recapitulate the observations made by Professor de Bary,[K] who, by
the bye, claims for them an affinity with _Peronospora_ (Mucedines but
too well known in connection with the potato disease), and _not_ with
the Uredines and their allies. In this genus there are two kinds of
reproductive organs, those produced on the surface of the plant
bursting through the cuticle in white pustules, and which De Bary
terms _conidia_, which are generated in chains, and certain globose
bodies termed _oogonia_, which are developed on the mycelium in the
internal tissues of the foster plant. When the conidia are sown on
water they rapidly absorb the moisture, and swell; the centre of one
of the extremities soon becomes a large obtuse papilla resembling the
neck of a bottle. This is filled with a granular protoplasm, in which
vacuoles are formed. Soon, however, these vacuoles disappear, and very
fine lines of demarcation separate the protoplasm into from five to
eight polyhedric portions, each presenting a little faintly-coloured
vacuole in the centre (_a_). Soon after this division the papilla at
the extremity swells, opens itself, and at the same time the five to
eight bodies which had formed in the interior are expelled one by one
(_b_). These are zoospores, which at first take a lenticular form,
and group themselves before the mouth of the parent cell in a globose
mass (_c._) Very soon, however, they begin to move, and then vibratile
cilia show themselves (_d_), and by means of these appendages the
entire globule moves in an oscillating manner as one by one the
zoospores disengage themselves, each becoming isolated and swimming
freely in the surrounding fluid. The movement is precisely that of the
zoospores of Algæ.
[Illustration: FIG. 92.--Resting spore of _Cystopus candidus_ with
zoospores escaped.]
The generation of the zoospores commences within from an hour and a
half to three hours after the sowing of the conidia on water. From the
oogonia, or resting spores, similar zoospores, but in greater number,
are generated in the same manner, and their conduct after becoming
free is identical. Their movements in the water usually last from two
to three hours, then they abate, the cilia disappear, and the spore
becomes immovable, takes a globose form, and covers itself with a
membrane of cellulose. Afterwards the spore emits, from any point
whatever of its surface, a thin, straight or flexuous tube, which
attains a length of from two to ten times the diameter of the spore.
The extremity becomes clavate or swollen, after the manner of a
vesicle, which receives by degrees the whole of the protoplasm.
De Bary then proceeds to describe experiments which he had performed
by watering growing plants with these zoospores, the result being that
the germinating tubes did not penetrate the epidermis, but entered by
the stomates, and there put forth an abundant mycelium which traversed
the intercellular passages. Altogether the germination of these
conidia or zoospores offers so many differences from the ordinary
germination of the Uredines, and is so like that which prevails in
_Peronospora_, in addition to the fact of both genera producing winter
spores or oogonia, that we cannot feel surprised that the learned
mycologist who made these observations should claim for _Cystopus_ an
affinity with _Peronospora_ rather than with the plants so long
associated with it amongst the _Coniomycetes_.
In passing from these to the _Mucedines_, therefore, we cannot do so
more naturally than by means of that genus of white moulds to which we
have just alluded. The erect branched threads bear at the tip of their
branchlets spores, or conidia, which conduct themselves in a like
manner to the organs so named in _Cystopus_, and oogonia or resting
spores developed on the mycelium within the tissues of the foster
plant also give origin to similar zoospores.
The conidia are borne upon erect, elongated filaments, originating
from the creeping mycelium. These threads are hollow, and rarely
septate; the upper portion divided into numerous branches, and these
again are subdivided, the ultimate ramuli each terminated by a single
conidium. This body when mature is oval or elliptical, filled with
protoplasm, but there is a diversity in their mode of germination. In
the greater part, of which _P. effusa_ may be taken as an example, the
conidia have the function of simple spores. Placed in favourable
conditions, each of them puts forth a germ-tube, the formation of
which does not differ in any essential point from what is known of the
spores of the greater part of fungi.
The short oval conidia of _P. gangliformis_ have little obtuse papillæ
at their apex, and it is at this point that germination commences.
The conidia of _P. densa_ are similar, but the germination is
different. When placed in a drop of water, under favourable
circumstances, the following changes may be observed in from four to
six hours. The protoplasm, at first uniformly distributed in all the
conidia, appears strewn with semi-lenticular, and nearly equidistant
vacuoles, of which the plane face is immediately in contact with the
periphery of the protoplasm. These vacuoles number from sixteen to
eighteen in _P. macrocarpa_, but are less numerous in _P. densa_. A
short time after the appearance of the vacuoles the entire conidium
extends itself so that the papilla disappears. Suddenly it reappears,
elongates itself, its attenuated membrane vanishes, and the protoplasm
is expelled by the narrow opening that remains in place of the
papilla. In normal cases the protoplasm remains united in a single
mass that shows a clear but very delicate outline. When it has reached
the front of the opening in the conidium, which is thus emptied, the
mass remains immovable. In _P. densa_ it is at first of a very
irregular form, but assumes by degrees a regular globose shape. This
is deprived of a distinct membrane, the vacuoles that disappeared in
the expulsion again become visible, but soon disappear for a second
time. The globule becomes surrounded with a membrane of cellulose, and
soon puts out from the point opposite to the opening of the conidium a
thick tube which grows in the same manner as the germ-tube of the
conidia in other species. Sometimes the expulsion of the protoplasm is
not completely accomplished; a portion of it remaining in the membrane
of the conidium detaches itself from the expelled portion, and while
this is undergoing changes takes the form of a vesicle, which is
destroyed with the membrane. It is very rare that the protoplasm is
not evacuated, and that the conidia give out terminal or lateral tubes
in the manner that is normal to other species without papillæ. The
germination just described does not take place unless the conidia are
entirely surrounded by water; it is not sufficient that they repose
upon its surface. Besides, there is another condition which, without
being indispensable, has a sensible influence on the germination of
_P. macrocarpa_, and that is the exclusion of light. To ascertain if
the light or the darkness had any influence, two equal sowings were
placed side by side, the one under a clear glass bell, the other under
a blackened glass bell. Repeated many times, these experiments always
gave the same result--germination in from four to six hours in the
conidia under the blackened glass; no change in those under the clear
glass up to the evening. In the morning germination was completed.
The conidia of _P. umbelliferarum_ and _P. infestans_[L] show an
analogous structure. These bodies, if their development be normal,
become zoosporangia. When they are sown upon water, one sees at the
end of some hours the protoplasm divided by very fine lines, and each
of the parts furnished with a small central vacuole. Then the papilla
of the conidium disappears. In its place appears a rounded opening, by
which the parts of the protoplasm are expelled rapidly, one after the
other. Each of these, when free, immediately takes the form of a
perfect zoospore, and commences to agitate itself. In a few moments
the sporangium is empty and the spores disappear from the field of the
microscope.
The zoospores are oval or semi-oval, and in _P. infestans_ the two
cilia spring from the same point on the inferior border of the
vacuole. Their number in a sporangium are from six to sixteen in _P.
infestans_, and from six to fourteen in _P. umbelliferarum_. The
movement of the zoospores ceases at the end of from fifteen to thirty
minutes. They become motionless, cover themselves with a membrane of
cellulose, and push out slender bent germ-tubes which are rarely
branched. It is but seldom that two tubes proceed from the same spore.
The same development of the zoospores in _P. infestans_ is favoured by
the exclusion of the light. Placed in a position moderately lighted or
protected by a blackened bell, the conidia very readily produced
zoospores.
A second form of germination of the conidia in _P. infestans_, when
sown upon a humid body or on the surface of a drop of water, consists
in the conidium emitting from its summit a simple tube, the extremity
of which swells itself into the form of an oval vesicle, drawing to
itself, little by little, all the protoplasm contained in the
conidium. Then it isolates itself from the germ-tube by a septum, and
takes all the essential characteristics of the parent conidium. This
secondary conidium can sometimes engender a third cellule by a similar
process. These secondary and tertiary productions have equally the
character of sporangia. When they are plunged into water, the ordinary
production of zoospores takes place.
Lastly, there is a third mode of germination which the conidia of _P.
infestans_ manifest, and which consists in the conidium emitting from
its summit a simple or branched germ-tube. This grows in a similar
manner to the conidia first named as of such species as _P. effusa_.
The conditions which control this form of germination cannot be
indicated, since some conidia which germinate after this manner will
sometimes be found mixed with others, the majority of which furnish
zoospores. It may be that the conidia themselves are in some sort of
abnormal condition.
In all the species examined the conidia possess the power of
germination from the moment of their maturity. The younger they are
the more freely they germinate. They can retain this power for some
days or weeks, provided they are not entirely dried. Dessication in an
ordinary temperature seemed sufficient to destroy the faculty of
germinating in twenty-four hours, when the conidia had been removed
from the leaves on which they were produced. They none of them
retained the faculty during a few months, hence they cannot preserve
it during the winter.
The germs of _Peronospora_ enter the foster plant if the spores are
sown upon a part suitable for the development of the parasite. It is
easy to convince one's self that the mycelium, springing from the
penetrating germs, soon takes all the characters that are found in the
adult state. Besides, when cultivated for some time, conidiiphorous
branches can be seen growing, identical with those to which it owes
its origin. Such cultivation is so readily accomplished that it can be
made upon cut leaves preserved fresh in a moist atmosphere.
In the species of _Peronospora_ that inhabit perennial plants, or
annual plants that last through the winter, the mycelium hidden in the
tissues of the foster-plant lasts with it. In the spring it
recommences vegetation, and emits its branches into the newly-formed
organs of its host, there to fructify. The _Peronospora_ of the potato
is thus perennial by means of its mycelium contained in the browned
tissue of the diseased tubers. When in the spring a diseased potato
begins to grow, the mycelium rises in the stalk, and soon betrays
itself by blackish spots. The parasites can fructify abundantly on
these little stalks, and in consequence propagate themselves in the
new season by the conidia coming from the vivacious mycelium.
The diseased tubers of the potato always contain the mycelium of _P.
infestans_, which never fructifies there as long as the skin of the
tuber is intact. But when, in cutting the tuber, the parenchyma
occupied by the mycelium is exposed to the contact of the air, it
covers itself with conidia-bearing branches at the end of from
twenty-four to forty-eight hours. Analogous results are obtained with
the stalks of the potato. It is evident that in these experiments
nothing is changed except the contact of the air; the specific
conditions particularly remain the same. It appears, therefore, that
it is this contact alone which determines generally the production of
the conidiiferous branches.[M]
The mode of germination and development in the Mucors has been studied
by several observers, but most recently by Van Tieghem and Le
Monnier.[N] In one of the common forms, the _Mucor phycomyces_ of some
authors, and the _Phycomyces nitens_ of others, the process is given
in detail. In this species germination will not take place in ordinary
water, but it readily takes place in orange juice and other media. The
spore loses colour, swells, and absorbs fluid around it until double
its original size and ovoid. Then a thick thread is emitted from one
or both extremities, which elongates and becomes branched in a pinnate
manner. Sometimes the exospore is ruptured and detached loosely from
the germinating spore. After about forty-eight hours from the first
sowing, the mycelium will send branches into the air, which again
become abundantly branched; other short submerged branches will also
remain simple, or have tuft-like ramifications, each terminating in a
point, so as to bristle with spiny hairs. In two or three days
abruptly swollen branches, of a club shape, will make their appearance
on the threads both in the air and in the fluid. Sometimes these
branches are prolonged into an equal number of sporangia-bearing
threads, but most frequently they divide first at their swollen
summits into numerous branches, of which usually one, sometimes two
or three, develop into sporangia-bearing threads, while the rest are
short, pointed, and form a tuft of rootlets. Sometimes these rootlets
reduce themselves to one or more rounded protuberances towards the
base of the sporangia-bearing threads.
[Illustration: FIG. 93.--Zygospores of _Mucor phycomyces_. (Van Tieghem.)]
There are often also a certain number of the branches which had
acquired a clavate shape, and do not erect themselves above the
surface, instead of producing a fertile thread, which would seem to
have been their first intention, become abruptly attenuated, and are
merely prolonged into a mycelial filament. Although in other species
chlamydospores are formed in such places on the mycelium, nothing of
the kind has been traced in this species, more than here indicated.
Occasionally, when germination is arrested prematurely, certain
portions of the hyphæ, in which the protoplasm maintains its vitality,
become partitioned off. This may be interpreted as a tendency towards
the formation of chlamydospores, but there is no condensation of
protoplasm, or investiture with a special membrane. Later on this
isolated protoplasm is gradually altered, separating into somewhat
regular ovoid or fusiform granules, which have, to a certain extent,
the appearance of spores in an ascus, but they seem to be incapable of
germination.
Another method of reproduction, not uncommon in _Mucorini_, is
described by Van Tieghem in this species. Conjugating threads on the
substratum by degrees elaborate zygospores, but these, contrary to
the mode in other species, are surrounded by curious branched
processes which emanate from the arcuate cells on either side of the
newly-developed zygospore. This system of reproduction is again
noticed more in detail in the chapter on polymorphism.
M. de Seynes has given the details of his examination of the sporidia
of _Morchella esculenta_ during germination.[O] A number of these
sporidia, placed in water in the morning, presented, at nine o'clock
of the same evening, a sprout from one of the extremities, measuring
half the length of the spore. In the morning of the next day this
sprout had augmented, and become a filament three or four times as
long. The next day these elongated filaments exhibited some transverse
divisions and some ramifications. On the third day, the germination
being more advanced, many more of the sporidia were as completely
changed, and presented, in consequence of the elongation, the
appearance of a cylindrical ruffle, the cellular prolongations arising
from the germination having a tendency towards one of the extremities
of the longer axis of the sporidium, and more often to the two opposed
extremities, either simultaneously or successively. Out of many
hundreds of sporidia examined during germination, he had only seen a
very few exceptions to this rule, among which he had encountered the
centrifugal tendency to vegetate by two opposed filaments, proving
that if it bears a second by the side of the primal filament situated
at one of the poles, a second would also be seen from the side of the
filament coming from the opposite pole.
Before being submitted to the action of water, the contents of the
sporidia seemed formed of two distinct parts, one big drop of yellow
oil of the same form as the sporidium, with the space between it and
the cell wall occupied by a clear liquid, more fluid and less
refractive, nearly colourless, or at times slightly roseate. As the
membrane absorbed the water by which it was surrounded, the quantity
of this clear liquid was augmented, and the rosy tint could be more
easily distinguished. All the contents of the spore, which up to this
time remained divided into two parts, presented altogether one aspect,
only containing numerous granulations, nearly of equal size,
completely filling it, and reaching the inner face of the sporic
membrane.
After this time the sporidium augments in size very rapidly, becoming
at times irregular, and sometimes even as much as from two to three
times its original dimensions, then there appears at the surface,
usually at one of the poles of the ellipse, a small prominence, with
an extremely fine membrane, which does not appear to separate itself
from that which surrounds the sporidium, and it is difficult to say
whether it is a prolongation of the internal membrane going across the
outside, or simply a prolongation caused by a continuation of tissue
of an unique membrane. Sometimes there may be seen at the point where
the primal filament issues from the sporidium a circular mark, which
appears to indicate the rupture of the external membrane. From this
time another change comes over the contents. We again find the yellow
oily liquid, now occupying the external position, with some drops of
colourless or roseate liquid in the centre, so that the oily liquid
and the more limpid fluid interchange the positions which they
occupied previous to the commencement of germination. Whether these
two fluids have undergone any change in their constitution is
difficult to determine, at all events the oily liquid appears to be
less refractive and more granular, and it may be that it is a product
of new formation, containing some of the elements of the primitive
oily drop. Having regard to the delicate character of the membrane of
the germinating filaments, De Seynes supposed that it might offer
greater facility for the entrance of water by endosmose, and account
for the rapid enlargement of the sporidia. By a series of experiments
he became satisfied that this was the case to a considerable extent,
but he adds:--"I cannot help supposing that a greater absorption of
greasy matter in the cell which is the first product of germination
raises an objection to an aqueous endosmose. One can also see in this
experience a proof of the existence of two special membranes, and so
suppose that the germinative cell is the continuation of the internal
membrane, the external membrane alone being susceptible of absorbing
the liquids, at least with a certain rapidity."
[Illustration: FIG. 94.--Sporidium of _Ascobolus_ germinating.]
In other _Discomycetes_ germination takes place in a similar manner.
Boudier[P] narrates that in _Ascobolus_, when once the spore reaches a
favourable place, if the circumstances are good, _i.e._, if the
temperature is sufficiently high and the moisture sufficient, it will
germinate. The time necessary for this purpose is variable, some hours
sufficing for some species; those of _A. viridis_, for example,
germinate in eight or ten hours, doubtless because, being terrestrial,
it has in consequence less heat. The spore slightly augments in size,
then opens, generally at one or other extremity, sometimes at two, or
at any point on its surface, in order to pass the mycelium tubes. At
first simple, without septa, and granular in the interior, above all
at the extremity, these tubes, the rudiment of the mycelium, are not
long in elongating, in branching, and later in having partitions.
These filaments are always colourless, only the spore may be coloured,
or not. Coemans has described them as giving rise to two kinds of
conidia,[Q] the one having the form of _Torula_, when they give rise
to continuous filaments, the other in the form of _Penicillium_, when
they give birth to partitioned filaments. De Seynes could never obtain
this result. Many times he had seen the _Penicillium glaucum_ invade
his sowings, but he feels confident that it had nothing to do with the
_Ascobolus_. M. Woronin[R] has detailed some observations on the
sexual phenomena which he has observed in _Ascobolus_ and _Peziza_,
and so far as the scolecite is concerned these have been confirmed by
M. Boudier.
There is no reason for doubt that in other of the _Discomycetes_ the
germination of the sporidia is very similar to that already seen and
described, whilst in the _Pyrenomycetes_, as far as we are aware,
although the production of germinating tubes is by no means difficult,
development has not been traced beyond this stage.[S]
[A] Seynes, J. de, "Essai d'une Flore Mycologique de la Montpellier,"
&c. (1863), p. 30.
[B] Hoffman, "Icones Analyticæ Fungorum."
[C] The spores of Agarics which are devoured by flies, however,
though returned in their dung in an apparently perfect state,
are quite effete. It is, we believe, principally by the
_Syrphidæ_, which devour pollen, that fungus spores are
consumed.
[D] All attempts at Chiswick failed with some of the more esculent
species, and Mr. Ingram at Belvoir, and the late Mr. Henderson
at Milton, were unsuccessful with native and imported spawn.
[E] Tulasne, "On the Organization of the Tremellini," "Ann. des. Sci.
Nat." 3^me sér. xix. (1853), p. 193.
[F] Tulasne, "Mémoire sur les Urédinées."
[G] Tulasne, in his "Memoirs on the Uredines."
[H] Mr. Berkeley has lately published a species under the name of _P.
Ellisii_, in which the gelatinous element is scarcely
discernible till the plant is moistened. There are two septa in
this species, and another species or form has lately been
received from Mr. Ellis which has much shorter pedicels, and
resembles more closely _Puccinia_, from which it is chiefly
distinguished by its revivescent character.
[I] Von Waldheim, on the "Development of the Ustilagineæ," in
"Pringsheim's Jahrbucher," vol. vii. (1869); translated in
"Transactions of N. Y. State Agricultural Society for 1870."
[J] Berkeley, on the "Propagation of Bunt," in "Trans. Hort. Soc.
London," ii. (1847), p. 113; Tulasne, second memoir, in "Ann.
des. Sci. Nat." ii. (4^me sér.), p. 77; Cooke, in "Journ.
Quekett Micro. Club," i. p. 170.
[K] De Bary, "Recherches," &c. in "Annales des Sciences Naturelles"
(4^me sér.), xx. p. 5; Cooke in "Pop. Sci. Rev." iii. (1864),
p. 459.
[L] This is the mould which produces the potato murrain.
[M] De Bary, "Champignons parasitiques," in "Annales des Sci. Nat."
(4^me sér.), xx. p. 5; Cooke, "Microscopic Fungi," cap. xi. p.
138; "Popular Science Review," iii. 193 (1864).
[N] Van Tieghem and Le Monnier, "Researches on Mucorini," in "Ann.
des Sci. Nat." (1873), xvii. p. 261; Summary in "Quart. Journ.
Micro. Science" (2nd ser.), xiv. p. 49.
[O] Seynes, "Essai d'une Flore Mycologique."
[P] Boudier, "Mémoire sur l'Ascoboles," pt. i. iv. f. 13-15.
[Q] Coemans, "Spicilége Mycologique," i. p. 6.
[R] Woronin, "Abhandlungen der Senchenbergischen Naturfor.
Gesellschaft" (1865), p. 333.
[S] In the very important observations made by Dr. Cunningham at
Calcutta, on substances floating in the atmosphere, it appeared
that the sporidia of many _Sphæriæ_ actually germinated after
being taken up by the air. The multitude of fungus spores which
were observed in every case was quite extraordinary.
VIII.
SEXUAL REPRODUCTION.