_DI-MORPHISM._
BEFORE entering further and more fully upon the subject of this volume,
it may be advisable to attempt an explanation of a phenomenon of no
uncommon occurrence in many groups of Fungi, and which is termed
_di-morphism_.
In the Uredines, or uredo-like fungi, as well as other of the
_Coniomycetes_ (in which the spores are the principal feature), the same
fungus appears under two or more distinct forms, not necessarily mere
differences of age, but so distinct that they have been regarded (and
some are so still) as different species belonging to different genera,
often far removed from each other, and bearing different names. One
plant, for instance, sprinkled over the under surface of a rose-leaf,
like turmeric powder, has its mycelium, or root-like threads,
penetrating the tissue, whilst bearing above its spherical
golden-coloured spores. Its vegetative system is complete, and,
apparently, its reproductive also; hence it seems to claim recognition
as a perfect plant, and under the name of _Uredo Rosæ_ was so
recognized, until microscopical investigation determined otherwise. Thus
it has been discovered that certain dark brown spots which appear later
in the season are produced upon the same mycelium, and are indeed
aggregations of more perfect and complex fruits of the same plant.
Before this point was satisfactorily decided, the brown spores, which
are borne on long stalks, and are themselves septate or divided
(apparently or really) by transverse partitions into a complex fruit,
received the name of _Puccinia Rosæ_. At this period, _Uredo Rosæ_ and
_Puccinia Rosæ_, or the yellow fungus and the dark brown fungus, were
believed to be distinct and different plants; now, on the contrary, they
are believed to be different forms of fruit produced by the same plant;
_i.e._, an instance of di-morphism. _Aregma mucronatum_, Fr., is the
present scientific name of what is regarded as the perfect fungus,
whilst the uredo-form either bears the name of _Lecythea Rosæ_, Lev., or
by some mycologists is rejected altogether as a spurious species.
During the summer it is not uncommon to find the leaves of some grasses,
of the hop, of roses, and many other plants, covered with a kind of
white mould, which appears under the microscope as a multitude of small
transparent colourless cellules, generally attached to each other in a
moniliform or beaded manner. These moulds were long known under the
generic name of _Oidium_, to which genus the vine disease was also
referred. More minute investigation and more careful examination proved
that these moulds were not in themselves perfect plants, but merely
conditions of other fungi of a higher order, little differing it is true
in external appearance to the naked eye, but offering material
differences in structure under the microscope. Upon the white mould-like
threads, spherical bodies are produced in the autumn, containing little
sacs or asci filled with spores; and in this condition the plants are
arranged under the genus _Erysiphe_, whilst the species of _Oidium_
which represented their imperfect condition, are excluded from the
system. Here, again, we have examples of _di-morphism_.
In the Journal of the Microscopical Society, Mr. F. Currey has detailed
several instances of di-morphism which have fallen within his
experience. In one instance he has shown that a small simple spored
fungus, termed _Cryptosporium Neesii_, Ca., is only a state or condition
of a fungus with compound fruit, belonging to the _Sphœria_ section of
ascigerous fungi, called _Valsa suffusa_, Fr. Both plants are exactly
alike externally, but the perithecium, or flask-like receptacle
containing the fructification, in one instance only holds naked spores,
and in the other the spores are contained in little elongated vesicular
bags or asci, which are packed within the perithecium.
Whilst writing this, one of the most wonderful books in a book-producing
age lies beside us; it is the second volume of a work on fungi, by the
brothers Tulasne; and this, as well as its predecessor, is devoted to
this very subject of a multiplicity of form in the fructification of
these plants. Illustrated by the most exquisite of engravings which art
has ever produced, it also unfolds many a page in the history of these
organisms, for which mycologists were not altogether unprepared. In
noticing this work, one of our most eminent authors on mycological
subjects quotes as an example _Dothidia ribis_, Fr., one of our most
common fungi, which occurs in the form of little black shields on dead
twigs of currants and goose-berries. Here we have, he says, naked spores
(_conidia_) growing on the external cells of the stroma; we have naked
spores of a second kind (_stylospores_) produced in distinct cysts
(_pycnides_); we have minute bodies of a third kind (_spermatia_)
produced again in distinct cysts, resembling very closely similar bodies
in lichens; and we have a third kind of cysts, containing the usual
sporidia in sausage-shaped hyaline sacs (_asci_). Even here, however, we
have not done with marvels; for if the stylospores are placed in water,
they produce in the course of twenty-four hours conidia of a second
order, exactly analogous to those which arise on the germination of the
spores of the rusts and mildews which affect our cereals and other
plants.
Further reference is also made to three species of moulds, which M.
Tulasne has shown to be only varied forms of the mycelium of a species
of _Sphœria_ common to various plants; these moulds having been hitherto
regarded as fungi perfect in themselves.
In the Uredines, to which much of this volume is devoted, the genera
known as _Lecythea_ and _Trichobasis_ are by some mycologists excluded
altogether, as containing only species which are mere forms of more
highly-developed uredines, such as _Puccinia_, _Aregma_, and others. On
the other hand, they are retained by those who possess a lingering doubt
whether both forms may not be distinct, though developed from the same
pustule. As the two forms are distinct in appearance, it will better
answer our present purpose to treat them separately, notwithstanding the
belief that, in a scientific point of view, the evidence is all in
favour of their union.
In fungi of this kind the mycelium, or delicate root-like threads,
consists of thin filaments, which are spread through all parts of the
plant occupied by the parasite, traversing the intercellular passages,
but rarely perforating and entering the cells. This compacted and
interwoven mycelium forms a kind of cushion beneath each pustule, on
which the fruits of the parasite rest. By the increase of this cushion
and the swelling of the fruit, the epidermis which covers them is
distended, and ultimately ruptured, so that, when ripened, the spores
escape. It must be remembered that the fruit is of from two to four
kinds. Small bodies, called _spermatia_, which are derived from the
spermogones, and which have not yet been known to germinate;
_Stylospores_, produced either singly, or in bead-like, or moniliform,
strings, and which either precede or are associated with the true
spores; _Spores_, sometimes simple, but often complex; and _Sporidia_,
or secondary sporules, which are produced on the germinating threads of
the true spores.
The various genera of these endophytes owe their distinctions to the
form, or mode of development of their true spores. In one instance these
spores are united in pairs, or divided by a septum, so that they are
two-celled: these are named _Puccinia_. In another instance the spores
are one-celled, and at first borne upon a stalk or peduncle, from which
they are detached in ripening: such are called _Trichobasis_. It is
unnecessary here to indicate all the variations to illustrate the fact
that the generic distinctions are based upon the characters of the true
spores. How unsatisfactory such a mode will appear, when subjected to
experience day by day, a botanist would suspect. In the same pustule,
resting upon the same cushion of mycelium, the spores of an _Aregma_
will be found with those of a _Lecythea_, and those of a _Puccinia_ with
_Trichobasis_. More than this has even been affirmed. The alternation of
generations, known to students in the animal world, is here repeated in
the vegetable. Dr. de Bary declares that certain data appear to indicate
that _Æcidium_ constitutes not a genus by itself, but are organs in the
development of some other germs and species, possessing its spermogonia,
its _Æcidium_; its _Uredo_, and its spores, properly speaking; whilst in
others the _Uredo_-form the _Puccinia_-form, and the _Æcidium_-form may
alternate. It is not our intention to enter deeply upon the discussion
of this subject, of so little interest to the beginner, and so out of
place in an introduction to the study. That forms and conditions are
multifarious, and that an entire revision of the classification is
inevitable, are facts which do not require many words to establish.
Already it is to be feared that in this brief chapter we have said too
much, and must recommend its perusal again, when the names and
characters of the genera alluded to have been rendered more familiar.
It could scarcely have been permitted that the student should go far
without being cautioned that there is such a thing as di-morphism in
microscopic fungi; and the explanation of such a phenomenon must
presuppose a certain amount of knowledge which, thus far, the reader
could not have acquired. Hence an anomaly, to escape from which an
ultimate return to the subject will be necessary.
In a recent account of Dr. de Bary’s experiments,[3] an interesting
history is given of the development of a rust-like fungus, which is
common on many plants of the pea and bean tribe. As it may serve to
illustrate some of the preceding, as well as subsequent, remarks on
development, an abstract shall close this chapter.
Footnote 3:
De Bary—“Annales des Sciences Naturelles,” ser. 4, vol. xx.
The spores of this species (_Uromyces appendiculatus_) are oboval cells,
terminated by a rounded point, provided with a deep brown, smooth,
_epispore_, or outer coating, and a distinct, but colourless
_endospore_, or inner coating. These enclose a granular matter, which
surrounds what has been termed the nucleus, but which appears to be a
vacuole. At the top of the epispore is a pore which is characteristic of
the genus. The spores are supported on a colourless, or slightly-tinted
pedicel of considerable length (Plate VII. fig. 150). By means of this
pedicel, the spores are attached to the fostering plant, on which they
form pustules or sori of a blackish colour, and variable extent. These
spores are ripened towards the end of summer or beginning of autumn.
During winter they remain in a state of repose, but in the following
spring the faculty of germination developes itself. At this period, when
moistened or placed on a humid soil, they germinate at the end of a few
days. The spore then emits a curved and obtuse tube, which soon ceasing
to elongate itself, gives origin to three or four sporidia, of a
reniform or kidney shape. When cultivated on moistened glass, these
sporidia also emit a short, thin, slender tube, which produce in turn
secondary sporidia. Here vegetation ends in the artificial culture above
indicated.
When the sporidia are sown upon the epidermis of a favourable plant, the
germ-tube being emitted, penetrates the wall of any approximate cellule,
swells and increases into a cylindrical tube equal in thickness to the
original sporidia, and therefore four or five times the diameter of the
germ-tube before it entered the cellule. The contents of the sporidia
and external portion of its germ-tube pass into the portion within the
cellule, and then these external portions perish, and all evidence of
the entry is obliterated, except a very minute point at which the tube
remains attached to the inner surface of the wall of the cellule. The
enclosed tube soon elongates, divides, and becomes branched. These
branches perforate the inner walls of the epidermis, and pass into the
intercellular spaces of the parenchyma to become mycelium. This takes
place within 24 hours. A few days afterwards the mycelium is spread
through the parenchyma. At length the surface of the same spots which
had been sown in the first instance with the sporidia, become of a
whitish tint, rapidly increasing and intensifying. Three days after,
little protuberances appear on the surface of the white spots. These are
of an orange colour, and many of them are surmounted by a little drop of
mucilaginous fluid. These are _spermogones_. Their number daily
increases, and a little time after appear numerous large globular
protuberances intermingled with them. These soon rupture the epidermis,
and take the orange colour and cylindrical form of cluster-cups
(_Æcidium_). At length the summit of the peridia opens to allow the
escape of the _stylospores_. It is easy to assure oneself that the
spermogones and cluster-cups proceed from the mycelium of the sporidia
which had been sown. During several days the length and number of the
peridia of the _Æcidium_ continue to increase. One month after sowing,
brownish or blackish points make their appearance upon the whitish
spots, around, or intermingled with the cluster-cups. These increase
rapidly in number and magnitude. Examined by the microscope, they
present the ordinary fructification of _Uromyces_, mingled with
stylospores. Thus the mycelium of the cluster-cups engenders at the end
of its vegetation fruits equal in all points to those from whence they
are in the first instance derived.
The stylospores of the cluster-cups possess the irregular, globular form
and structure of their congeners. They are filled with orange granular
matter, and provided with a colourless, finely-punctated epispore. When
these stylospores are sown on the moistened epidermis of a favourable
plant, the germ-tube at first creeps along the surface, but as soon as
its extremities find a stomate, it enters it and elongates itself in the
air-cavity below the orifice, receives the contents of the original
stylospore and exposed portion of its tube, then separates itself from
those parts, which become dispersed. The active part increases and
ramifies, and produces a mycelium which spreads through the
intercellular passages of the parenchyma. At the end of from six to
eight days, the whitish spots appear on the surface of the fostering
plant, and indicate that the fructification of the parasite is about to
commence. The epidermis is elevated and broken, and little brown
pustules appear through the openings. These are the _stylospores_ of
_Uredo_, which are produced in immense quantities, and soon cover the
pustules with a deep brown dust. Later, the formation of the stylospores
is arrested, and the true germinating spores appear in the same
pustules.
The stylospores of _Uredo_ are borne singly at the top of short
filaments. On arriving at maturity they detach themselves. They are of a
globular form, with a reddish-brown epispore, provided with little
pointed prominences, and three pores at equal distances. After maturity
they germinate in precisely the same manner as the stylospores of the
cluster-cups. They enter only through the stomata of the epidermis. The
pulvinules are identical with those which the stylospores of _Æcidium_
originate, and they also produce true spores at the end of their
vegetation. No other fruit arises from them. These organs, therefore,
always reproduce the same form to which they owe their origin. The
result of these investigations shows that the bean rust (_Uromyces
appendiculatus_), besides spermogones, possesses four sorts of
reproductive organs, which all serve to propagate the species, but that
one alone of them produces it in a form always identical, whilst the
others present well-marked alternations of generation. Hence it is
concluded that there are,
I. _Spores_ which produce in germinating the promycelium, and
II. _Sporidia._—These give place to a mycelium, which bears afterwards—
III. _Æcidium._—Particular organs which engender stylospores, and which
produce—
IV. _Uredo_, the second form of the stylospores, and later spores (No.
I.), which are always associated with _Uredo_ in the same pustule. The
spores and stylospores of _Uredo_ come also upon the old mycelium, which
has previously produced _Æcidium_. The _Uredo_ stylospores always
produce _Uredo_, and true spores.
------------------------------------------------------------------------