SUB-KINGDOM III.
FUNGI.
The name "Fungi" has been given to a vast assemblage of plants,
varying much among themselves, but on the whole of about the same
structural rank as the algæ. Unlike the algæ, however, they are
entirely destitute of chlorophyll, and in consequence are dependent
upon organic matter for food, some being parasites (growing upon
living organisms), others saprophytes (feeding on dead matter). Some
of them show close resemblances in structure to certain algæ, and
there is reason to believe that they are descended from forms that
originally had chlorophyll; others are very different from any green
plants, though more or less evidently related among themselves.
Recognizing then these distinctions, we may make two divisions of the
sub-kingdom: I. The Alga-Fungi (_Phycomycetes_), and II. The True
Fungi (_Mycomycetes_).
CLASS I.--_Phycomycetes_.
These are fungi consisting of long, undivided, often branching tubular
filaments, resembling quite closely those of _Vaucheria_ or other
_Siphoneæ_, but always destitute of any trace of chlorophyll. The
simplest of these include the common moulds (_Mucorini_), one of which
will serve to illustrate the characteristics of the order.
If a bit of fresh bread, slightly moistened, is kept under a bell jar
or tumbler in a warm room, in the course of twenty-four hours or so it
will be covered with a film of fine white threads, and a little later
will produce a crop of little globular bodies mounted on upright
stalks. These are at first white, but soon become black, and the
filaments bearing them also grow dark-colored.
These are moulds, and have grown from spores that are in the
atmosphere falling on the bread, which offers the proper conditions
for their growth and multiplication.
One of the commonest moulds is the one here figured (Fig. 32), and
named _Mucor stolonifer_, from the runners, or "stolons," by which it
spreads from one point to another. As it grows it sends out these
runners along the surface of the bread, or even along the inner
surface of the glass covering it. They fasten themselves at intervals
to the substratum, and send up from these points clusters of short
filaments, each one tipped with a spore case, or "sporangium."
For microscopical study they are best mounted in dilute glycerine
(about one-quarter glycerine to three-quarters pure water). After
carefully spreading out the specimens in this mixture, allow a drop
of alcohol to fall upon the preparation, and then put on the cover
glass. The alcohol drives out the air, which otherwise interferes
badly with the examination.
The whole plant consists of a very long, much-branched, but
undivided tubular filament. Where it is in contact with the
substratum, root-like outgrowths are formed, not unlike those
observed in _Vaucheria_. At first the walls are colorless, but later
become dark smoky brown in color. A layer of colorless granular
protoplasm lines the wall, becoming more abundant toward the growing
tips of the branches. The spore cases, "sporangia," arise at the
ends of upright branches (Fig. 32, _C_), which at first are
cylindrical (_a_), but later enlarge at the end (_b_), and become
cut off by a convex wall (_c_). This wall pushes up into the young
sporangium, forming a structure called the "columella." When fully
grown, the sporangium is globular, and appears quite opaque, owing
to the numerous granules in the protoplasm filling the space between
the columella and its outer wall. This protoplasm now divides into a
great number of small oval cells (spores), which rapidly darken,
owing to a thick, black wall formed about each one, and at the same
time the columella and the stalk of the sporangium become
dark-colored.
When ripe, the wall of the sporangium dissolves, and the spores
(Fig. 32, _E_) are set free. The columella remains unchanged, and
some of the spores often remain sticking to it (Fig. 32, _D_).
[Illustration: FIG. 32.--_A_, common black mould (_Mucor_), × 5. _B_,
three nearly ripe spore cases, × 25. _C_, development of the spore
cases, i-iv, × 150; v, × 50. _D_, spore case which has discharged its
spores. _E_, spores, × 300. _F_, a form of _Mucor mucedo_, with small
accessory spore cases, × 5. _G_, the spore cases, × 50. _H_, a single
spore case, × 300. _I_, development of the zygospore of a black mould,
× 45 (after De Bary).]
Spores formed in a manner strongly recalling those of the pond scums
are also known, but only occur after the plants have grown for a
long time, and hence are rarely met with (Fig. 32, _I_).
Another common mould (_M. mucedo_), often growing in company with the
one described, differs from it mainly in the longer stalk of the
sporangium, which is also smaller, and in not forming runners. This
species sometimes bears clusters of very small sporangia attached to
the middle of the ordinary sporangial filament (Fig. 32, _F_, _H_).
These small sporangia have no columella.
Other moulds are sometimes met with, parasitic upon the larger species
of _Mucor_.
Related to the black moulds are the insect moulds (_Entomopthoreæ_),
which attack and destroy insects. The commonest of these attacks the
house flies in autumn, when the flies, thus infested, may often be
found sticking to window panes, and surrounded by a whitish halo of
the spores that have been thrown off by the fungus.
ORDER II.--WHITE RUSTS AND MILDEWS (_Peronosporeæ_)
These are exclusively parasitic fungi, and grow within the tissues of
various flowering plants, sometimes entirely destroying them.
As a type of this group we will select a very common one (_Cystopus
bliti_), that is always to be found in late summer and autumn growing
on pig weed (_Amarantus_). It forms whitish, blister-like blotches
about the size of a pin head on the leaves and stems, being commonest
on the under side of the leaves (Fig. 33, _A_). In the earlier stages
the leaf does not appear much affected, but later becomes brown and
withered about the blotches caused by the fungus.
If a thin vertical section of the leaf is made through one of these
blotches, and mounted as described for _Mucor_, the latter is found
to be composed of a mass of spores that have been produced below the
epidermis of the leaf, and have pushed it up by their growth. If the
section is a very thin one, we may be able to make out the structure
of the fungus, and then find it to be composed of irregular,
tubular, much-branched filaments, which, however, are not divided by
cross-walls. These filaments run through the intercellular spaces of
the leaf, and send into the cells little globular suckers, by means
of which the fungus feeds.
The spores already mentioned are formed at the ends of crowded
filaments, that push up, and finally rupture the epidermis (Fig. 33,
_B_). They are formed by the ends of the filaments swelling up and
becoming constricted, so as to form an oval spore, which is then cut
off by a wall. The portion of the filament immediately below acts in
the same way, and the process is repeated until a chain of half a
dozen or more may be produced, the lowest one being always the last
formed. When ripe, the spores are separated by a thin neck, and
become very easily broken off.
In order to follow their germination it is only necessary to place a
few leaves with fresh patches of the fungus under a bell jar or
tumbler, inverted over a dish full of water, so as to keep the air
within saturated with moisture, but taking care to keep the leaves
out of the water. After about twenty-four hours, if some of the
spores are scraped off and mounted in water, they will germinate in
the course of an hour or so. The contents divide into about eight
parts, which escape from the top of the spore, which at this time
projects as a little papilla. On escaping, each mass of protoplasm
swims away as a zoöspore, with two extremely delicate cilia. After a
short time it comes to rest, and, after developing a thin cell wall,
germinates by sending out one or two filaments (Fig. 33, _C_, _E_).
[Illustration: FIG. 33.--_A_, leaf of pig-weed (_Amarantus_), with
spots of white rust (_c_), one-half natural size. _B_, non-sexual
spores (conidia). _C_, the same germinating. _D_, zoöspores. _E_,
germinating zoöspores. _sp._ the spore. _F_, young. _G_, mature sexual
organs. In _G_, the tube may be seen connecting the antheridium
(_an._), with the egg cell (_o_). _H_, a ripe resting spore still
surrounded by the wall of the oögonium. _I_, a part of a filament of
the fungus, showing its irregular form. All × 300.]
Under normal conditions the spores probably germinate when the
leaves are wet, and the filaments enter the plant through the
breathing pores on the lower surface of the leaves, and spread
rapidly through the intercellular spaces.
Later on, spores of a very different kind are produced. Unlike those
already studied, they are formed some distance below the epidermis,
and in order to study them satisfactorily, the fungus must be freed
from the host plant. In order to do this, small pieces of the leaf
should be boiled for about a minute in strong caustic potash, and
then treated with acetic or hydrochloric acid. By this means the
tissues of the leaf become so soft as to be readily removed, while
the fungus is but little affected. The preparation should now be
washed and mounted in dilute glycerine.
The spores (oöspores) are much larger than those first formed, and
possess an outer coat of a dark brown color (Fig. 33, _H_). Each
spore is contained in a large cell, which arises as a swelling of
one of the filaments, and becomes shut off by a wall. At first
(Fig. 33, _F_) its contents are granular, and fill it completely,
but later contract to form a globular mass of protoplasm (G.
_o_), the germ cell or egg cell. The whole is an oögonium, and
differs in no essential respect from that of _Vaucheria_.
Frequently a smaller cell (antheridium), arising from a neighboring
filament, and in close contact with the oögonium, may be detected
(Fig. 33, _F_, _G_, _an._), and in exceptionally favorable cases a
tube is to be seen connecting it with the germ cell, and by means of
which fertilization is effected.
After being fertilized, the germ cell secretes a wall, at first thin
and colorless, but later becoming thick and dark-colored on the
outside, and showing a division into several layers, the outermost
of which is dark brown, and covered with irregular reticulate
markings. These spores do not germinate at once, but remain over
winter unchanged.
[Illustration: FIG. 34.--Fragment of a filament of the white rust of
the shepherd's-purse, showing the suckers (_h_), × 300.]
It is by no means impossible that sometimes the germ cell may develop
into a spore without being fertilized, as is the case in many of the
water moulds.
Closely related to the species above described is another one
(_C. candidus_), which attacks shepherd's-purse, radish, and others of
the mustard family, upon which it forms chalky white blotches, and
distorts the diseased parts of the plant very greatly.
For some reasons this is the best species for study, longitudinal
sections through the stem showing very beautifully the structure of
the fungus, and the penetration of the cells of the host[4] by the
suckers (Fig. 34).
[4] "Host," the plant or animal upon which a parasite lives.
[Illustration: FIG. 35.--Non-sexual spores of the vine mildew
(_Peronospora viticola_), × 150.]
Very similar to the white rusts in most respects, but differing in the
arrangement of the non-sexual spores, are the mildews (_Peronospora_,
_Phytophthora_). These plants form mouldy-looking patches on the
leaves and stems of many plants, and are often very destructive. Among
them are the vine mildew (_Peronospora viticola_) (Fig. 35), the
potato fungus (_Phytophthora infestans_), and many others.
ORDER III.--_Saprolegniaceæ_ (WATER MOULDS).
These plants resemble quite closely the white rusts, and are probably
related to them. They grow on decaying organic matter in water, or
sometimes on living water animals, fish, crustaceans, etc. They may
usually be had for study by throwing into water taken from a stagnant
pond or aquarium, a dead fly or some other insect. After a few days it
will probably be found covered with a dense growth of fine, white
filaments, standing out from it in all directions (Fig. 36, _A_).
Somewhat later, if carefully examined with a lens, little round, white
bodies may be seen scattered among the filaments.
[Illustration: FIG. 36.--_A_, an insect that has decayed in water, and
become attacked by a water mould (_Saprolegnia_), natural size. _B_, a
ripe zoösporangium, × 100. _C_, the same discharging the spores. _D_,
active. _E_, germinating zoöspores, × 300. _F_, a second sporangium
forming below the empty one. _G_ i-iv, development of the oögonium,
× 100. _H_, ripe oögonium filled with resting spores, × 100.]
On carefully removing a bit of the younger growth and examining it
microscopically, it is found to consist of long filaments much like
those of _Vaucheria_, but entirely destitute of chlorophyll. In
places these filaments are filled with densely granular protoplasm,
which when highly magnified exhibits streaming movements. The
protoplasm contains a large amount of oil in the form of small,
shining drops.
In the early stages of its growth the plant multiplies by zoöspores,
produced in great numbers in sporangia at the ends of the branches.
The protoplasm collects here much as we saw in _V. sessilis_, the
end of the filament becoming club-shaped and ending in a short
protuberance (Fig. 36, _B_). This end becomes separated by a wall,
and the contents divide into numerous small cells that sometimes are
naked, and sometimes have a delicate membrane about them. The first
sign of division is the appearance in the protoplasm of delicate
lines dividing it into numerous polygonal areas which soon become
more distinct, and are seen to be distinct cells whose outlines
remain more or less angular on account of the mutual pressure. When
ripe, the end of the sporangium opens, and the contained cells are
discharged (Fig. 36, _C_). In case they have no membrane, they swim
away at once, each being provided with two cilia, and resembling
almost exactly the zoöspores of the white rust (Fig. 36, _D_, _E_).
When the cells are surrounded by a membrane they remain for some
time at rest, but finally the contents escape as a zoöspore, like
those already described. By killing the zoöspores with a little
iodine the granular nature of the protoplasm is made more evident,
and the cilia may be seen. They soon come to rest, and germinate in
the same way as those of the white rusts and mildews.
As soon as the sporangium is emptied, a new one is formed, either by
the filament growing up through it (Fig. 36, _F_) and the end being
again cut off, or else by a branch budding out just below the base
of the empty sporangium, and growing up by the side of it.
Besides zoöspores there are also resting spores developed. Oögonia
like those of _Vaucheria_ or the _Peronosporeæ_ are formed usually
after the formation of zoöspores has ceased; but in many cases,
perhaps all, these develop without being fertilized. Antheridia are
often wanting, and even when they are present, it is very doubtful
whether fertilization takes place.[5]
[5] The antheridia, when present, arise as branches just below the
oögonium, and become closely applied to it, sometimes sending tubes
through its wall, but there has been no satisfactory demonstration of
an actual transfer of the contents of the antheridium to the egg cell.
The oögonia (Fig. 36, _G_, _H_) arise at the end of the main
filaments, or of short side branches, very much as do the sporangia,
from which they differ at this stage in being of globular form. The
contents contract to form one or several egg cells, naked at first,
but later becoming thick-walled resting spores (_H_).