[1] E.S. Hartland, _Legend of Perseus_ (1895), ii. 278.
[2] Mary Kingsley, _West African Studies_ (1901), p. 178.
[3] B. Spencer and F.J. Gillen, _The Native Tribes of Central
Australia_ (1899), p. 48.
FUNGI (pl. of Lat. _fungus_, a mushroom), the botanical name covering in
the broad sense all the lower cellular Cryptogams devoid of chlorophyll,
which arise from spores, and the thallus of which is either unicellular
or composed of branched or unbranched tubes or cell-filaments (hyphae)
with apical growth, or of more or less complex wefted sheets or
tissue-like masses of such (mycelium). The latter may in certain cases
attain large dimensions, and even undergo cell-divisions in their
interior, resulting in the development of true tissues. The spores,
which may be uni- or multicellular, are either abstricted free from the
ends of hyphae (acrogenous), or formed from segments in their course
(_chlamydospores_) or from protoplasm in their interior (endogenous).
The want of chlorophyll restricts their mode of life--which is rarely
aquatic--since they are therefore unable to decompose the carbon dioxide
of the atmosphere, and renders them dependent on other plants or
(rarely) animals for their carbonaceous food-materials. These they
obtain usually in the form of carbohydrates from the dead remains of
other organisms, or in this or other forms from the living cells of
their hosts; in the former case they are termed saprophytes, in the
latter parasites. While some moulds (_Penicillium_, _Aspergillus_) can
utilize almost any organic food-materials, other fungi are more
restricted in their choice--e.g. insect-parasites, horn- and
feather-destroying fungi and parasites generally. It was formerly the
custom to include with the Fungi the Schizomycetes or Bacteria, and the
Myxomycetes or Mycetozoa; but the peculiar mode of growth and division,
the cilia, spores and other peculiarities of the former, and the
emission of naked amoeboid masses of protoplasm, which creep and fuse to
streaming plasmodia, with special modes of nutrition and spore-formation
of the latter, have led to their separation as groups of organisms
independent of the true Fungi. On the other hand, lichens, previously
regarded as autonomous plants, are now known to be dual organisms--fungi
symbiotic with algae.
The number of species in 1889 was estimated by Saccardo at about 32,000,
but of these 8500 were so-called _Fungi imperfecti_--i.e. forms of which
we only know certain stages, such as conidia, pycnidia, &c., and which
there are reasons for regarding as merely the corresponding stages of
higher forms. Saccardo also included about 400 species of Myxomycetes
and 650 of Schizomycetes. Allowing for these and for the cases,
undoubtedly not few, where one and the same fungus has been described
under different names, we obtain Schroeter's estimate (in 1892) of
20,000 species. In illustration of the very different estimates that
have been made, however, may be mentioned that of De Bary in 1872 of
150,000 species, and that of Cooke in 1895 of 40,000, and Massee in 1899
of over 50,000 species, the fact being that no sufficient data are as
yet to hand for any accurate census. As regards their geographical
distribution, fungi, like flowering plants, have no doubt their centres
of origin and of dispersal; but we must not forget that every exchange
of wood, wheat, fruits, plants, animals, or other commodities involves
transmission of fungi from one country to another; while the migrations
of birds and other animals, currents of air and water, and so forth, are
particularly efficacious in transmitting these minute organisms. Against
this, of course, it may be argued that parasitic forms can only go where
their hosts grow, as is proved to be the case by records concerning the
introduction of _Puccinia malvacearum_, _Peronospora viticola_,
_Hemileia vastatrix_, &c. Some fungi--e.g. moulds and yeasts--appear to
be distributed all over the earth. That the north temperate regions
appear richest in fungi may be due only to the fact that North America
and Europe have been much more thoroughly investigated than other
countries; it is certain that the tropics are the home of very numerous
species. Again, the accuracy of the statement that the fleshy Agaricini,
Polyporei, _Pezizae_, &c., are relatively rarer in the tropics may
depend on the fact that they are more difficult to collect and remit for
identification than the abundantly recorded woody and coriaceous forms
of these regions. When we remember that many parts of the world are
practically unexplored as regards fungi, and that new species are
constantly being discovered in the United States, Australia and northern
Europe--the best explored of all--it is clear that no very accurate
census of fungi can as yet be made, and no generalizations of value as
to their geographical distribution are possible.
The existence of fossil fungi is undoubted, though very few of the
identifications can be relied on as regards species or genera. They
extend back beyond the Carboniferous, where they occur as hyphae, &c.,
preserved in the fossil woods, but the best specimens are probably those
in amber and in siliceous petrifactions of more recent origin.
[Illustration: FIG. 1.--1, _Peronospora parasitica_ (De Bary). Mycelium
with haustoria (h); 2, _Erysiphe_; A and B, mycelium (m), with haustoria
(h). (After De Bary.)]
_Organs._--Individual hyphae or their branches often exhibit
specializations of form. In many Basidiomycetes minute branches arise
below the septa; their tips curve over the outside of the latter, and
fuse with the cell above just beyond it, forming a _clamp-connexion_.
Many parasitic hyphae put out minute lateral branches, which pierce
the cell-wall of the host and form a peg-like (_Trichosphaeria_),
sessile (_Cystopus_), or stalked (_Hemileia_), knot-like, or a more or
less branched (_Peronospora_) or coiled (_Protomyces_) haustorium. In
_Rhizopus_ certain hyphae creep horizontally on the surface of the
substratum, and then anchor their tips to it by means of a tuft of
short branches (_appressorium_), the walls of which soften and gum
themselves to it, then another branch shoots out from the tuft and
repeats the process, like a strawberry-runner. Appressoria are also
formed by some parasitic fungi, as a minute flattening of the tip of a
very short branch (_Erysiphe_), or the swollen end of any hypha which
comes in contact with the surface of the host (_Piptocephalis_,
_Syncephalis_), haustoria piercing in each case the cell-wall below.
In _Botrytis_ the appressoria assume the form of dense tassels of
short branches. In _Arthrobotrys_ side-branches of the mycelium sling
themselves around the host (_Tylenchus_) much as tendrils round a
support.
Many fungi (_Phallus_, _Agaricus_, _Fumago_, &c.) when strongly
growing put out ribbon-like or cylindrical cords, or sheet-like
mycelial plates of numerous parallel hyphae, all growing together
equally, and fusing by anastomoses, and in this way extend long
distances in the soil, or over the surfaces of leaves, branches, &c.
These mycelial strands may be white and tender, or the outer hyphae
may be hard and black, and very often the resemblance of the
subterranean forms to a root is so marked that they are termed
rhizomorphs. The outermost hyphae may even put forth thinner hyphae,
radiating into the soil like root-hairs, and the convergent tips may
be closely appressed and so divided by septa as to resemble the
root-apex of a higher plant (_Armillaria mellea_).
_Sclerotia._--Fungi, like other plants, are often found to store up
large quantities of reserve materials (oil, glycogen, carbohydrates,
&c.) in special parts of their vegetative tissues, where they lie
accumulated between a period of active assimilation and one of renewed
activity, forming reserves to be consumed particularly during the
formation of large fructifications. These reserve stores may be packed
away in single hyphae or in swollen cells, but the hyphae containing
them are often gathered into thick cords or mycelial strands
(_Phallus_, mushroom, &c.), or flattened and anastomosing ribbons and
plates, often containing several kinds of hyphae (_Merulius
lacrymans_). In other cases the strands undergo differentiation into
an outer layer with blackened, hardened cell-walls and a core of
ordinary hyphae, and are then termed rhizomorphs (_Armillaria
mellea_), capable not only of extending the fungus in the soil, like
roots, but also of lying dormant, protected by the outer casing. Such
aggregations of hyphae frequently become knotted up into dense masses
of interwoven and closely packed hyphae, varying in size from that of
a pin's head or a pea (_Peziza_, _Coprinus_) to that of a man's fist
or head, and weighing 10 to 25 lb. or more (_Polyporus Mylittae_, _P.
tumulosus_, _Lentinus Woermanni_, _P. Sapurema_, &c.). The interwoven
hyphae fuse and branch copiously, filling up all interstices. They
also undergo cutting up by numerous septa into short cells, and these
often divide again in all planes, so that a pseudoparenchyma results,
the walls of which may be thickened and swollen internally, or
hardened and black on the exterior. In many cases the swollen
cell-walls serve as reserves, and sometimes the substance is so
thickly deposited in strata as to obliterate the lumen, and the hyphae
become nodular (_Polyporus sacer_, _P. rhinoceros_, _Lentinus
Woermanni_). The various sclerotia, if kept moist, give rise to the
fructifications of the fungi concerned, much as a potato tuber does to
a potato plant, and in the same way the reserve materials are
consumed. They are principally Polyporei, Agaricini, Pezizae; none are
known among the Phycomycetes, Uredineae or Ustilagineae. The functions
of mycelial strands, rhizomorphs and sclerotia are not only to collect
and store materials, but also to extend the fungus, and in many cases
similar strands act as organs of attack. The same functions of storage
in advance of fructification are also exercised by the stromata so
common in Ascomycetes.
_Tissue Differentiations._--The simpler mycelia consist of hyphae all
alike and thin-walled, or merely differing in the diameter of the
branches of various orders, or in their relations to the environment,
some plunging into the substratum like roots, others remaining on its
surface, and others (aerial hyphae) rising into the air. Such hyphae
may be multicellular, or they may consist of simple tubes with
numerous nuclei and no septa (_Phycomycetes_), and are then
non-cellular. In the more complex tissue-bodies of higher fungi,
however, we find considerable differences in the various layers or
strands of hyphae.
An epidermis-like or cortical protective outer layer is very common,
and is usually characterized by the close septation of the densely
interwoven hyphae and the thickening and dark colour of their outer
walls (sclerotia, _Xylaria_, &c.). Fibre-like hyphae with the lumen
almost obliterated by the thick walls occur in mycelial cords
(_Merulius_). Latex-tubes abound in the tissues of _Lactarius_,
_Stereum_, _Mycena_, _Fistulina_, filled with white or coloured milky
fluids, and Istvanffvi has shown that similar tubes with fluid or oily
contents are widely spread in other Hymenomycetes. Sometimes fatty oil
or watery sap is found in swollen hyphal ends, or such tubes contain
coloured sap. Cystidia and paraphyses may be also classed here. In
_Merulius lacrymans_ Hartig has observed thin-walled hyphae with large
lumina, the septa of which are perforated like those of sieve-tubes.
As regards its composition, the cell-wall of fungi exhibits variations
of the same kind as those met with in higher plants. While the
fundamental constituent is a cellulose in many Mucorini and other
Phycomycetes, in others bodies like pectose, callose, &c., commonly
occur, and Wisselingh's researches show that chitin, a gluco-proteid
common in animals, forms the main constituent in many cases, and is
probably deposited directly as such, though, like the other
substances, it may be mixed with cellulose. As in other cell-walls, so
here the older membranes may be altered by deposits of various
substances, such as resin, calcium oxalate, colouring matters; or more
profoundly altered throughout, or in definite layers, by
lignification, suberization (_Trametes_, _Daedalea_), or swelling to a
gelatinous mucilage (_Tremella_, _Gymnosporangium_), while
cutinization of the outer layers is common. One of the most striking
alterations of cell-walls is that termed _carbonization_, in which the
substance gradually turns black, hard and brittle, as if charred--e.g.
_Xylaria_, _Ustulina_, some sclerotia. At the other extreme the
cell-walls of many lichen-fungi are soft and colourless, but turn blue
in iodine, as does starch. The young cell-wall is always tenuous and
flexible, and may remain so throughout, but in many cases thickenings
and structural differentiations, as well as the changes referred to
above, alter the primary wall considerably. Such thickening may be
localized, and _pits_ (e.g. _Uredospores_, septa of Basidiomycetes),
_spirals_, _reticulations_, _rings_, &c. (capillitium fibres of
_Podaxon_, _Calostoma_, _Battarrea_), occur as in the vessels of
higher plants, while sculptured networks, pittings and so forth are as
common on fungus-spores as they are on pollen grains.
_Cell-Contents._--The cells of fungi, in addition to protoplasm,
nuclei and sap-vacuoles, like other vegetable cells, contain formed
and amorphous bodies of various kinds. Among those directly visible to
the microscope are oil drops, often coloured (_Uredineae_) crystals of
calcium oxalate (_Phallus_, _Russula_), proteid crystals (_Mucor_,
_Pilobolus_, &c.) and resin (Polyporei). The oidia of Erysipheae
contain fibrosin bodies and the hyphae of Saprolegnieae cellulin
bodies, but starch apparently never occurs. Invisible to the
microscope, but rendered visible by reagents, are glycogen, _Mucor_,
Ascomycetes, yeast, &c. In addition to these cell-contents we have
good indirect evidence of the existence of large series of other
bodies, such as proteids, carbohydrates, organic acids, alkaloids,
enzymes, &c. These must not be confounded with the numerous substances
obtained by chemical analysis of masses of the fungus, as there is
often no proof of the manner of occurrence of such bodies, though we
may conclude with a good show of probability that some of them also
exist preformed in the living cell. Such are sugars (glucose, mannite,
&c.), acids (acetic, citric and a whole series of lichen-acids),
ethereal oils and resinous bodies, often combined with the intense
colours of fungi and lichens, and a number of powerful alkaloid
poisons, such as muscarin (_Amanita_), ergotin (_Claviceps_), &c.
Among the enzymes already extracted from fungi are _invertases_
(yeasts, moulds, &c.), which split cane-sugar and other complex sugars
with hydrolysis into simpler sugars such as dextrose and levulose;
_diastases_, which convert starches into sugars (_Aspergillus_, &c.);
_cytases_, which dissolve cellulose similarly (_Botrytis_, &c.);
_peptases_, using the term as a general one for all enzymes which
convert proteids into peptones and other bodies (_Penicillium_, &c.);
lipases, which break up fatty oils (_Empusa_, _Phycomyces_, &c.);
oxydases, which bring about the oxidations and changes of colour
observed in _Boletus_, and _zymase_, extracted by Buchner from yeast,
which brings about the conversion of sugar into alcohol and
carbon-dioxide. That such enzymes are formed in the protoplasm is
evident from the behaviour of hyphae, which have been observed to
pierce cell-membranes, the chitinous coats of insects, artificial
collodion films and layers of wax, &c. That a fungus can secrete more
than one enzyme, according to the materials its hyphae have to attack,
has been shown by the extraction of diastase, inulase, trehalase,
invertase, maltase, raffinase, malizitase, emulsin, trypsin and lipase
from _Aspergillus_ by Bourquelot, and similar events occur in other
fungi. The same fact is indicated by the wide range of organic
substances which can be utilized by _Penicillium_ and other moulds,
and by the behaviour of parasitic fungi which destroy various
cell-contents and tissues. Many of the coloured pigments of fungi are
fixed in the cell-walls or excreted to the outside (_Peziza
aeruginosa_). Matruchot has used them for staining the living
protoplasm of other fungi by growing the two together. Striking
instances of coloured mycella are afforded by _Corticium sanguineum_,
blood-red; _Elaphomyces Leveillei_, yellow-green; _Chlorosplenium
aeruginosum_, verdigris green; and the _Dematei_, brown or black.
_Nuclei._--Although many fungi have been regarded as devoid of nuclei,
and all have not as yet been proved to contain them, the numerous
investigations of recent years have revealed them in the cells of all
forms thoroughly examined, and we are justified in concluding that the
nucleus is as essential to the cell of a fungus as to that of other
organisms. The hyphae of many contain numerous, even hundreds of
nuclei (Phycomycetes); those of others have several (_Aspergillus_) in
each segment, or only two (_Exoascus_) or one (_Erysiphe_) in each
cell. Even the isolated cells of the yeast plant have each one
nucleus. As a rule the nuclei of the mycelium are very minute (1.5-2
[mu] in _Phycomyces_), but those of many asci and spores are large and
easily rendered visible. As with other plants, so in fungi the
essential process of fertilization consists in the fusion of two
nuclei, but owing to the absence of well-marked sexual organs from
many fungi, a peculiar interest attaches to certain nuclear fusions in
the vegetative cells or in young spores of many forms. Thus in
Ustilagineae the chlamydospores, and in Uredineae the teleutospores,
each contain two nuclei when young, which fuse as the spores mature.
In young asci a similar fusion of two nuclei occurs, and also in
basidia, in each case the nucleus of the ascus or of the basidium
resulting from the fusion subsequently giving rise by division to the
nuclei of the ascospores and basidiospores respectively. The
significance of these fusions will be discussed under the various
groups. Nuclear division is usually accompanied by all the essential
features of karyokinesis.
_Spores._--No agreement has ever been arrived at regarding the
consistent use of the term spore. This is apparently owing to the
facts that too much has been attempted in the definition, and that
differences arise according as we aim at a morphological or a
physiological definition. Physiologically, any cell or group of cells
separated off from a hypha or unicellular fungus, and capable of
itself growing out--germinating--to reproduce the fungus, is a spore;
but it is evident that so wide a definition does not exclude the
ordinary vegetative cells of sprouting fungi, such as yeasts, or small
sclerotium like cell-aggregates of forms like _Coniothecium_.
Morphologically considered, spores are marked by peculiarities of
form, size, colour, place of origin, definiteness in number, mode of
preparation, and so forth, such that they can be distinguished more or
less sharply from the hyphae which produce them. The only
physiological peculiarity exhibited in common by all spores is that
they germinate and initiate the production of a new fungus-plant.
Whether a spore results from the sexual union of two similar gametes
(zygospore) or from the fertilization of an egg-cell by the protoplasm
of a male organ (oospore); or is developed asexually as a motile
(zoospore) or a quiescent body cut off from a hypha (conidium) or
developed along its course (oidium or chlamydospore), or in its
protoplasm (endospore), are matters of importance which have their
uses in the classification and terminology of spores, though in many
respects they are largely of academic interest.
[Illustration: FIG. 2.--_Peronospora parasitica_ (De Bary).
Conidiophore with conidia.]
Klebs has attempted to divide spores into three categories as follows:
(1) kinospores, arising by relatively simple cell-divisions and
subserving rapid dissemination and propagation, e.g. zoospores,
conidia, endogonidia, stylospores, &c.; (2) paulospores, due to simple
rearrangement of cell-contents, and subserving the persistence of the
fungus through periods of exigency, e.g. gemmae, chlamydospores,
resting-cells, cysts, &c.; (3) carpospores, produced by a more or less
complex formative process, often in special fructifications, and
subserving either or both multiplication and persistence, e.g.
zygospores, oospores, brand-spores, aecidiospores, ascospores,
basidiospores, &c. Little or nothing is gained by these definitions,
however, which are especially physiological. In practice these various
kinds of spores of fungi receive further special names in the separate
groups, and names, moreover, which will appear, to those unacquainted
with the history, to have been given without any consistency or regard
to general principles; nevertheless, for ordinary purposes these names
are far more useful in most cases, owing to their descriptive
character, than the proposed new names, which have been only partially
accepted.
_Sporophores._--In some of the simpler fungi the spores are not borne
on or in hyphae which can be distinguished from the vegetative parts
or mycelium, but in the vast majority of cases the sporogenous hyphae
either ascend free into the air or radiate into the surrounding water
as distinct branches, or are grouped into special columns, cushions,
layers or complex masses obviously different in colour, consistency,
shape and other characters from the parts which gather up and
assimilate the food-materials. The term "receptacle" sometimes applied
to these spore-bearing hyphae is better replaced by sporophore. The
sporophore is obsolete when the spore-bearing hyphae are not sharply
distinct from the mycelium, simple when the constituent hyphae are
isolated, and compound when the latter are conjoined. The chief
distinctive characters of the sporogenous hyphae are their
orientation, usually vertical; their limited apical growth; their
peculiar branching, form, colour, contents, consistency; and their
spore-production. According to the characters of the last, we might
theoretically divide them into conidiophores, sporangiophores,
gametophores, oidiophores, &c.; but since the two latter rarely occur,
and more than one kind of spore or spore-case may occur on a
sporophore, it is impossible to carry such a scheme fully into
practice.
A simple sporophore may be merely a single short hypha, the end of
which stops growing and becomes cut off as a conidium by the formation
of a septum, which then splits and allows the conidium to fall. More
generally the hypha below the septum grows forwards again, and repeats
this process several times before the terminal conidium falls, and so
a chain of conidia results, the oldest of which terminates the series
(_Erysiphe_); when the primary branch has thus formed a basipetal
series, branches may arise from below and again repeat this process,
thus forming a tuft (_Penicillium_). Or the primary hypha may first
swell at its apex, and put forth a series of short peg-like branches
(_sterigmata_) from the increased surface thus provided, each of which
develops a similar basipetal chain of conidia (_Aspergillus_), and
various combinations of these processes result in the development of
numerous varieties of exquisitely branched sporophores of this type
(_Botrytis_, _Botryosporium_, _Verticillium_, &c.).
[Illustration: FIG. 3.--_Cystopus candidus_.
A. a, Conidia.
b, Conidiophores.
c, Conidium emitting zoospores.
d, Free zoospore.
B.og, Oogonium.
os, Oosphere.
an, Antheridium.
C. Formation of zoospores by oospores.
z, Free zoospores. (After De Bary.)]
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.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.
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).]
[Illustration: FIG. 5.--_Phytophthora infestans_. Fungus of Potato
Disease.
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.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.
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).