THE TRUE FUNGI (_Mycomycetes_).
The great majority of the plants ordinarily known as _fungi_ are
embraced under this head. While some of the lower forms show
affinities with the _Phycomycetes_, and through them with the algæ,
the greater number differ very strongly from all green plants both in
their habits and in their structure and reproduction. It is a
much-disputed point whether sexual reproduction occurs in any of them,
and it is highly probable that in the great majority, at any rate, the
reproduction is purely non-sexual.
Probably to be reckoned with the _Mycomycetes_, but of doubtful
affinities, are the small unicellular fungi that are the main causes
of alcoholic fermentation; these are the yeast fungi (_Saccharomycetes_).
They cause the fermentation of beer and wine, as well as the incipient
fermentation in bread, causing it to "rise" by the giving off of
bubbles of carbonic acid gas during the process.
If a little common yeast is put into water containing starch or sugar,
and kept in a warm place, in a short time bubbles of gas will make
their appearance, and after a little longer time alcohol may be
detected by proper tests; in short, alcoholic fermentation is taking
place in the solution.
If a little of the fermenting liquid is examined microscopically, it
will be found to contain great numbers of very small, oval cells,
with thin cell walls and colorless contents. A careful examination
with a strong lens (magnifying from 500-1000 diameters) shows that
the protoplasm, in which are granules of varying size, does not fill
the cell completely, but that there are one or more large vacuoles
or spaces filled with colorless cell sap. No nucleus is visible in
the living cell, but it has been shown that a nucleus is present.
If growth is active, many of the cells will be seen dividing. The
process is somewhat different from ordinary fission and is called
budding (Fig. 37, _B_). A small protuberance appears at the bud or
at the side of the cell, and enlarges rapidly, assuming the form of
the mother cell, from which it becomes completely separated by the
constriction of the base, and may fall off at once, or, as is more
frequently the case, may remain attached for a time, giving rise
itself to other buds, so that not infrequently groups of half a
dozen or more cells are met with (Fig. 37, _B_, _C_).
[Illustration: FIG. 37.--_A_, single cells of yeast. _B_, _C_, similar
cells, showing the process of budding, × 750.]
That the yeast cells are the principal agents of alcoholic
fermentation may be shown in much the same way that bacteria are shown
to cause ordinary decomposition. Liquids from which they are excluded
will remain unfermented for an indefinite time.
There has been much controversy as to the systematic position of the
yeast fungi, which has not yet been satisfactorily settled, the
question being whether they are to be regarded as independent plants
or only one stage in the life history of some higher fungi (possibly
the _Smuts_), which through cultivation have lost the power of
developing further.
CLASS I.--THE SMUTS (_Ustillagineæ_).
The smuts are common and often very destructive parasitic fungi,
living entirely within the tissues of the higher plants. Owing to
this, as well as to the excessively small spores and difficulty in
germinating them, the plants are very difficult of study, except in a
general way, and we will content ourselves with a glance at one of the
common forms, the corn smut (_Ustillago maydis_). This familiar fungus
attacks Indian corn, forming its spores in enormous quantities in
various parts of the diseased plant, but particularly in the flowers
("tassel" and young ear).
The filaments, which resemble somewhat those of the white rusts,
penetrate all parts of the plant, and as the time approaches for the
formation of the spores, these branch extensively, and at the same
time become soft and mucilaginous (Fig. 38, _B_). The ends of these
short branches enlarge rapidly and become shut off by partitions,
and in each a globular spore (Fig. 38, _C_) is produced. The outer
wall is very dark-colored and provided with short spines. To study
the filaments and spore formation, very thin sections should be made
through the young kernels or other parts in the vicinity, before
they are noticeably distorted by the growth of the spore-bearing
filaments.
[Illustration: FIG. 38.--_A_, "tassel" of corn attacked by smut
(_Ustillago_). _B_, filaments of the fungus from a thin section of a
diseased grain, showing the beginning of the formation of the spores,
× 300. _C_, ripe spores, × 300.]
As the spores are forming, an abnormal growth is set up in the cells
of the part attacked, which in consequence becomes enormously enlarged
(Fig. 38, _A_), single grains sometimes growing as large as a walnut.
As the spores ripen, the affected parts, which are at first white,
become a livid gray, due to the black spores shining through the
overlying white tissues. Finally the masses of spores burst through
the overlying cells, appearing like masses of soot, whence the popular
name for the plant.
The remaining _Mycomycetes_ are pretty readily divisible into two
great classes, based upon the arrangement of the spores. The first of
these is known as the _Ascomycetes_ (Sac fungi), the other the
_Basidiomycetes_ (mushrooms, puff-balls, etc.).
CLASS II.--_Ascomycetes_ (SAC FUNGI).
This class includes a very great number of common plants, all
resembling each other in producing spores in sacs (_asci_, sing.
_ascus_) that are usually oblong in shape, and each containing eight
spores, although the number is not always the same. Besides the spores
formed in these sacs (ascospores), there are other forms produced in
various ways.
There are two main divisions of the class, the first including only a
few forms, most of which are not likely to be met with by the student.
In these the spore sacs are borne directly upon the filaments without
any protective covering. The only form that is at all common is a
parasitic fungus (_Exoascus_) that attacks peach-trees, causing the
disease of the leaves known as "curl."
All of the common _Ascomycetes_ belong to the second division, and
have the spore sacs contained in special structures called spore
fruits, that may reach a diameter of several centimetres in a few
cases, though ordinarily much smaller.
Among the simpler members of this group are the mildews
(_Perisporiaceæ_), mostly parasitic forms, living upon the leaves and
stems of flowering plants, sometimes causing serious injury by their
depredations. They form white or grayish downy films on the surface of
the plant, in certain stages looking like hoar-frost. Being very
common, they may be readily obtained, and are easily studied. One of
the best species for study (_Podosphæra_) grows abundantly on the
leaves of the dandelion, especially when the plants are growing under
unfavorable conditions. The same species is also found on other plants
of the same family. It may be found at almost any time during the
summer; but for studying, the spore fruits material should be
collected in late summer or early autumn. It at first appears as
white, frost-like patches, growing dingier as it becomes older, and
careful scrutiny of the older specimens will show numerous brown or
blackish specks scattered over the patches. These are the spore
fruits.
[Illustration: FIG. 39.--_A_, spore-bearing filaments of the dandelion
mildew (_Podosphæra_), × 150. _B_, a germinating spore, × 150. _C-F_,
development of the spore fruit, × 300. _ar._ archicarp. _G_, a ripe
spore fruit, × 150. _H_, the spore sac removed from the spore fruit,
× 150. _I_, spore-bearing filament attacked by another fungus
(_Cicinnobulus_), causing the enlargement of the basal cell, × 150.
_J_, a more advanced stage, × 300. _K_, spores, × 300.]
For microscopical study, fresh material may be used, or, if
necessary, dried specimens. The latter, before mounting, should be
soaked for a short time in water, to which has been added a few
drops of caustic-potash solution. This will remove the brittleness,
and swell up the dried filaments to their original proportions. A
portion of the plant should be carefully scraped off the leaf on
which it is growing, thoroughly washed in pure water, and
transferred to a drop of water or very dilute glycerine, in which it
should be carefully spread out with needles. If air bubbles
interfere with the examination, they may be driven off with alcohol,
and then the cover glass put on. If the specimen is mounted in
glycerine, it will keep indefinitely, if care is taken to seal it
up. The plant consists of much-interlaced filaments, divided at
intervals by cross-walls.[6] They are nearly colorless, and the
contents are not conspicuous. These filaments send up vertical
branches (Fig. 39, _A_), that become divided into a series of short
cells by means of cross-walls. The cells thus formed are at first
cylindrical, but later bulge out at the sides, becoming broadly
oval, and finally become detached as spores (_conidia_). It is these
spores that give the frosty appearance to the early stages of the
fungus when seen with the naked eye. The spores fall off very easily
when ripe, and germinate quickly in water, sending out two or more
tubes that grow into filaments like those of the parent plant
(Fig. 39, _B_).
[6] The filaments are attached to the surface of the leaf by suckers,
which are not so readily seen in this species as in some others. A
mildew growing abundantly in autumn on the garden chrysanthemum,
however, shows them very satisfactorily if a bit of the epidermis of a
leaf on which the fungus is just beginning to grow is sliced off with
a sharp razor and mounted in dilute glycerine, or water, removing the
air with alcohol. These suckers are then seen to be globular bodies,
penetrating the outer wall of the cell (Fig. 40).
[Illustration: FIG. 40.--Chrysanthemum mildew (_Erysiphe_), showing
the suckers (_h_) by which the filaments are attached to the leaf.
_A_, surface view. _B_, vertical section of the leaf, × 300.]
The spore fruits, as already observed, are formed toward the end of
the season, and, in the species under consideration at least, appear
to be the result of a sexual process. The sexual organs (if they are
really such) are extremely simple, and, owing to their very small
size, are not easily found. They arise as short branches at a point
where two filaments cross; one of them (Fig. 39, _C_, _ar._), the
female cell, or "archicarp," is somewhat larger than the other and
nearly oval in form, and soon becomes separated by a partition from
the filament that bears it. The other branch (antheridium) grows up
in close contact with the archicarp, and like it is shut off by a
partition from its filament. It is more slender than the archicarp,
but otherwise differs little from it. No actual communication can be
shown to be present between the two cells, and it is therefore still
doubtful whether fertilization really takes place. Shortly after
these organs are full-grown, several short branches grow up about
them, and soon completely envelop them (_D_, _E_). These branches
soon grow together, and cross-walls are formed in them, so that the
young spore fruit appears surrounded by a single layer of cells,
sufficiently transparent, however, to allow a view of the interior.
The antheridium undergoes no further change, but the archicarp soon
divides into two cells,--a small basal one and a larger upper cell.
There next grow from the inner surface of the covering cells, short
filaments, that almost completely fill the space between the
archicarp and the wall. An optical section of such a stage (Fig. 39,
_F_) shows a double wall and the two cells of the archicarp. The
spore fruit now enlarges rapidly, and the outer cells become first
yellow and then dark brown, the walls becoming thicker and harder as
they change color. Sometimes special filaments or appendages grow
out from their outer surfaces, and these are also dark-colored.
Shortly before the fruit is ripe, the upper cell of the archicarp,
which has increased many times in size, shows a division of its
contents into eight parts, each of which develops a wall and becomes
an oval spore. By crushing the ripe spore fruit, these spores still
enclosed in the mother cell (ascus) may be forced out (Fig. 39,
_H_). These spores do not germinate at once, but remain dormant
until the next year.
[Illustration: FIG. 41.--Forms of mildews (_Erysiphe_). _A_,
_Microsphæra_, a spore fruit, × 150. _B_, cluster of spore sacs of the
same, × 150. _C_, a single appendage, × 300. _D_, end of an appendage
of _Uncinula_, × 300. _E_, appendage of _Phyllactinia_, × 150.]
Frequently other structures, resembling somewhat the spore fruits,
are found associated with them (Fig. 39, _I_, _K_), and were for a
long time supposed to be a special form of reproductive organ; but
they are now known to belong to another fungus (_Cicinnobulus_),
parasitic upon the mildew. They usually appear at the base of the
chains of conidia, causing the basal cell to enlarge to many times
its original size, and finally kill the young conidia, which shrivel
up. A careful examination reveals the presence of very fine
filaments within those of the mildew, which may be traced up to the
base of the conidial branch, where the receptacle of the parasite is
forming. The spores contained in these receptacles are very small
(Fig. 39, _K_), and when ripe exude in long, worm-shaped masses, if
the receptacle is placed in water.
The mildews may be divided into two genera: _Podosphæra_, with a
single ascus in the spore fruit; and _Erysiphe_, with two or more. In
the latter the archicarp branches, each branch bearing a spore sac
(Fig. 41, _B_).
The appendages growing out from the wall of the spore fruit are often
very beautiful in form, and the two genera given above are often
subdivided according to the form of these appendages.
A common mould closely allied to the mildews is found on various
articles of food when allowed to remain damp, and is also very common
on botanical specimens that have been poorly dried, and hence is often
called "herbarium mould" (_Eurotium herbariorum_).
[Illustration: FIG. 42.--_A_, spore bearing filament of the herbarium
mould (_Eurotium_), × 150. _B_, _C_, another species showing the way
in which the spores are borne--optical section--× 150. _D_, spore
fruit of the herbarium mould, × 150. _E_, spore sac. _F_, spores,
× 300. _G_, spore-bearing filament of the common blue mould
(_Penicillium_), × 300. _sp._ the spores.]
The conidia are of a greenish color, and produced on the ends of
upright branches which are enlarged at the end, and from which grow
out little prominences, which give rise to the conidia in the same
way as we have seen in the mildews (Fig. 42, _A_).
Spore fruits much like those of the mildews are formed later, and
are visible to the naked eye as little yellow grains (Fig. 42, _D_).
These contain numerous very small spore sacs (_E_), each with eight
spores.
There are numerous common species of _Eurotium_, differing in color
and size, some being yellow or black, and larger than the ordinary
green form.
Another form, common everywhere on mouldy food of all kinds, as well
as in other situations, is the blue mould (_Penicillium_). This, in
general appearance, resembles almost exactly the herbarium mould, but
is immediately distinguishable by a microscopic examination (Fig. 42,
_G_).
In studying all of these forms, they may be mounted, as directed for
the black moulds, in dilute glycerine; but must be handled with
great care, as the spores become shaken off with the slightest jar.
Of the larger _Ascomycetes_, the cup fungi (_Discomycetes_) may be
taken as types. The spore fruit in these forms is often of
considerable size, and, as their name indicates, is open, having the
form of a flat disc or cup. A brief description of a common one will
suffice to give an idea of their structure and development.
_Ascobolus_ (Fig. 43) is a small, disc-shaped fungus, growing on horse
dung. By keeping some of this covered with a bell jar for a week or
two, so as to retain the moisture, at the end of this time a large
crop of the fungus will probably have made its appearance. The part
visible is the spore fruit (Fig. 43, _A_), of a light brownish color,
and about as big as a pin-head.
Its development may be readily followed by teasing out in water the
youngest specimens that can be found, taking care to take up a
little of the substratum with it, as the earliest stages are too
small to be visible to the naked eye. The spore fruits arise from
filaments not unlike those of the mildews, and are preceded by the
formation of an archicarp composed of several cells, and readily
seen through the walls of the young fruit (Fig. 43, _B_). In the
study of the early stages, a potash solution will be found useful in
rendering them transparent.
The young fruit has much the same structure as that of the mildews,
but the spore sacs are much more numerous, and there are special
sterile filaments developed between them. If the young spore fruit
is treated with chlor-iodide of zinc, it is rendered quite
transparent, and the young spore sacs colored a beautiful blue, so
that they are readily distinguishable.
[Illustration: FIG. 43.--_A_, a small cup fungus (_Ascobolus_), × 5.
_B_, young spore fruit, × 300. _ar._ archicarp. _C_, an older one,
× 150. _ar._ archicarp. _sp._ young spore sacs. _D_, section through a
full-grown spore fruit (partly diagrammatic), × 25. _sp._ spore sacs.
_E_, development of spore sacs and spores: i-iii, × 300; iv, × 150.
_F_, ripe spores. _G_, a sterile filament (paraphysis), × 300. _H_,
large scarlet cup fungus (_Peziza_), natural size.]
The development of the spore sacs may be traced by carefully
crushing the young spore fruits in water. The young spore sacs
(Fig. 43, _E_ i) are colorless, with granular protoplasm, in which a
nucleus can often be easily seen. The nucleus subsequently divides
repeatedly, until there are eight nuclei, about which the protoplasm
collects to form as many oval masses, each of which develops a wall
and becomes a spore (Figs. ii-iv). These are imbedded in protoplasm,
which is at first granular, but afterwards becomes almost
transparent. As the spores ripen, the wall acquires a beautiful
violet-purple color, changing later to a dark purple-brown, and
marked with irregular longitudinal ridges (Fig. 43, _F_). The
full-grown spore sacs (Fig. 43, _E_, _W_) are oblong in shape, and
attached by a short stalk. The sterile filaments between them often
become curiously enlarged at the end (_G_). As the spore fruit
ripens, it opens at the top, and spreads out so as to expose the
spore sacs as they discharge their contents (Fig. 43, _D_).
Of the larger cup fungi, those belonging to the genus _Peziza_
(Fig. 43, _H_) are common, growing on bits of rotten wood on the
ground in woods. They are sometimes bright scarlet or orange-red, and
very showy. Another curious form is the morel (_Morchella_), common in
the spring in dry woods. It is stalked like a mushroom, but the
surface of the conical cap is honeycombed with shallow depressions,
lined with the spore sacs.
ORDER _Lichenes_.
Under the name of lichens are comprised a large number of fungi,
differing a good deal in structure, but most of them not unlike the
cup fungi. They are, with few exceptions, parasitic upon various forms
of algæ, with which they are so intimately associated as to form
apparently a single plant. They grow everywhere on exposed rocks, on
the ground, trunks of trees, fences, etc., and are found pretty much
the world over. Among the commonest of plants are the lichens of the
genus _Parmelia_ (Fig. 44, _A_), growing everywhere on tree trunks,
wooden fences, etc., forming gray, flattened expansions, with much
indented and curled margins. When dry, the plant is quite brittle, but
on moistening becomes flexible, and at the same time more or less
decidedly green in color. The lower surface is white or brown, and
often develops root-like processes by which it is fastened to the
substratum. Sometimes small fragments of the plant become detached in
such numbers as to form a grayish powder over certain portions of it.
These, when supplied with sufficient moisture, will quickly produce
new individuals.
Not infrequently the spore fruits are to be met with flat discs of a
reddish brown color, two or three millimetres in diameter, and closely
resembling a small cup fungus. They are at first almost closed, but
expand as they mature (Fig. 44, _A_, _ap._).
[Illustration: FIG. 44.--_A_, a common lichen (_Parmelia_), of the
natural size. _ap._ spore fruit. _B_, section through one of the spore
fruits, × 5. _C_, section through the body of a gelatinous lichen
(_Collema_), showing the _Nostoc_ individuals surrounded by the fungus
filaments, × 300. _D_, a spermagonium of _Collema_, × 25. _E_, a
single _Nostoc_ thread. _F_, spore sacs and paraphyses of _Usnea_,
× 300. _G_, _Protococcus_ cells and fungus filaments of _Usnea_.]
If a thin vertical section of the plant is made and sufficiently
magnified, it is found to be made up of somewhat irregular,
thick-walled, colorless filaments, divided by cross-walls as in the
other sac-fungi. In the central parts of the plant these are rather
loose, but toward the outside become very closely interwoven and
often grown together, so as to form a tough rind. Among the
filaments of the outer portion are numerous small green cells, that
closer examination shows to be individuals of _Protococcus_, or some
similar green algæ, upon which the lichen is parasitic. These are
sufficiently abundant to form a green line just inside the rind if
the section is examined with a simple lens (Fig. 44, _B_).
The spore fruits of the lichens resemble in all essential respects
those of the cup fungi, and the spore sacs (Fig. 44, _F_) are much
the same, usually, though not always, containing eight spores, which
are sometimes two-celled. The sterile filaments between the spore
sacs usually have thickened ends, which are dark-colored, and give
the color to the inner surface of the spore fruit.
In Figure 45, _H_, is shown one of the so-called "_Soredia_,"[7] a
group of the algæ, upon which the lichen is parasitic, surrounded by
some of the filaments, the whole separating spontaneously from the
plant and giving rise to a new one.
[7] Sing. _soredium_.
Owing to the toughness of the filaments, the finer structure of the
lichens is often difficult to study, and free use of caustic potash is
necessary to soften and make them manageable.
[Illustration: FIG. 45.--Forms of lichens. _A_, a branch with lichens
growing upon it, one-half natural size. _B_, _Usnea_, natural size.
_ap._ spore fruit. _C_, _Sticta_, one-half natural size. _D_,
_Peltigera_, one-half natural size. _ap._ spore fruit. _E_, a single
spore fruit, × 2. _F_, _Cladonia_, natural size. _G_, a piece of bark
from a beech, with a crustaceous lichen (_Graphis_) growing upon it,
× 2. _ap._ spore fruit. _H_, _Soredium_ of a lichen, × 300.]
According to their form, lichens are sometimes divided into the bushy
(fruticose), leafy (frondose), incrusting (crustaceous), and
gelatinous. Of the first, the long gray _Usnea_ (Fig. 45, _A_, _B_),
which drapes the branches of trees in swamps, is a familiar example;
of the second, _Parmelia_, _Sticta_ (Fig. 45, _C_) and _Peltigera_
(_D_) are types; of the third, _Graphis_ (_G_), common on the trunks
of beech-trees, to which it closely adheres; and of the last,
_Collema_ (Fig. 44, _C_, _D_, _E_), a dark greenish, gelatinous form,
growing on mossy tree trunks, and looking like a colony of _Nostoc_,
which indeed it is, but differing from an ordinary colony in being
penetrated everywhere by the filaments of the fungus growing upon it.
Not infrequently in this form, as well as in other lichens, special
cavities, known as spermogonia (Fig. 44, _D_), are found, in which
excessively small spores are produced, which have been claimed to
be male reproductive cells, but the latest investigations do not
support this theory.
[Illustration: FIG. 46.--Branch of a plum-tree attacked by black knot.
Natural size.]
The last group of the _Ascomycetes_ are the "black fungi,"
_Pyrenomycetes_, represented by the black knot of cherry and plum
trees, shown in Figure 46. They are mainly distinguished from the cup
fungi by producing their spore sacs in closed cavities. Some are
parasites; others live on dead wood, leaves, etc., forming very hard
masses, generally black in color, giving them their common name. Owing
to the hardness of the masses, they are very difficult to manipulate;
and, as the structure is not essentially different from that of the
_Discomycetes_, the details will not be entered into here.
Of the parasitic forms, one of the best known is the "ergot" of rye,
more or less used in medicine. Other forms are known that attack
insects, particularly caterpillars, which are killed by their attacks.