are still delicate, more flexuous, with much thinner walls, and never
invested with an external cortical layer. One of the most important
and highly developed genera is _Peronospora_, the members of which are
parasitic upon and destructive of living vegetables. It is to this
genus that the mould of the too famous potato disease belongs.
Professor De Bary has done more than any other mycologist in the
investigation and elucidation of this genus; and his monograph is a
masterpiece in its way.[o] He was, however, preceded by Mr. Berkeley,
and more especially by Dr. Montagne, by many years in elucidation of
the structure of the flocci and conidia in a number of species.[p] In
this genus, there is a delicate mycelium, which penetrates the
intercellular passages of living plants, giving rise to erect branched
threads, which bear at the tips of their ultimate ramuli, sub-globose,
ovate, or elliptic spores, or, as De Bary terms them--conidia. Deeply
seated on the mycelium, within the substance of the foster plant,
other reproductive bodies, called oogonia, originate. These are
spherical, more or less warted and brownish, the contents of which
become differentiated into vivacious zoospores, capable, when
expelled, of moving in water by the aid of vibratile cilia. A similar
structure has already been indicated in _Cystopus_, otherwise it is
rare in fungi, if the _Saprolegniei_ be excluded. In _Botrytis_ and in
_Polyactis_, the flocci and spores are similar, but the branches of
the threads are shorter and more compact, and the septa are more
common and numerous; the oogonia also are absent. De Bary has selected
_Polyactis cinerea_, as it occurs on dead vine leaves, to illustrate
his views of the dualism which he believes himself to have discovered
in this species. "It spreads its mycelium in the tissue which is
becoming brown," he writes, "and this shows at first essentially the
same construction and growth as that of the mycelium filaments of
_Aspergillus_." On the mycelium soon appear, besides those which are
spread over the tissue of the leaves, strong, thick, mostly
fasciculate branches, which stand close to one another, breaking forth
from the leaf and rising up perpendicularly, the conidia-bearers. They
grow about 1 _mm._ long, divide themselves, by successively rising
partitions, into some prominent cylindrical linked cells, and then
their growth is ended, and the upper cell produces near its point
three to six branches almost standing rectangularly. Of these the
under ones are the longest, and they again shoot forth from under
their ends one or more still shorter little branches. The nearer they
are to the top, the shorter are the branches, and less divided; the
upper ones are quite branchless, and their length scarcely exceeds the
breadth of the principal stem. Thus a system of branches appears, upon
which, on a small scale, a bunch of grapes is represented. All the
twigs soon end their growth; they all separate their inner space from
the principal stem, by means of a cross partition placed close to it.
All the ends, and also that of the principal stem, swell about the
same time something like a bladder, and on the upper free half of each
swelling appear again, simultaneously, several fine protuberances,
close together, which quickly grow to little oval bladders filled with
protoplasm, and resting on their bearers with a sub-sessile,
pedicellate, narrow basis, and which at length separate themselves
through a partition as in _Aspergillus_. The detached cells are the
conidia of our fungus; only one is formed on each stalk. When the
formation is completed in the whole of the panicle, the little
branches which compose it are deprived of their protoplasm in favour
of the conidia; it is the same with the under end of the principal
stem, the limits of which are marked by a cross partition. The
delicate wall of these parts shrinks up until it is unrecognizable;
all the conidia of the panicle approach one another to form an
irregular grape-like bunch, which rests loosely on the bearer, and
from which it easily falls away as dust. If they be brought into water
they fall off immediately; only the empty, shrivelled, delicate skins
are to be found on the branch which bore them, and the places on which
they are fixed to the principal stem clearly appear as round
circumscribed hilums, generally rather arched towards the exterior.
The development of the main stem is not ended here. It remains solid
and filled with protoplasm as far as the portion which forms the end
through its conidia. Its end, which is to be found among these pieces,
becomes pointed after the ripening of the first panicle, pushes the
end of the shrivelled member on one side, and grows to the same length
as the height of one or two panicles, and then remains still, to form
a second panicle similar to the first. This is later equally
perfoliated as the first, then a third follows, and thus a large
number of panicles are produced after and over one another on the same
stem. In perfect specimens, every perfoliated panicle hangs loosely to
its original place on the surface of the stem, until by shaking or the
access of water to it, it falls immediately into the single conidia,
or the remains of branches, and the already-mentioned oval hilums are
left behind. Naturally, the stem becomes longer by every perfoliation;
in luxuriant specimens the length can reach that of some lines. Its
partition is already, by the ripening of the first panicle from the
beginning of its foundation, strong and brown; it is only colourless
at the end which is extending, and in all new formations. During all
these changes the filament remains either unbranched, except as
regards the transient panicles, or it sends out here and there, at the
perfoliated spots, especially from the lower ones, one or two strong
branches, standing opposite one another and resembling the principal
stem.
[Illustration: FIG. 28.--_Polyactis cinerea._ _a._ Apex of hypha.]
The mycelium, which grows so exuberantly in the leaf, often brings
forth many other productions, which are called _sclerotia_, and are,
according to their nature, a thick bulbous tissue of mycelium
filaments. Their formation begins with the profuse ramification of the
mycelium threads in some place or other; generally, but not always, in
the veins of the leaf; the intertwining twigs form an uninterrupted
cavity, in which is often enclosed the shrivelling tissue of the leaf.
The whole body swells to a greater thickness than that of the leaf,
and protrudes on the surface like a thickened spot. Its form varies
from circular to fusiform; its size is also very unequal, ranging
between a few lines and about half a millimetre in its largest
diameter. At first it is colourless, but afterwards its outer layers
of cells become round, of a brown or black colour, and it is
surrounded by a black rind, consisting of round cells, which separate
it from the neighbouring tissue. The tissue within the rind remains
colourless; it is an entangled uninterrupted tissue of fungus
filaments, which gradually obtain very solid, hard, cartilaginous
coats. The sclerotium, which ripens as the rind becomes black, loosens
itself easily from the place of its formation, and remains preserved
after the latter is decayed.
[Illustration: FIG. 29.--_Peziza Fuckeliana._ _a._ Natural size. _b._
Section enlarged. _c._ Ascus and sporidia.]
The sclerotia are, here as in many other fungi, biennial organs,
designed to begin a new vegetation after a state of apparent quietude,
and to send forth special fruit-bearers. They may in this respect be
compared to the bulbs and perennial roots of under shrubs. The usual
time for the development of the sclerotia is late in the autumn,
after the fall of the vine leaves. As long as the frost does not set
in, new ones continually spring up, and each one attains to ripeness
in a few days. If frost appears, it can lie dry a whole year, without
losing its power of development. This latter commences when the
sclerotium is brought into contact with damp ground during the usual
temperature of our warmer seasons. If this occur soon, at the latest
some weeks after it is ripe, new vegetation grows very quickly,
generally after a few days; in several parts the colourless filaments
of the inner tissue begin to send out clusters of strong branches,
which, breaking through the black rind, stretch themselves up
perpendicularly towards the surface, separate from one another, and
then take all the characteristics of the conidia-bearers. Many such
clusters can be produced on one sclerotium, so that soon the greater
part of the surface is covered by filamentous conidia-bearers with
their panicles. The colourless tissue of the sclerotium disappears in
the same degree as the conidia-bearers grow, and at last the black
rind remains behind empty and shrivelled. If we bring, after many
months, for the first time, the ripe sclerotium, in damp ground, in
summer or autumn, after it has ripened, the further development takes
place more slowly, and in an essentially different form. It is true
that from the inner tissue numerous filamentous branches shoot forth
at the cost of this growing fascicle, and break through the black
rind, but its filaments remain strongly bound, in an almost parallel
situation, to a cylindrical cord, which for a time lengthens itself
and spreads out its free end to a flat plate-like disc. This is always
formed of strongly united threads, ramifications of the cylindrical
cord. On the free upper surface of the disc, the filaments shoot forth
innumerable branches, which, growing to the same height, thick and
parallel with one another, cover the before-named disc. Some remain
narrow and cylindrical, are very numerous, and produce fine hairs
(paraphyses); others, also very numerous, take the form of club-like
ampulla cells, and each one forms in its interior eight free swimming
oval spores. Those ampulla cells are sporidiiferous asci. After the
spores have become ripe, the free point of the utricle bursts, and the
spores are scattered to a great distance by a mechanism which we will
not here further describe. New ampullas push themselves between those
which are ripening and withering; a disc can, under favourable
circumstances, always form new asci for weeks at a time. The number of
the already described utricle-bearers is different, according to the
size of the sclerotium; smaller specimens usually produce only one,
larger two to four. The size is regulated by that of the sclerotia,
and ranges, in full-grown specimens, between one and more millimetres
for the length of the stalk, and a half to three (seldom more)
millimetres for the breadth of the disc.[q] For some time the conidia
form, belonging to the Mucedines, has been known as _Botrytis cinerea_
(or _Polyactis cinerea_). The compact mycelium, or sclerotium, as an
imperfect fungus, bore the name of _Sclerotium echinatum_, whilst to
the perfect and cup-like form has been given the name of _Peziza
Fuckeliana_. We have reproduced De Bary's life-history of this mould
here, as an illustration of structure in the _Mucedines_, but
hereafter we shall have to write of similar transformations when
treating of polymorphism.
The form of the threads, and the form and disposition of the spores,
vary according to the genera of which this order is composed. In
_Oidium_ the mostly simple threads break up into joints. Many of the
former species are now recognized as conditions of _Erysiphe_. In
_Aspergillus_, the threads are simple and erect, with a globose head,
around which are clustered chains of simple spores. In _Penicillium_,
the lower portion of the threads is simple, but they are shortly
branched at the apex, the branches being terminated by necklaces of
minute spores. In _Dactylium_, the threads are branched, but the
spores are collected in clusters usually, and are moreover septate. In
other genera similar distinctions prevail. These two groups of black
moulds and white moulds are the noblest, and contain the largest
number of genera and species amongst the _Hyphomycetes_. There is,
however, the small group of _Isariacei_, in which the threads are
compacted, and a semblance of such hymenomycetal forms as _Clavaria_
and _Pterula_ is the result, but it is doubtful if this group contains
many autonomous species. In another small group, the _Stilbacei_,
there is a composite character in the head, or receptacle,[r] and in
the stem when the latter is present. Many of these, again, as
_Tubercularia_, _Volutella_, _Fusarium_, &c., contain doubtful
species. In _Sepedoniei_ and _Trichodermacei_, the threads are reduced
to a minimum, and the spores are such a distinctive element that
through these groups the _Hyphomycetes_ are linked with the
_Coniomycetes_. These groups, however, are not of sufficient size or
importance to demand from us, in a work of this character, anything
more than the passing allusion which we have given to them.
[Illustration: FIG. 30.--_Penicillium chartarum_, Cooke.]
We come now to consider the structure in the Sporidiifera, in which
the fructifying corpuscles or germs, whether called spores or
sporidia, are generated within certain privileged cysts, usually in
definite numbers. In systematic works, these are included under two
orders, the _Physomycetes_ and the _Ascomycetes_. The former of these
consists of cyst-bearing moulds, and from their nearest affinity to
the foregoing will occupy the first place.
[Illustration: FIG. 31.--_Mucor mucedo_, with three sporangia. _a._
Portion of frill with sporangiola.]
PHYSOMYCETES include, especially amongst the _Mucorini_, many most
interesting and instructive species for study, which even very lately
have occupied the attention of continental mycologists. Most of these
phenomena are associated more or less with reproduction, and as such
will have to be adverted to again, but there are points in the
structure which can best be alluded to here. Again taking Professor de
Bary's researches as our guide,[s] we will illustrate this by the
common _Mucor mucedo_: If we bring quite fresh horse-dung into a damp
confined atmosphere, for example, under a bell-glass, there appears on
its surface, after a few days, an immense white mildew. Upright strong
filaments of the breadth of a hair raise themselves over the surface,
each of them soon shows at its point a round little head, which
gradually becomes black, and a closer examination shows us that in all
principal points it perfectly agrees with the sporangia of other
species. Each of these white filaments is a sporangia-bearer. They
spring from a mycelium which is spread in the dung, and appear singly
upon it. Certain peculiarities in the form of the sporangium, and the
little long cylindrical spores, which, when examined separately, are
quite flat and colourless, are characteristic of the species. If the
latter be sown in a suitable medium, for example, in a solution of
sugar, they swell, and shoot forth germinating utricles, which quickly
grow to mycelia, which bear sporangia. This is easily produced on the
most various organic bodies, and _Mucor mucedo_ is therefore found
spontaneously on every substratum which is capable of nourishing
mildew, but on the above-named the most perfect and exuberant
specimens are generally to be found. The sporangia-bearers are at
first always branchless and without partitions. After the sporangium
is ripe, cross partitions in irregular order and number often appear
in the inner space, and on the upper surface branches of different
number and size, each of which forms a sporangium at its point. The
sporangia which are formed later are often very similar, but sometimes
very different, to those which first appeared, because their partition
is very thick and does not fall to pieces when it is ripe, but
irregularly breaks off, or remains entire, enclosing the spores, and
at last falls to the ground, when the fungus withers. The cross
partition which separates the sporangia from its bearers is in those
which are first formed (which are always relatively thicker sporangia)
very strongly convex, while those which follow later are often
smaller, and in little weak specimens much less arched, and sometimes
quite straight. After a few days, similar filaments generally show
themselves on the dung between the sporangia-bearers, which appear to
the naked eye to be provided with delicate white frills. Where such an
one is to be found, two to four rectangular expanding little branches
spring up to the same height round the filament. Each of these, after
a short and simple process, branch out into a furcated form; the
furcations being made in such a manner that the ends of the branch at
last so stand together that their surface forms a ball. Finally, each
of the ends of a branch swells to a little round sporangium, which is
limited by a partition (called sporangiolum, to distinguish it from
the larger ones), in which some, generally four, spores are formed in
the manner already known. When the sporangiola are alone, they have
such a peculiar appearance, with their richly-branched bearers, that
they can be taken for something quite different to the organs of the
_Mucor mucedo_, and were formerly not considered to belong to it. That
they really belong to the _Mucor_ is shown by the principal filament
which it bears, not always, but very often, ending with a large
sporangium, which is characteristic of the _Mucor mucedo_; it is still
more evident if we sow the spores of the sporangiolum, for, as it
germinates, a mycelium is developed, which, near a simple bearer, can
form large sporangia, and those form sporangiola, the first always
considerably preponderating in number, and very often exclusively. If
we examine a large number of specimens, we find every possible middle
form between the simple or less branched sporangia-bearers and the
typical sporangiola frills; and we arrive at last at the conclusion
simply to place the latter among the varieties of form which the
sporangia-bearer of the _Mucor mucedo_ shows, like every other typical
organic form within certain limits. On the other hand, propagation
organs, differing from those of the sporangia and their products,
belong to _Mucor mucedo_, which may be termed conidia. On the dung
(they are rare on any other substance) these appear at the same time,
or generally somewhat later, than the sporangia-bearers, and are not
unlike those to the naked eye. In a more accurate examination, they
appear different; a thicker, partition-less filament rises up and
divides itself, generally three-forked, at the length of one
millimetre, into several series of branchlets. The forked branches of
the last series bear under their points, which are mostly capillary,
short erect little ramuli, and these, with which the ends of the
principal branches articulate on their somewhat broad tops, several
spores and conidia, near one another; about fifteen to twenty are
formed at the end of each little ramulus. The peculiarities and
variations which so often appear in the ramification need not be
discussed here. After the articulation of the conidia, their bearers
sink together by degrees, and are quite destroyed. The ripe conidia
are round like a ball, their surface is scarcely coloured, and almost
wholly smooth. These conidioid forms were at first described as a
separate species under the name of _Botrytis Jonesii_. How, then, do
they belong to the _Mucor_?[t] That they appear gregariously is as
little proof of an original relation to one another, here as
elsewhere. Attempts to prove that the conidia and sporangia-bearers
originate on one and the same mycelium filament may possibly hereafter
succeed. Till now this has not been the case, and he who has ever
tried to disentangle the mass of filaments which exuberantly covers
the substratum of a _Mucor_ vegetation, which has reached so far as to
form conidia, will not be surprised that all attempts have hitherto
proved abortive. The suspicion of the connection founded on the
gregariously springing up, and external resemblance, is fully
justified, if we sow the conidia in a suitable medium, for example, in
a solution of sugar. They here germinate and produce a mycelium which
exactly resembles that of the _Mucor mucedo_, and, above all, they
produce in profusion the typical sporangia of the same on its bearers.
The latter are till now alone reproductions of conidia-bearers, and
have never been observed on mycelia which have grown out of conidia.
[Illustration: FIG. 32.--Small portion of _Botrytis Jonesii_.]
These phenomena of development appear in the _Mucor_ when it dwells on
a damp substance, which must naturally contain the necessary
nourishment for it, and is exposed to the atmospheric air. Its
mycelium represents at first strong branched utricles without
partitions; the branches are of the higher order, mostly divided into
rich and very fine-pointed ramuli. In old mycelium, and also in the
sporangia-bearers, the contents of which are mostly used for the
formation of spores, and the substratum of which is exhausted for our
fungus, short stationary pieces, filled with protoplasm, are very
often formed into cells through partitions in order to produce spores,
that is, grow to a new fruitful mycelium. These cells are called
gemmules, brooding cells, and resemble such vegetable buds and sprouts
of foliaceous plants which remain capable of development after the
organs of vegetation are dead, in order to grow, under suitable
circumstances, to new vegetating plants, as, for example, the bulbs of
onions, &c.
If we bring a vegetating mycelium of _Mucor mucedo_ into a medium
which contains the necessary nourishment for it, but excluded from the
free air, the formation of sporangia takes place very sparingly or not
at all, but that of gemmules is very abundant. Single interstitial
pieces of the ramuli, or even whole systems of branches, are quite
filled with a rich greasy protoplasm; the short pieces and ends are
bound by partitions which form particular, often tun-like or globular
cells; the longer ones are changed, through the formation of cross
partitions, into chains of similar cells; the latter often attain by
degrees strong, thick walls, and their greasy contents often pass into
innumerable drops of a very regular globular form and of equal size.
Similar appearances show themselves after the sowing of spores, which
are capable of germinating in the medium already described, from which
the air is excluded. Either short germinating utricles shoot forth,
which soon form themselves into rows of gemmules, or the spores swell
to large round bladders filled with protoplasm, and shoot forth on
various parts of their surface innumerable protuberances, which,
fixing themselves with a narrow basis, soon become round vesiculate
cells, and on which the same sprouts which caused their production are
repeated, formations which remind us of the fungus of fermentation
called globular yeast. Among all the known forms of gemmules we find a
variety which are intermediate, all of which show, when brought into a
normal condition of development, the same proportion, and the same
germination, as those we first described.
We have detailed rather at length the structure and development of one
of the most common of the Mucors, which will serve as an illustration
of the order. Other distinctions there may be which are of more
interest as defining the limits of genera, except such as may be
noticed when we come to write more specially of reproduction.
ASCOMYCETES.--Passing now to the _Ascomycetes_, which are especially
rich in genera and species, we must first, and but superficially,
allude to _Tuberacei_, an order of sporidiiferous fungi of subterranean
habit, and rather peculiar structure.[u] In this order an external
stratum of cells forms a kind of perithecium, which is more or less
developed in different genera. This encloses the hymenium, which is
sinuous, contorted, and twisted, often forming lacunæ. The hymenium in
some genera consists of elongated, nearly cylindrical asci, enclosing a
definite number of sporidia; in the true truffles and their
immediate allies, the asci are broad sacs, containing very large and
beautiful, often coloured, sporidia. These latter have either a
smooth, warted, spinulose, or lacunose epispore, and, as will be seen
from the figures in Tulasne's Monograph,[v] or those in the last
volume of Corda's great work,[w] are attractive microscopical objects.
In some cases, it is not difficult to detect paraphyses, but in
others they would seem to be entirely absent. A comparatively large
number have been discovered and recorded in Great Britain,[x] but of
those none are more suitable for study of general structure than the
ordinary truffle of the markets.
The structure of the remaining Ascomycetes can be studied under two
groups, _i.e._, the fleshy Ascomycetes, or, as they have been termed,
the Discomycetes, and the hard, or carbonaceous Ascomycetes,
sometimes called the Pyrenomycetes. Neither of these names gives an
accurate idea of the distinctions between the two groups, in the
former of which the discoid form is not universal, and the latter
contains somewhat fleshy forms. But in the Discomycetes the
hymenium soon becomes more or less exposed, and in the latter it is
enclosed in a perithecium. The Discomycetes are of two kinds, the
pileate and the cup-shaped. Of the pileate such a genus as _Gyromitra_
or _Helvella_ is, in a certain sense, analogous to the Agarics
amongst _Hymenomycetes_, with a superior instead of an inferior
hymenium, and enclosed, not naked, spores. Again, _Geoglossum_ is
somewhat analogous to _Clavaria_. Amongst the cup-shaped, _Peziza_
is an Ascomycetous _Cyphella_. But these are perhaps more fanciful
than real analogies.
Recently Boudier has examined one group of the cup-shaped Discomycetes,
the _Ascobolei_, and, by making a somewhat free use of his Memoir,[y] we
may arrive at a general idea of the structure in the cupulate
Discomycetes. They present themselves at first under the form of a
small rounded globule, and almost entirely cellular. This small
globule, the commencement of the receptacle, is not long in increasing,
preserving its rounded form up to the development of the asci. At this
period, under the influence of the rapid growth of these organs, it soon
produces at its summit a fissure of the external membrane, which
becomes a more marked depression in the marginate species. The
receptacle thus formed increases rapidly, becomes plane, more convex,
or more or less undulated at the margin, if at all of large size.
Fixed to the place where it is generated by some more or less
abundant mycelioid filaments, the receptacle becomes somewhat
cup-shaped and either stipitate or sessile, composed of the
receptacle proper and the hymenium.
[Illustration: FIG. 33.--Section of cup of _Ascobolus_. _a._ External
cells. _b._ Secondary layer. _c._ Subhymenial tissue (Janczenski).]
The receptacle proper comprehends the subhymenial tissue, the
parenchyma, and the external membrane. The subhymenial tissue is
composed of small compact cells, forming generally a more coloured and
dense stratum, the superior cells of which give rise to the asci and
paraphyses. The parenchyma is seated beneath this, and is generally of
interlaced filaments, of a looser consistency than the preceding,
united by intermediate cellules. The external membrane, which
envelopes the parenchyma, and limits the hymenium, differs from the
preceding by the cells often being polyhedric, sometimes transverse,
and united together, and sometimes separable. Externally it is
sometimes smooth, and sometimes granular or hairy.