parasites_. There is, however, no strong line
of demarcation between these; many Fungi commence as true parasites,
but only attain their full development upon or in dead plants or
animals (_Rhytisma_, _Empusa_). Many saprophytes may occasionally
appear as parasites, and are then designated “_facultative parasites_”
(_Nectria cinnabarina_, _Lophodermium pinastri_), in contradistinction
to those which only appear as parasites, “_obligate parasites_”
(Mildew, Brand-and Rust-Fungi, _Cordyceps_).
The parasites which live on the surface of the host-plant are termed
_epiphytic_ (Mildew, _Fusicladium_); and those living in its tissues
are termed _endophytic_ (_Ustilago_, _Peronospora_). _Epizoic_
(_Oidium tonsurans_, _Laboulbenia_) and _endozoic_ Fungi (_Cordyceps_,
_Entomophthora_), are distinguished, in the same manner, as those
which live on the surface or in the interior of animals. The Fungi
designated _pathogenic_ are especially those which produce disease in
human beings and in animals.
Most of the diseases of plants are attributed to the parasitic Fungi.
These force their way into the host-plant by piercing the outer wall
of the epidermis, as in the Potato-disease; or by growing in through
the stomata, _e.g._ the summer generations of the Rust of Wheat; or
they can only penetrate through a wound, _e.g. Nectria_. Some effect
an entrance into the host-plant by the secretion of a poisonous matter
or ferment, which softens and destroys the cell-walls (_Sclerotinia_).
Some Yeast and Mould Fungi secrete ferments (enzymes), which, for
example, convert cane-sugar into a sugar capable of fermentation.
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.
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.