FUNGI--_Continued_.
CLASS _Basidiomycetes_.
The _Basidiomycetes_ include the largest and most highly developed of
the fungi, among which are many familiar forms, such as the mushrooms,
toadstools, puff-balls, etc. Besides these large and familiar forms,
there are other simpler and smaller ones that, according to the latest
investigations, are probably related to them, though formerly regarded
as constituting a distinct group. The most generally known of these
lower _Basidiomycetes_ are the so-called rusts. The larger
_Basidiomycetes_ are for the most part saprophytes, living in decaying
vegetable matter, but a few are true parasites upon trees and others
of the flowering plants.
All of the group are characterized by the production of spores at the
top of special cells known as basidia,[8] the number produced upon a
single basidium varying from a single one to several.
[8] Sing. _basidium_.
Of the lower _Basidiomycetes_, the rusts (_Uredineæ_) offer common and
easily procurable forms for study. They are exclusively parasitic in
their habits, growing within the tissues of the higher land plants,
which they often injure seriously. They receive their popular name
from the reddish color of the masses of spores that, when ripe, burst
through the epidermis of the host plant. Like many other fungi, the
rusts have several kinds of spores, which are often produced on
different hosts; thus one kind of wheat rust lives during part of its
life within the leaves of the barberry, where it produces spores quite
different from those upon the wheat; the cedar rust, in the same way,
is found at one time attacking the leaves of the wild crab-apple and
thorn.
[Illustration: FIG. 47.--_A_, a branch of red cedar attacked by a rust
(_Gymnosporangium_), causing a so-called "cedar apple," × ½. _B_,
spores of the same, one beginning to germinate, × 300. _C_, a spore
that has germinated, each cell producing a short, divided filament
(basidium), which in turn gives rise to secondary spores (_sp._),
× 300. _D_, part of the leaf of a hawthorn attacked by the cluster cup
stage of the same fungus, upper side showing spermogonia, natural
size. _E_, cluster cups (_Roestelia_) of the same fungus, natural
size. _F_, tip of a leaf of the Indian turnip (_Arisæma_), bearing the
cluster cup (_Æcidium_) stage of a rust, × 2. _G_, vertical section
through a young cluster cup. _H_, similar section through a mature
one, × 50. _I_, germinating spores of _H_, × 300. _J_, part of a corn
leaf, with black rust, natural size. _K_, red rust spore of the wheat
rust (_Puccinia graminis_), × 300. _L_, forms of black-rust spores: i,
_Uromyces_; ii, _Puccinia_; iii, _Phragmidium_.]
The first form met with in most rusts is sometimes called the
"cluster-cup" stage, and in many species is the only stage known. In
Figure 47, _F_, is shown a bit of the leaf of the Indian turnip
(_Arisæma_) affected by one of these "cluster-cup" forms. To the naked
eye, or when slightly magnified, the masses of spores appear as bright
orange spots, mostly upon the lower surface. The affected leaves are
more or less checked in their growth, and the upper surface shows
lighter blotches, corresponding to the areas below that bear the
cluster cups. These at first appear as little elevations of a
yellowish color, and covered with the epidermis; but as the spores
ripen they break through the epidermis, which is turned back around
the opening, the whole forming a little cup filled with a bright
orange red powder, composed of the loose masses of spores.
Putting a piece of the affected leaf between two pieces of pith so
as to hold it firmly, with a little care thin vertical sections of
the leaf, including one of the cups, may be made, and mounted,
either in water or glycerine, removing the air with alcohol. We find
that the leaf is thickened at this point owing to a diseased growth
of the cells of the leaf, induced by the action of the fungus. The
mass of spores (Fig. 47, _G_) is surrounded by a closely woven mass
of filaments, forming a nearly globular cavity. Occupying the bottom
of the cup are closely set, upright filaments, each bearing a row of
spores, arranged like those of the white rusts, but so closely
crowded as to be flattened at the sides. The outer rows have
thickened walls, and are grown together so as to form the wall of
the cup.
The spores are filled with granular protoplasm, in which are
numerous drops of orange-yellow oil, to which is principally due
their color. As the spores grow, they finally break the overlying
epidermis, and then become rounded as the pressure from the sides is
relieved. They germinate within a few hours if placed in water,
sending out a tube, into which pass the contents of the spore
(Fig. 47, _I_).
One of the most noticeable of the rusts is the cedar rust
(_Gymnosporangium_), forming the growths known as "cedar apples,"
often met with on the red cedar. These are rounded masses, sometimes
as large as a walnut, growing upon the small twigs of the cedar
(Fig. 47, _A_). This is a morbid growth of the same nature as those
produced by the white rusts and smuts. If one of these cedar apples is
examined in the late autumn or winter, it will be found to have the
surface dotted with little elevations covered by the epidermis, and on
removing this we find masses of forming spores. These rupture the
epidermis early in the spring, and appear then as little spikes of a
rusty red color. If they are kept wet for a few hours, they enlarge
rapidly by the absorption of water, and may reach a length of four or
five centimetres, becoming gelatinous in consistence, and sometimes
almost entirely hiding the surface of the "apple." In this stage the
fungus is extremely conspicuous, and may frequently be met with after
rainy weather in the spring.
This orange jelly, as shown by the microscope, is made up of
elongated two-celled spores (teleuto spores), attached to long
gelatinous stalks (Fig. 47, _B_). They are thick-walled, and the
contents resemble those of the cluster-cup spores described above.
To study the earlier stages of germination it is best to choose
specimens in which the masses of spores have not been moistened. By
thoroughly wetting these, and keeping moist, the process of
germination may be readily followed. Many usually begin to grow
within twenty-four hours or less. Each cell of the spore sends out a
tube (Fig. 47, _C_), through an opening in the outer wall, and this
tube rapidly elongates, the spore contents passing into it, until a
short filament (basidium) is formed, which then divides into several
short cells. Each cell develops next a short, pointed process, which
swells up at the end, gradually taking up all the contents of the
cell, until a large oval spore (_sp._) is formed at the tip,
containing all the protoplasm of the cell.
Experiments have been made showing that these spores do not germinate
upon the cedar, but upon the hawthorn or crab-apple, where they
produce the cluster-cup stage often met with late in the summer. The
affected leaves show bright orange-yellow spots about a centimetre in
diameter (Fig. 47, _D_), and considerably thicker than the other parts
of the leaf. On the upper side of these spots may be seen little black
specks, which microscopic examination shows to be spermogonia,
resembling those of the lichens. Later, on the lower surface, appear
the cluster cups, whose walls are prolonged so that they form little
tubular processes of considerable length (Fig. 47, _E_).
In most rusts the teleuto spores are produced late in the summer or
autumn, and remain until the following spring before they germinate.
They are very thick-walled, the walls being dark-colored, so that in
mass they appear black, and constitute the "black-rust" stage
(Fig. 47, _J_). Associated with these, but formed earlier, and
germinating immediately, are often to be found large single-celled
spores, borne on long stalks. They are usually oval in form, rather
thin-walled, but the outer surface sometimes provided with little
points. The contents are reddish, so that in mass they appear of the
color of iron rust, and cause the "red rust" of wheat and other
plants, upon which they are growing.
The classification of the rusts is based mainly upon the size and
shape of the teleuto spores where they are known, as the cluster-cup
and red-rust stages are pretty much the same in all. Of the commoner
genera _Melampsora_, and _Uromyces_ (Fig. 47, _L_ i), have unicellular
teleuto spores; _Puccinia_ (ii) and _Gymnosporangium_, two-celled
spores; _Triphragmium_, three-celled; and _Phragmidium_ (iii), four or
more.
The rusts are so abundant that a little search can scarcely fail to
find some or all of the stages. The cluster-cup stages are best
examined fresh, or from alcoholic material; the teleuto spores may be
dried without affecting them.
Probably the best-known member of the group is the wheat rust
(_Puccinia graminis_), which causes so much damage to wheat and
sometimes to other grains. The red-rust stage may be found in early
summer; the black-rust spores in the stubble and dead leaves in the
autumn or spring, forming black lines rupturing the epidermis.
Probably to be associated with the lower _Basidiomycetes_ are the
large fungi of which _Tremella_ (Fig. 51, _A_) is an example. They are
jelly-like forms, horny and somewhat brittle when dry, but becoming
soft when moistened. They are common, growing on dead twigs, logs,
etc., and are usually brown or orange-yellow in color.
Of the higher _Basidiomycetes_, the toadstools, mushrooms, etc., are
the highest, and any common form will serve for study. One of the most
accessible and easily studied forms is _Coprinus_, of which there are
several species growing on the excrement of various herbivorous
animals. They not infrequently appear on horse manure that has been
kept covered with a glass for some time, as described for _Ascobolus_.
After two or three weeks some of these fungi are very likely to make
their appearance, and new ones continue to develop for a long time.
[Illustration: FIG. 48.--_A_, young. _B_, full-grown fruit of a
toadstool (_Coprinus_), × 2. _C_, under side of the cap, showing the
radiating "gills," or spore-bearing plates. _D_, section across one of
the young gills, × 150. _E_, _F_, portions of gills from a nearly ripe
fruit, × 300. _sp._ spores. _x_, sterile cell. In _F_, a basidium is
shown, with the young spores just forming. _G_, _H_, young fruits,
× 50.]
The first trace of the plant, visible to the naked eye, is a little
downy, white speck, just large enough to be seen. This rapidly
increases in size, becoming oblong in shape, and growing finally
somewhat darker in color; and by the time it reaches a height of a few
millimetres a short stalk becomes perceptible, and presently the whole
assumes the form of a closed umbrella. The top is covered with little
prominences, that diminish in number and size toward the bottom. After
the cap reaches its full size, the stalk begins to grow, slowly at
first, but finally with great rapidity, reaching a height of several
centimetres within a few hours. At the same time that the stalk is
elongating, the cap spreads out, radial clefts appearing on its upper
surface, which flatten out very much as the folds of an umbrella are
stretched as it opens, and the spaces between the clefts appear as
ridges, comparable to the ribs of the umbrella (Fig. 48, _B_). The
under side of the cap has a number of ridges running from the centre
to the margin, and of a black color, due to the innumerable spores
covering their surface (_C_). Almost as soon as the umbrella opens,
the spores are shed, and the whole structure shrivels up and
dissolves, leaving almost no trace behind.
If we examine microscopically the youngest specimens procurable,
freeing from air with alcohol, and mounting in water or dilute
glycerine, we find it to be a little, nearly globular mass of
colorless filaments, with numerous cross-walls, the whole arising
from similar looser filaments imbedded in the substratum (Fig. 48,
_G_). If the specimen is not too young, a denser central portion can
be made out, and in still older ones (Fig. 48, _H_) this central
mass has assumed the form of a short, thick stalk, crowned by a flat
cap, the whole invested by a loose mass of filaments that merge more
or less gradually into the central portion. By the time the spore
fruit (for this structure corresponds to the spore fruit of the
_Ascomycetes_) reaches a height of two or three millimetres, and is
plainly visible to the naked eye, the cap grows downward at the
margins, so as to almost entirely conceal the stalk. A longitudinal
section of such a stage shows the stalk to be composed of a
small-celled, close tissue becoming looser in the cap, on whose
inner surface the spore-bearing ridges ("gills" or _Lamellæ_) have
begun to develop. Some of these run completely to the edge of the
cap, others only part way. To study their structure, make
cross-sections of the cap of a nearly full-grown, but unopened,
specimen, and this will give numerous sections of the young gills.
We find them to be flat plates, composed within of loosely
interwoven filaments, whose ends stand out at right angles to the
surface of the gills, forming a layer of closely-set upright cells
(basidia) (Fig. 48, _D_). These are at first all alike, but later
some of them become club-shaped, and develop at the end several
(usually four) little points, at the end of which spores are formed
in exactly the same way as we saw in the germinating teleuto spores
of the cedar rust, all the protoplasm of the basidium passing into
the growing spores (Fig. 48, _E_, _F_). The ripe spores (_E_, _sp._)
are oval, and possess a firm, dark outer wall. Occasionally some of
the basidia develop into very large sterile cells (E, _x_),
projecting far beyond the others, and often reaching the neighboring
gill.
Similar in structure and development to _Coprinus_ are all the large
and common forms; but they differ much in the position of the
spore-bearing tissue, as well as in the form and size of the whole
spore fruit. They are sometimes divided, according to the position of
the spores, into three orders: the closed-fruited (_Angiocarpous_)
forms, the half-closed (_Hemi-angiocarpous_), and the open or
naked-fruited forms (_Gymnocarpous_).
[Illustration: FIG. 49.--_Basidiomycetes_. _A_, common puff-ball
(_Lycoperdon_). _B_, earth star (_Geaster_). _A_, × ¼. _B_, one-half
natural size.]
Of the first, the puff-balls (Fig. 49) are common examples. One
species, the giant puff-ball (_Lycoperdon giganteum_), often reaches a
diameter of thirty to forty centimetres. The earth stars (_Geaster_)
have a double covering to the spore fruit, the outer one splitting at
maturity into strips (Fig. 49, _B_). Another pretty and common form is
the little birds'-nest fungus (_Cyathus_), growing on rotten wood or
soil containing much decaying vegetable matter (Fig. 50).
[Illustration: FIG. 50.--Birds'-nest fungus (_Cyathus_). _A_, young.
_B_, full grown. _C_, section through _B_, showing the "sporangia"
(_sp._). All twice the natural size.]
In the second order the spores are at first protected, as we have seen
in _Coprinus_, which belongs to this order, but finally become
exposed. Here belong the toadstools and mushrooms (Fig. 51, _B_), the
large shelf-shaped fungi (_Polyporus_), so common on tree trunks and
rotten logs (Fig. 51, _C_, _D_, _E_), and the prickly fungus
(_Hydnum_) (Fig. 51, _G_).
[Illustration: FIG. 51.--Forms of _Basidiomycetes_. _A_, _Tremella_,
one-half natural size. _B_, _Agaricus_, natural size. _C_, _E_,
_Polyporus_: _C_, × ½; _E_, × ¼. _D_, part of the under surface of
_D_, natural size. _F_, _Clavaria_, a small piece, natural size. _G_,
_Hydnum_, a piece of the natural size.]
Of the last, or naked-fruited forms, the commonest belong to the
genus _Clavaria_ (Fig. 51, _F_), smooth-branching forms, usually of a
brownish color, bearing the spores directly upon the surface of the
branches.