suspected, although in earlier instances upon insufficient grounds; but
of late years observations have multiplied and facts accumulated which
leave no doubt of its existence. If the _Saprolegniæ_ are left out of the
question as disputed Fungi, there still remain a number of well
authenticated instances of the phenomena of copulation, and many other
facts which indicate some sort of sexual relationship. The precise
manner in which those minute bodies, so common amongst the _Sphæronemei_,
which we prefer to call stylospores, perform their functions is still to a
great extent a mystery; yet it is no longer doubted that certain species
of _Aposphæria_, _Phoma_, _Septoria_, &c., are only conditions of some
species of _Sphæria_, often developed and matured in close proximity to
them on the same host. In _Æcidium_, _Roestelia_, &c., spermogonia are
produced plentifully on or near the same spots on which the
fructification appears, either simultaneously or at a later period.[A] The
relation of _Cytispora_ to _Valsa_ was suspected by Fries very many
years ago, and, as since demonstrated, with very good reason. All
attempts, however, to establish anything like sexual reproduction in
the higher forms of _Hymenomycetes_ have at present been unsuccessful; and
the same may be said of the _Gasteromycetes_; but in _Ascomycetes_ and
_Physomycetes_ instances abound.
We know not whether any importance is to be attached to the views of
M. A. S. Oersted,[B] which have not since been confirmed, but which
have been cited with some approval by Professor de Bary, as to a trace
of sexual organs in _Hymenomycetes_. He is supposed to have seen in
_Agaricus variabilis_, P., oocysts or elongated reniform cells, which
spring up like rudimentary branches of the filaments of the mycelium,
and enclose an abundant protoplasm, if not even a nucleus. At the base
of these oocysts appear the presumed antheridia, that is to say, one
or two slender filaments, which generally turn their extremities
towards the oocysts, and which more rarely are applied to them. Then,
without ulteriorily undergoing any appreciable modifications, the
fertile cell or oocyst becomes enveloped in a network of filaments of
mycelium which proceed from the one which bears it, and this tissue
forms the rudiments of the cap. The reality of some kind of
fecundation in this circumstance, and the mode of the phenomena, if
there is one, are for the present equally uncertain. If M. Oersted's
opinion is confirmed, naturally the whole of the cap will be the
product of fecundation. Probably Karsten (Bonplandia, 1862, p. 62) saw
something similar in _Agaricus campestris_, but his account is
obscure.
[Illustration: FIG. 95.--Zygospore of _Mucor phycomyces_.]
In _Phycomyces_ the organs of reproduction have been subjected to
close examination by Van Tieghem,[C] and although he failed to
discover chlamydospores in this, he describes them in other Mucors. In
this species, besides the regular sexual development, by means of
sporangia, there is a so-called sexual reproduction by means of
zygospores, which takes place in this wise. The threads which
conjugate to form the zygospores are slender and erect on the surface
of the substratum. Two of these threads come into close contact
through a considerable length, and clasp each other by alternate
protuberances and depressions. Some of the protuberances are prolonged
into slender tubes. At the same time the free extremities of the
threads dilate, and arch over one towards the other until their tops
touch like a vice, each limb of which rapidly increases in size. Each
of these arcuate, clavate cells has now a portion of its extremity
isolated by a partition, by means of which a new hemispherical cell is
formed at the end of each thread at its point of junction with the
opposed thread. These cells become afterwards cylindrical by pressure,
the protoplasm is aggregated into a mass, the double membrane at the
point of first contact is absorbed, and the two confluent masses of
protoplasm form a zygospore invested with a tubercular coat and
enveloped by the primary wall of the two conjugating cells. During
this formation of the zygospore, the two arched cells whence the
zygospore originated develop a series of dichotomous processes in
close proximity to the walls which separate them from the zygospore.
These processes appear at first on one of the arcuate cells in
successive order. The first makes its appearance above upon the convex
side; the succeeding ones to the right and left in descending order;
the last is in the concavity beneath. It is only after the development
of this that the first process appears on the opposite cell, which is
followed by others in the same order. These dichotomous processes are
nothing more than branches developed from the arcuate, or mother
cells. During all these changes, while the zygospore enlarges, the
wall of the arcuate cells becomes coloured brown. This colouring is
more marked on the convex side, and it shows itself first in the cell
on which the dichotomous branches are first produced, and which
retains the darker tint longer than the other. The zone from whence
the processes issue, and also the processes themselves, have their
walls blackened deeply, while the walls of the conjugated cells, which
continue to clothe the zygospore during the whole of its development,
are bluish-black. By pressure, the thin brittle coat which envelopes
the zygospore is ruptured, and the coat of the zygospore exposed,
formed of a thick cartilaginous membrane, studded with large irregular
warts.
The germination of the zygospores in this species has not as yet been
observed, but it is probably the same or very similar to that observed
in other species of _Mucor_. In these the rough tuberculate epispore
splits on one side, and its internal coat elongates itself and
protrudes as a tube filled with protoplasm and oil globules,
terminating in an ordinary sporangium. Usually the amount of nutriment
contained in the zygospore is exhausted by the formation of the
terminal sporangium, according to Brefeld;[D] but Van Tieghem and Le
Monnier remark that in their examinations they have often seen a
partition formed at about a third of the length of the principal
filament from the base, below which a strong branch is given off, and
this is also terminated by a large sporangium.
[Illustration: FIG. 96.--Zygospore of _Rhizopus_ in different stages. (De
Bary.)]
De Bary has given a precise account of the formation of the zygospore
in another of the Mucors, _Rhizopus nigricans_, in which he says that
the filaments which conjugate are solid rampant tubes, which are
branched without order and confusedly intermingled. Where two of these
filaments meet each of them pushes towards the other an appendage
which is at first cylindrical and of the same diameter. From the first
these two processes are applied firmly one to the other by their
extremities; they increase in size, become clavate, and constitute
together a fusiform body placed across the two conjugated filaments.
Between the two halves of this body there exists no constant
difference of size; often they are both perfectly equal. In each
there is collected an abundance of protoplasm, and when they have
attained a certain development the largest extremity of each is
isolated by a septum from the clavule, which thus becomes the support
or suspender of the copulative cell. The two conjugated cells of the
fusiform body are generally unequal; the one is a cylinder as long as
it is broad, the other is disciform, and its length is only equal to
half its breadth. The primitive membrane of the clavule forms between
the copulative cells a solid partition of two membranes, but soon
after the cells have become defined the medial partition becomes
pierced in the centre, and then soon entirely disappears, so that the
two twin cells are confounded in one single zygospore, which is due to
the union of two more or less similar utricles. After its formation
the zygospore still increases considerably in size, and acquires a
diameter of more than one-fifth of a millimetre. Its form is generally
spherical, and flattened on the faces which are united to the
suspenders, or it resembles a slightly elongated cask. The membrane
thickens considerably, and consists at the time of maturity of two
superposed integuments; the exterior or epispore is solid, of a dark
blackish-blue colour, smooth on the plane faces in contact with the
suspenders, but covered everywhere else with thick warts, which are
hollow beneath. The endospore is thick and composed of several layers,
colourless, and covered with warts, which correspond and fit into
those of the epispore. The contents of the zygospore are a coarsely
granular protoplasm, in which float large oleaginous drops. While the
zygospore is increasing in size, the suspender of the smaller
copulative cell becomes a rounded and stipitate utricle, often divided
at the base by a septum, and which attains almost to the size of the
zygospore. The suspender of the larger copulative cell preserves its
primitive form and becomes scarcely any larger. It is rare that there
is not a considerable difference of size between the two conjugated
cells and the suspenders.[E]
Similar conjugation with like results also takes place in _Syzygites
megalocarpus_. In this species the germination of the zygospores has
been observed. If, after a certain time of repose, these bodies are
placed on a moist substratum, they emit a germ-like tube, which,
without originating a proper mycelium, develops at the expense of the
nutritive material stored in the zygospore into a carpophore or fruit
bearer, which is many times dichotomously branched, bearing terminal
sporangia characteristic of the species.
It has already been remarked by us that the _Saprolegnei_ are claimed
by some authors as Algæ, whilst we are more disposed to regard them as
closely allied to the Mucors, and as they exhibit in themselves strong
evidence in support of the existence of sexual reproduction, we cannot
forbear giving a summary of what has been observed by De Bary and
others in this very interesting and singular group of plants, to which
M. Cornu has recently dedicated an exhaustive monograph.[F]
In _Saprolegnia monoica_, and others, the female organs consist of
oogonia--that is to say, of cells which are at first globose and rich
in plastic matter, which most generally terminate short branches of
the mycelium, and which are rarely seen in an interstitial position.
The constitutive membrane of the adult oogonia is reabsorbed in a
great many points, and is there pierced with rounded holes. At the
same time the plasma is divided into a larger or smaller number of
distinct portions, which are rounded into little spheres, and separate
from the walls of the conceptacle in order to group themselves at the
centre, where they float in a watery fluid. These gonospheres are then
smooth and bare, with no membrane on their surface of the nature of
cellulose.
[Illustration: FIG. 97.--Conjugation in _Achlya racemosa_. (Cornu.)]
During the formation of the oogonia there arise from its pedicel
or from neighbouring filaments slight cylindrical curved branches,
sometimes turned round the support of the oogonia, and which all tend
towards this organ. Their superior extremity is intimately applied
to its wall, then ceases to be elongated, becomes slightly inflated,
and is limited below by a partition; it is then an oblong cell,
slightly curved, filled with protoplasm, and intimately applied to
the oogonia--in fact, an antheridium or organ of the male sex.
Each oogonium possesses one or several antheridia. Towards the time
when the gonospheres are formed it may be observed that each
antheridium sends to the interior of the oogonia one or several
tubular processes, which have crossed its side wall, and which open
at their extremity in order to discharge their contents. These,
while they are flowing out, present some very agile corpuscles, and
which, considering their resemblance to those in _Vaucheria_, to
which the name of spermatozoids are applied, ought to be considered
as the fecundating corpuscles. After the evacuation of the antheridia
the gonospheres are found to be covered with cellulose; they then
constitute so many oospores, with solid walls. De Bary considers
that, bearing in mind analogous phenomena observed in _Vaucheria_, and
the direct observations of Pringsheim,[G] the cellulose membrane on
the surface of the gonospheres is only the consequence of a sexual
fecundation.
In _Achlya dioica_ the antheridium is cylindrical, the plasma which it
encloses is divided into particles, which attain nearly the size of
the zoospores of the same plant. These particles become globose
cells, grouped in the centre of the antheridium. Afterwards the
contents of these latter cells become divided into numerous bacillary
spermatozoids, which first break the wall of their mother cell, and
then issue from the antheridium. These rod-like corpuscles, which
resemble the spermatozoids in _Vaucheria_, have their movements
assisted by a long cilium. It is presumable that here, as in the
Algæ, the spermatozoids introduce themselves into the cavity of
the oogonium, and unite with the gonospheres.
Amongst obscure and doubtful bodies are those described by Pringsheim,
which have their origin in thick filaments or tubes, similar to those
which form the zoosporangia, and represent so many distinct little
masses of plasma within an homogeneous parietal ganglion. The contour
of these plastic masses is soon delineated in a more precise manner.
We see in their interior some homogeneous granules, which are at first
globose, then oval, and finally travel to the enlarged and ampullæform
extremity of the generating tube. There they become rounded or oval
cells covered with cellulose, and emit from their surface one or
several cylindrical processes, which elongate towards the wall of the
conceptacle, and pierce it, without, however, ever projecting very far
beyond it. At the same time the lacunose protoplasm of each cell
becomes divided into a number of corpuscles, which escape by the open
extremity of the cylindrical neck. They resemble in their organization
and agility the spermatozoids of _Achlya dioica_. They soon become
motionless in water, and do not germinate. During the development of
these organs, the protoplasm of the utricle which contains them offers
at first completely normal characteristics, and disappears entirely by
degrees as they increase. De Bary and Pringsheim believe that these
organs constitute the antheridia of the species of _Saprolegnia_ to
which they belong.
The oospores of the _Saprolegniæ_, when arrived at maturity, possess a
tolerably thick double integument, consisting of an epispore and an
endospore. After a considerable time of repose they give rise to
tubular or vesicular germs, which, without being much elongated,
produce zoospores.[H]
De Bary has claimed for the oogonia in _Cystopus_ and _Peronospora_ a
kind of fecundation which deserves mention here.[I] These same fruits,
he says, which owe their origin to sexual organs, should bear the
names of _oogonia_ and _antheridia_, according to the terminology
proposed by Pringsheim for analogous organs in the Algæ. The formation
of the oogonia, or female organs, commences by the terminal or
interstitial swelling of the tubes of the mycelium, which increase and
take the form of large spherical or oboval cells, and which separate
themselves by septa from the tube which carries them. Their membrane
encloses granules of opaque protoplasm, mingled with numerous bulky
granules of colourless fatty matter.
[Illustration: FIG. 98.--Conjugation in _Peronospora; a. antheridium_.
(De Bary.)]
The branches of the mycelium which do not bear oogonia apply their
obtuse extremities against the growing oogonia; this extremity swells,
and, by a transverse partition, separates itself from the supporting
tube. It is the antheridium, or male organ, which is formed by this
process; it takes the form of an obliquely clavate or obovate cellule,
which is always considerably smaller than the oogonium, and adheres to
its walls by a plane or convex area. The slightly thickened membrane
of the antheridia encloses protoplasm which is finely granular. It is
seldom that more than one antheridium applies itself to an oogonium.
The two organs having together achieved their development, the large
granules contained in the oogonium accumulate at its centre to group
themselves under the form of an irregular globule deprived of a proper
membrane, and surrounded by a bed of almost homogeneous protoplasm.
This globule is the _gonosphere_, or reproductive sphere, which,
through the means of fecundation, should become the reproductive
body, vegetable egg, or oospore. The gonosphere having been formed,
the antheridium shoots out from the centre of its face, close against
the oogonium, a straight tube, which perforates the walls of the
female cell, and traversing the protoplasm of its periphery, directs
itself to the gonosphere. It ceases to elongate itself as soon as it
touches it, and the gonosphere becomes clothed with a membrane of
cellulose, and takes a regular spheroidal form.
[Illustration: FIG. 99.--Antheridia and oogonium of _Peronospora_. (De
Bary.)]
Considering the great resemblance of these organs with the sexual
organs of the Saprolegniæ, which are closely allied to the Algæ, and
of which the sexuality has been proved, De Bary adds, we have no doubt
whatever that the phenomena just described represent an act of
fecundation, and that the tube pushed out by the antheridium should be
regarded as a fecundating tube. It is remarkable that amongst these
fungi the tube projected by the antheridium effects fecundation only
by contact. Its extremity never opens, and we never find antherozoids;
on the contrary, the antheridium presents, up to the maturity of the
oospore, the appearance which it presented at the moment of
fecundation.
The primitive membrane of the oospore, at first very thin, soon
acquires a more sensible thickness, and becomes surrounded by an
external layer (epospore), which is formed at the expense of the
protoplasm of the periphery. This disappears in proportion as the
epispore attains maturity, and finally there only remains a quantity
of granules, suspended in a transparent watery fluid. At the period of
maturity, the epispore is a slightly thickened, resistant membrane, of
a yellowish-brown colour, and finely punctate. The surface is almost
always provided with brownish warts, which are large and obtuse,
sometimes isolated, and sometimes confluent, forming irregular crests.
These warts are composed of cellulose, which reagents colour of a deep
blue, whilst the membrane which bears them preserves its primitive
colour. One of the warts, larger than the rest, and recognizable by
its cylindrical form, always forms a kind of thick sheath around the
fecundating tube. The ripe endospore is a thick, smooth, colourless
membrane, composed of cellulose containing a bed of finely granulated
protoplasm, which surrounds a great central vacuole. This oospore, or
resting spore, may remain dormant in this state within the tissues of
the foster plant for some months. Its ultimate development by
production of zoospores is similar to the production of zoospores from
conidia, which it is unnecessary to repeat here. The oospore becomes
an oosporangium, and from it at least a hundred germinating bodies are
at length expelled.
Amongst the principal observers of certain phenomena of copulation in
cells formed in the earliest stages of the _Discomycetes_ are
Professor de Bary,[J] Dr. Woronin,[K] and Messrs. Tulasne.[L] In the
_Ascobolus pulcherrimus_ of Crouan, Woronin ascertained that the cup
derives its origin from a short and flexible tube, thicker than the
other branches of the mycelium, and which is soon divided by
transverse septa into a series of cells, the successive increase of
which finally gives to the whole a torulose and unequal appearance.
The body thus formed he calls a "vermiform body." The same observer
also seems to have convinced himself that there exists always in
proximity to this body certain filaments, the short arched or
inflected branches of which, like so many antheridia, rest their
anterior extremities on the utriform cells. This contact seems to
communicate to the vermiform body a special vital energy, which is
immediately directed towards the production of a somewhat filamentous
tissue, on which the hymenium is at a later period developed. This
"vermiform body" of M. Woronin has since come to be recognized under
the name of "scolecite."
Tulasne observes that this "scolecite" or ringed body can be readily
isolated in _Ascobolus furfuraceus_. When the young receptacles are
still spherical and white, and have not attained a diameter exceeding
the one-twentieth of a millimetre, it is sufficient to compress them
slightly in order to rupture them at the summit and expel the
"scolecite." This occupies the centre of the little sphere, and is
formed of from six to eight cells, curved in the shape of a comma.
In _Peziza melanoloma_, A. and S., the same observer succeeded still
better in his searches after the scolecite, which he remarks is in
this species most certainly a lateral branch of the filaments of the
mycelium. This branch is isolated, simple, or forked at a short
distance from its base, and in diameter generally exceeding that of
the filament which bears it. This branch is soon arcuate or bent, and
often elongated in describing a spiral, the irregular turns of which
are lax or compressed. At the same time its interior, at first
continuous, becomes divided by transverse septa into eight or ten or
more cells. Sometimes this special branch terminates in a crozier
shape, which is involved in the bent part of another crozier which
terminates a neighbouring filament. In other cases the growing branch
is connected, by its extremity, with that of a hooked branch. These
contacts, however, did not appear to Tulasne to be so much normal as
accidental. But of the importance of the ringed body, or "scolecite,"
there was no room for doubt, as being the certain and habitual
rudiment of the fertile cup. In fact, inferior cells are produced from
the flexuous filaments which creep about its surface, cover and
surround it on all sides, while joining themselves to each other. At
first continuous, then septate, these cells by their union constitute
a cellular tissue, which increases little by little until the
scolecite is so closely enveloped that only its superior extremity can
be seen. These cellular masses attain a considerable volume before the
hymenium begins to show itself in a depression of their summit. So
long as their smallness permits of their being seen in the field of
the microscope, it can be determined that they adhere to a single
filament of the mycelium by the base of the scolecite which remains
naked.