THE
SLIME
ORGANISMS
by John E. Peterson
THE KAN$A$ $CHOOl NATURAl/$T Vol. 36
No.2
Emporia State University
Emporia, Kansas
Dec.
1989
The Kansas School Naturalist Published by
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Prepared and Issued by
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3
THE SLIME ORGANISMS
by John E. Peterson The expression "slime ball" (as in
"He is a slime ball!") is always used
disparagingly. No one ever calls
someone they like or admire a "slime
ball." Use of the word, slime, in any
context often brings a "yuck," or a frown
at the very least, from a listener. Why is
that? What are the connotations of
"slime" in most minds?
My dictionary gives three definitions
for the word, slime. They are: (1) Soft,
moist earth or clay; viscous mud; (2)
Any dirty substance that is moist, soft,
and adhesive; (3) The mucous or
mucus-like secretion of the skin of
slugs, land snails, etc. No wonder the
word gets such bad press! One of my
scientific dictionaries, however, does the
word much more credit. It has only two
simple defmitions: (1) A wet, generally
sticky, substance; (2) mucus. Now, that
is mOre like it.
The outer cell layers of most
organisms manufacture and exude
materials which protect the organism
from desiccation, from chemical and
physical damage, and which assist it in
adapting to its environment. In so-called
"lower organisms," the production of
wet, viscous slimes is commonplace.
Practically all organisms which exist in
soil and decaying organic debris habitats
do so. We are all familiar with the slime
traces left on sidewalks and other
surfaces by slugs after their nightly
forays out to eat our plants. We are all
familiar with the slimy character of
earthworms and insect larvae when we
dig them out of the soil. Interestingly,
some insect larvae in the soil and
organic debris are quite slimy, whereas
the adult stage of the same insect,
which will spend its existence out in the
atmopshere, is quite "dry."
Slime is a very common biological
material. It varies chemically, but many
cells and tissues produce it. Most of the
microscopic organisms which we
generally lump together as "microbes"­
-bacteria, protozoa, molds and other
fungi, algae, tiny worms and other
simple animals with which the soils and
waters of the world literally teem--are
invariably slime producers. They
produce slime, in greater or lesser
amounts, as a shield against their
environment, as a layer which keeps
them from drying out, from being
damaged by temperature fluctuations,
from being affected by various
chemicals in their environment.
If one contemplates the situation a
Dedication
My graduate work on slime molds was directed by a wonderful teacher, Prof. Constantine J.
Alexopoulos, whose influence has bolstered my work and my life in subsequent years. I wish to
dedicate this issue of the Naturalist to him--and to the public school biology teachers, whose conduct
and contributions are too often not recognized nor commended. JEP
Dr. John E. Peterson is Emeritus Professor of Biology at ESU. He served as Dean of Liberal Arts
and Sciences for 12 years before retiring. His personal publication, The Life of the Mind, has reached
international proportions and has received praise from a wide variety of recipients. He is the author
of several issues of the Naturalist.
Illustrations are by Dr. Robert F. Clarke.
4
moment, it soon becomes clear, in view
of the tremendous populations of these
microbes in our world, that there must
be a heck of a lot of slime
produced.Indeed, we may say that we
live in a slimy world. It is of special
interest, then, that, even under such
circumstances, there are three groups of
organisms which are such outstanding
slime producers that this characteristic
has become an integral part of their
names.
The purpose of the following pages
is to acquaint you--the teacher, the
student, the general reader--with these
three groups of very common, but
generally unseen and unknown,
absolutely fascinating organisms. They
may be thought of collectively as "the
slime organisms."
The Names and Sizes of the Groups of
Slime Organisms
. The common names for these three
groups of slime organisms are the slime
bacteria, the cellular slime molds, and
the acellular, or true, slime molds.
The technical name for the slime
bacteria is myxobacteria, or the
Myxobacterales, to make it even more
technical and give it its proper ending
as an order of the bacteria. The "myxo"
part of the name means "slime" in the
Greek. Only about 30 species of
myxobacteria are recognized, but they
occur in great profusion in soils and
decaying organic debris all over the
world.
The technical name for the cellular
slime molds is Acrasiae, Acrasiales, or
various other designations, depending
on the authority. That for the true slime
molds is Myxomycetes, though the
ending may vary, depending on the
authority doing the classifying. Note the
"Myxo" part, again meaning "slime." The
"mycetes" portion means "fungus" or
"mold" in the Greek, so the name
means "slime mold," as we have said
above.
About 40 species of the cellular
slime molds have been described. They
are generally found in soil, but they are
also found in considerable profusion in
and on decaying plant materials. There
are about 500 species of true slime
molds recognized.
Habitats and Modes of Existence
Though they all need moisture for
growth and the completion of their life
cycles, there are no truly aquatic
members of the slime organisms, as
they are viewed today. Nor are any of
them found in decaying animal tissues.
All of them are found in soils and in or
on organic debris near the soil. Various
of them prefer different types of soil, of
course. For example, some species may
most commonly be found in cultivated
soils in great profusion, whereas others
prefer undisturbed forest or field soils.
A few prefer desert soils; even though
they may be found in all soils, they can
most readily be found in profusion in
arid and semi-arid soils. They are better
able to compete, no doubt, in those soil
environments where there are fewer
total organisms.
The "organic debris" in which they
are found is predominately of plant
origin, though animal dungs are a rich
source for them. Rabbit dung pellets
are such a source for slime bacteria and
for some of the true slime molds, but
other dungs are always worth the
gathering.
A habitat which has proven to be
very rich in many species of the slime
bacteria and of the true slime molds is
the bark of living trees. The cellular
slime molds are not common here, but
one can find one once in a while. I did
my masters thesis on the myxomycetes
on the bark of living trees and, while
doing that, found so many myxobacteria
5
there that I proceeded to do my
doctoral research on the myxobacteria
on the bark of living trees. What is tree
bark anyway? Non-living cells of plant
origin in various stages of
decomposition. Each piece of bark is a
micro-environment in its own right.
What are the slime organisms doing
in these soil and organic debris
habitats? What is their mode of
existence here? most of them are busily
devouring true bacteria. They get their
energy to grow, to move, to make their
slime, to multiply, and to complete their
life cycles from bacteria. They are, as
we say, eubacteriolytic. The "eu" means
"true" and the "lytic" means "dissolve," so
the word simply says they "dissolve true
bacteria." In doing so, they get the
energy for all of their activities.
A few of the slime organisms are
able to utilize large molecules such as
cellulose, the main constituent of plant
cell walls. Their source of energy, then­
-their mode of existence--would be from
these large molecules.
General Characteristics
Later, we will examine the
characteristics of each of the three
groups of the slime organisms in some
detail. At this point, let us simply
generalize on what they are like, on
what are the parts of their life cycles.
They all have a vegetative stage, a
sporulating stage, and a resistant stage.
The vegetative stage is where the
feeding and growing occurs. This stage
is motile. The slime is produced here.
Cell division occurs here.
In the slime bacteria, this vegetative
stage is a single bacterial cell. It
produces enzymes which dissolve the
true bacteria near it and then it absorbs
the nutrient "goodies" into itself. As the
cell grows, it divides into two cells, both
of which continue to feed and grow and,
ultimately, divide. Consequently, there
.
would soon be literally millions of these
cells at work.
The vegetative stage in the cellular
slime molds is a single amoeba. It
engulfs bacterial cells inside itself in
typical amoeboid fashion. The
enzymatic dissolving of the bacteria-­
the "digestion"--then takes place
internally. Of course, it grows and
divides and produces populations of
millions of amoebae like itself.
The vegetative stage of the true
slime molds is a naked, multinucleate
mass of protoplasm. It is like a
multicellular organism, or tissue, of
several thousand cells, but there are no
walls or membranes dividing it into
separate cells, nor is there a membrane
around the entire mass. It is a naked,
flowing mass of protoplasm. It feeds by
engulfmg bacteria--many at a time-­
and, then, digesting them within itself.
this vegetative stage in the true slime
molds is called a plasmodium.
To summarize the vegetative stages
of the three slime organism groups: that
of the slime bacteria is a bacterial cell,
in the cellular slime molds, it is an
amoeba, and the true slime mold
vegetative stage is a mass of naked
protoplasm called plasmodium. This
stage is responsible for the feeding,
growth, division, and slime production
in all three cases.
When the food supply is depleted,
when it becomes too dry, when it
becomes too hot or too cold, or when
the environment otherwise becomes
unsatisfactory, the slime organism
begins to form resistant spores. This is
the sporulating, or spore-forming, or
fruiting stage.
In both the slime bacteria and the
cellular slime molds, several thousand­
-even several million--cells do this
collectively. At a given signal, cells
begin to move toward a center, they
begin to pile up on each other to form
6
intricate spore-bearing structures. Do
contemplate this process! Thousands of
individual cells with no connection to
each other, at a given time, aggregate
through an intricate maneuver which
always ends up with a magnificent,
though tiny, structure of exactly the
same form for that particular species!
What controls this process? It is still an
unanswered question, though we have
some ideas about how it works.
Since the plasmodium of the true
slime molds is already a multinucleate
structure of some size, the spore­
bearing process is simply a matter of it
clumping-up, raising itself above the
substrate, and forming its distinctive
spore-bearing structure . When this
process is complete, it is in its resistant
stage.
The resistant stage in all three
groups of slime organisms is a single
cell called a spore. Sometimes it is
called a "myxospore" because the
protective wall around it is produced
from slime. Many of these spores are
produced in a single "fruiting body" or
"sporangium." The "angium" part of the
term means "vessel," so a sporangium is
a "spore vessel." The fruiting body
produced by a given species is always
the same for that species. Our
taxonomy of the organisms is based on
what the fruiting body looks like. They
vary from simple dry-slime-encrusted
clumps to tiny, intricate tree-like
structures. We shall consider the various
forms of them in more detail shortly.
The spores themselves are
microscopic single-cells enclosed in a
wall of dried slime to make them highly
resistant to adverse conditions. We have
records of some of them lasting for over
100 years in storage. When taken out of
storage and provided with moisture and
proper temperature, they go ahead and
germinate into living, vegetative cells.
After bringing the soil into the
laboratory, I have been able to get them
to grow out of soil which was so hot
that I could not hold it in my hand
when I collected it--185-190°F. And out
of soils where there had been no rain
for over a year. So, the spores are very
resistant structures, indeed. They are
the mechanism of the slime organisms
by which they sit and rest until adverse
conditions again become satisfactory for
growth, feeding, and division.
TIlE SLIME BACfERIA
As we have said--and as the name
implies--the vegetative structure of the
myxobacteria is a bacterial cell. Bacteria
are described as being prokaryotes (pro
- before; karyo - nucleus) because they
do not possess organized nuclei or other
cell organelles. Consequently, though
areas of lightness and darkness can be
seen within the cell, few definitive
characteristics can be seen under the
microscope. There is little to say,
therefore, about the structure of the cell
itself.
There are, however, two distinctly
different myxobacterial cells on the
basis of shape. One is long, pointed at
the ends, and flexuous. The other is
shorter, rounded at the ends, and rigid .
These cell types are shown in Figure 1.
FIG 1.
Both types of cells glide across solid
surfaces, such as that on an agar plate.
The exact mode of locomotion is not
known, but it is thought that they
secrete slime off a rear quarter of the
7
ceU. When that slime begins to gel, it
pushes the ceU forward. Though
patience is required because they are
slow, with the aid of a microscope one
can easily enough watch these ceUs
move. Often a lead ceU will move off,
leaving a slime trail which other ceUs
readily glide along. The slime is a
polysaccharide in nature.
The first purpose of this ceU
motility is feeding. Their movement,
therefore, is in search of, and toward, a
food supply.
As the cells feed and grow, they
divide by pulling into two pieces. Under
good conditions, division will occur
every hour or so. Consequently, there
wiU soon be many, many cells gliding
about, feeding, dividing, and secreting
slime. There will be so many, in fact,
that the mass, which we will now call a
colony, can be seen with the unaided
eye. The colonies of most species are
clear and without color, but a number
of species produce pigments which color
them various shades of yeUow, orange,
red, brown, and, even, black.
A second purpose of the cell
motility is to permit them to move into
appropriate position as they collectively
produce the spore-bearing structure.
This movement of thousands of celis
toward a common goal is called
are continually embedded in slime. As
the entire mass dries out, the long,
flexuous ceUs all condense to spheres
(the "coccus" part of the name) and
produce a heavy wall of dried slime
about themselves. They are called
myxospores. The mature fruiting bodies,
then, are dried-slime-enclosed globes
fiUed with spherical myxospores. They
will be found sitting on the surface of a
piece of bark, animal dung, or other
plant debris (Figure 2).
In other genera, short stalks of
slime are produced so that the globe of
myxospores is raised up off the surface
(Figure 3). In one group, the fruiting
body is a coral-like, rather than globose,
structure (Figure 4). in another, the
interior of the mass is divided into a
system of intestine-like tubules within
which are contained the myxospores
(Figure 5).
FIG 4
FIG 3
FIG 2
aggregation.
The Fruiting Structure
The systematics of the thirty-some
species, eight genera, and four families
of the myxobacteria is based on the
form of the fruiting body. It would serve
no purpose to cover all the intricacies of
that systematics here; rather, we will
focus our attention on a few
representative forms.
In the simplest fruiting structure,
that found in species of the genus
Myxococcus, the vegetative cells
aggregate into a rounded clump. They
FIG 5
FIG 6
Those myxobacteria which possess
the short, rounded-end, rigid vegetative
cells produce the most complex fruiting
bodies. These cells do not shorten
appreciably, as did the long, flexuous
ones. They do produce a slightly heavier
wall around themselves; however, and
they, too, are called myxospores.
The fruiting bodies produced by
members of the genus Polyangium (poly
-many; angium - vessel) are often
clumps of sufficient size to be seen with
8
the unaided eye. They are produced on
various plant debris and dung surfaces.
Those which decompose cellulose
produce this type of fruiting body. They
may be found on, and in, cellulose
fibers. The clumps are bounded by a
heavy sheath of slime within which are
many smaller, circular slime-sheathed
containers (the "many vessels") within
which are myxospores (Figure 6). They
come in a variety of colors from yellow
to black. When mature, they are highly
resistant and readily disseminated
structures.
Members of the genus
Chondromyces produce the most
magnificent of fruiting bodies. They are
like little trees, big enough to be
spotted by the trained, but unaided, eye.
And in color, too! (Figure 7)
FIG 7
Now, do turn your imagination to
the coordinated effort that must be
going here for thousands of individual
cells--with no intimate contact between
them, remember--to collectively build
these structures of themselves and the
slime they secrete. It is mind-boggling
to envision such cooperative effort!
The entire purpose of these fruiting
structures, of course, is to get those
cells which end up as myxospores up in
the air where they can be best
preserved and best spread over the
countryside. To that end, the vegetative
cells aggregate into clumps, but the
clumps continue to grow into pillars as
cells swarm on top of cells and produce
masses of slime while doing so. These
pillars become the trunks of the tiny
trees. More cells glide into the process,
the pillar is pushed higher and branches
at the apex. And branches again. And
agatn.
At the ends of the branches, masses
of cells destined to be myxospores
surround themselves with a common
layer of slime. This layer is the wall of
the cyst, or sporangium, in which the
myxospores are contained. Not all cells
make it into the cysts, by any means.
The bulk of them give their all in the
process and are left behind, embedded
in the slime of the trunk and the
branches, where they ultimately perish.
This production of cysts at the ends
of the branches is distinctive for various
species. In one, the cysts are sessile and
cylindrical. Another, Chondrol7lyces
pedicu latus , always produces a small
stalk, or pedicel, under each cyst. In
Chondromyces apicelatus, a pointed
apex of dried slime is always left on the
extremity of each cyst. And in
Chondromyces catenulatus, the cysts are
always produced in chains. How can
such intricacy regularly be produced by
a mass of unconnected, independent
cells in a wallow of slime?
The Life Cycle
Now that we have become
acquainted with the vegetative stages
and the fruiting stages of the
myxobacteria, it will be a relatively
simple matter to summarize the life
cycle of the group. The vegetative cells
move about and feed, grow, produce
9
slime, and divide. This continues as long
as there are plenty of eubacteria on
which to feed and as long as all
conditions are favorable. Depletion of
the food supply or the coming of
unfavorable environmental conditions
causes a shift toward the resistant state.
The cells aggregate, produce lots of
slime, and begin to form clumps. In
those species with the long, flexuous
cells, the cells shorten to spheres and
produce heavy walls around themselves
to form myxospores. This happens
within slime coverings in the clumps.
The fruiting bodies are relatively simple.
When the mass dries out, it tends to
break apart and the myxospores are
spread about. They, themselves, are the
resistant structures.
In those forms with the short, rigid
cells, the aggregation clumps tend to go
further and produce fruiting bodies with
many compartments within which the
myxospores are encased. The
compartments and cysts here tend to be
the resistant units and the units which
are spread about. Various of the many­
vessel and tree-like fruiting bodies are
produced by these species. When
mature and dry, they tend to break
apart so that they are more readily
disseminated.
As myxospores--the resistant state­
-myxobacteria are able to tolerate long
periods of extremely unfavorable
conditions. They can sit for years
without water (we kept some of them
in dry soil for 10 years). They can
tolerate freezing (we kept others frozen
for 10 years) . They can tolerate high
temperatures (the desert soils too hot to
hold in the hand, remember?).
When conditions again become
appropriate--when temperatures are
reasonable and moisture is present--the
protoplasm of the myxospore becomes
active, the dry slime coat softens and
disappears, the cell becomes an active
vegetative bacterial cell. One single cell
will soon become an immense
population ready to make the whole
thing go around again.
Role in Nature and Importance to Man
Since most myxobacteria feed on
various bacteria, they must, obviously,
be important in regulating the numbers
of such organisms in nature. They are.
They have developed a range of
enzymes which decompose bacteria and
bacterial parts. Some of these have
been studied and are useful in
laboratory work, but none has been
found to be directly useful to man in
other ways.
Because of their habitats in soil and
decaying debris and their feeding on
various other bacteria, these
myxobacteria must compete with many
other organisms. They help keep their
place in this busy scheme of things by
having developed a system of antibiotics
to keep other organisms--mostly other
types of bacteria--from encroaching on,
and overwhelming,
them. These
antibiotics, though of obvious benefit to
the myxobacteria, have not been found
to be of value to man.
Those few myxobacteria which get
their sustenance by decomposing
cellulose certainly play an important
role in the scheme of things. Much
carbon and energy is tied-up in the
cellulose of plant cell walls. This carbon
must be released to be used over and
over again; relatively few organisms can
do this. These few myxobacteria which
can, therefore, are important in keeping
the carbon cycle going. And particularly
so in desert and arid situations. In such
arid habitats, these myxobacteria must
compete for cellulose and their
ecological niche with fungi. Therefore,
they have elaborated a range of
antifungal antibiotics. Some of these do
look promising for medicinal use.
•
10
The myxobacteria, then, playa role
in the scheme of nature by assisting in
the control of bacterial populations and
by helping keep the carbon cycle going.
They are of no direct use--or trouble
either--to man. We must not forget, of
course, that without the balance we find
in nature and a functioning carbon
cycle, man would be unable to survive.
In this respect, the myxobacteria, like
all of the components of the natural
system, are quite important to man.
TIIE CELLULAR SLIME MOLDS
Many aspects of the cellular slime
molds, the Arcrasiae, are similar to
those of the myxobacteria. Now that we
have some familiarity with the
myxobacteria, it will be a simpler matter
to discuss the cellular slime molds.
The Vegetative Strncture
The vegetative structure in the
cellular slime molds is a single cell, a
single amoeba. As is the case with all
other amoeba, this is a eukaryote (eu ­
true; karyote - nucleus). That means
that each cell .possesses a true,
organized nucleus and various other
organelles.
With usual nuclear-staining
techniques, then, the nucleus in each
cell can be seen with the aid of a
microscope. And, at the right time,
chromosomes can be seen, counted, and
watched undergo the usual mitotic
process. Other cell organelles-­
mitochondria, membranes, various
vacuoles--are also present and can be
seen. Food vacuoles, in particular, are
prominent.
The amoebae of the cellular slime
molds feed, primarily, on bacteria.
Yeast cells and other particulate organic
debris may also be engulfed and
digested. Sometimes they appear to feed
even on each other as cannibals.
Bacteria constitute the bulk of the diet,
however. The amoeba simply flows
around a bacterium, produces a
.membrane around it and, thereby,
incorporates it inside itself as a food
vacuole. It is digested by a spectrum of
enzymes produced by the protoplasm of
the amoeba.
The amoebae flow about in typical
amoeboid fashion, produce the slime
which gives them their common name,
grow, and divide. In a short period of
time, large populations of them are
produced when conditions are
favorable. Depletion of the food supply
or
the coming of unfavorable
environmental
conditions
causes
aggregation of many amoebae into the
fruiting structures.
The Frnitillg Strncture
Compared to the other two groups
of slime organisms, the fruiting
structures of the cellular slime molds
are characteristically unshowy. Most of
them consist of a simple structure of a
mass of encysted cells, the myxospores,
at the apex of a stalk of slime and
amoebae. A few of them produce
rudimentary branches and one genus
produces chains of spores. Some are
very tiny with only one spore on top of
a slime stalk. Others are of considerable
size with a mass of spores on the end of
a stalk which is composed of both slime
and cells. They are all quite simple,
however. The best way to describe them
is with a series of sketches (Figure 8).
The Life Cycle
As we have said, the vegetative
cells, the amoebae, move about, produce
their slime, engulf bacteria on which to
feed,
grow, and
divide.
Large
populations are produced. When food
supply becomes depleted
or
environmental conditions become
unfavorable, the amoebae begin to
aggregate.
11
idea of the form of this migrating
pseudoplasmodium.
FIG 8
Aggregation is a matter of many
amoebae moving toward a central point.
It is known that they move along
pathways of chemical gradient to these
centers. The chemical is even known
and named. It is called acrasin, a name
coined from the technical name for the
group, the Acrasiae. One cell,
apparently, begins to produce acrasin
and this becomes the focal point to
which other amoebae migrate. Any
large population will, of course, produce
several aggregation centers.
In some of the cellular slime molds,
the aggregation results in a clump of
one or more cells, which become the
spores, and that is the end of it. In
others, a stalk may be produced which
will raise the developing spores up in
the air. In several of them, however,
aggregation is just the start of the
fruiting process.
In these latter forms, the aggregated
clump of many amoebae takes on an
elongate form and begins to glide over
the surface in a migration phase. It
looks very much like a small garden
slug and is sometimes called a "slug."
More correctly, it is a
pseudoplasmodium which means "false
plasmodium." Figure 9 will give some
FIG 9
Much is known about the condition
of cells during aggregation and during
migration. It has been thought that this
is a time of sexual reproduction and
genetic recombination, but such has
never been demonstrated to the
satisfaction of most students of these
organisms. Suffice it to say, for our
purpose, that migration stops and a
portion of the pseudoplasmodium is
raised up on a stalk to become the
spore mass. Those cells in the mass at
the top become spherical and produce
walls of slime around themselves. When
dry and mature, they are the resistant
structures, the spores, of the organism.
The spores are quite tolerant of
adverse
condi tions -- desiccation,
freezing, etc. When conditions are again
favorable, each becomes an active
amoeba and the life cycle commences
agam.
Role in Nature and Importance to Man
As with the myxobacteria, the
cellular slime molds play a role in the
balance of nature. Their feeding on true
bacteria certainly has something to do
with keeping our world from being
inundated by these rapidly-multiplying,
voracious microbes. Past that, however,
one is hard-pressed to assign them any
other natural function.
Nor do they play a role in man's
affairs. They cause no diseases of
anything. They produce no products of
value to us or our devices as far as we
know at this time.
I ask you, again, to contemplate this
situation where dozens of individual
12
entities--the amoebae--begin to work
together in a pre-orchestrated pattern
of aggregation,
migration,
and
production of the fruiting structure.
There is never any connection between
the individuals, yet it always goes as it
should in a beautiful process of
cooperation toward a common goal.
Think of the questions such a
process raises about nature, about the
nature of life, about how individuals can
operate together. Certainly, there are
lessons to be learned here about the
basic meaning of existence. If we can
unravel some of such secrets from this
slime organism source, that, indeed,
would be of immense importance to
Homo sapiens, would it not?
THE TRUE SLIME MOLDS
Myxomycetes, the true slime molds,
may be properl'y called the grandest of
the three groups of slime organisms.
They are much more commonly seen
across the face of the world and they
are much better known. They constitute
a much 'l arger group in that some 500
species have been described and
generally agreed upon. Though clearly
microorganisms, they nonetheless form
structures, both in the vegetative and
the fruiting states, which are big enough
and showy enough to often be seen with
the unaided eye. That, of course, is why
they are more widely known throughout
the world. Let us have a closer look at
them.
The Vegetative Strncture
As we have learned earlier, the
vegetative structure of the myxomycetes
is a multinucleate, naked mass of
protoplasm called the plasmodium.
Before the plasmodium comes into
being, however, there are other
structures which are of sufficiently
vegetative nature for a portion of their
existence to warrant their being
discussed here. These are the swarm
cells.
When the myxomycete spore
germinates after lying about during
periods of drought, cold, excessively
high temperature, etc., it splits open and
out come one-to-four swarm cells. Each
possesses a single nucleus and each
nucleus contains the haploid number of
chromosomes. The spore contained a
single diploid nucleus when it went into
the resistant state. As it prepared to
germinate, that diploid nucleus
underwent meiosis.
Meiosis is often called reduction­
division because the purpose is to
produce four nuclei, each of the
haploid, or reduced, number of
chromosomes, from the original diploid
nucleus. Once this has happened within
the spore, the protoplasm divides into
four portions, each of which surrounds
a nucleus, the spore wall cracks open
and out come the swarm cells. If all
occurs as it should, there will be four,
but it is not uncomnion for something
to go wrong and only one, two, or three
swarm cells emerge.
The myxomycetes are eukaryotic.
Each swarm cell has two flagella on one
end. One of these is of such length that
it can be seen readily under the light
microscope. The other is so short that it
must be seen under the electron
microscope.
cells
are
Since the swarm
flagellated, they readily swim about,
engulf bacteria, grow, and, ultimately,
divide by fission into two cells. The
swarm cells may lose their flagella and
glide about as amoebae; they are, then,
called myxamoebae. The myxamoebae,
also, engulf bacteria, grow, and divide
by fission. Some of them may again
form flagella if conditions are more
appropriate
for
that
form
of
locomotion. A swarm cell and a
myxamoebae may be seen in Figure 10.
13
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Q'
:
D'
,()•.. ~ • . ~/.O·
~"~ ,,~O
'jJ'"
,'lJ '.'
.
'0'
FIG 10
At some point, two myxamoebae or
two swarm cells fuse to form a tiny
protoplasmic mass with two haploid
nuclei in it. This is certainly a sexual
fusion, though the two cells are
isogametes, which means "like gametes."
Not all swarm cells or myxamoebae are
compatible with each other; they are
fussy about with whom they will mate.
They will not, for example, fuse with
another cell from the same spore or,
even, from the same fruiting body. This,
of course, makes sense because there
will be better genetic recombination
potential from such "unrelated" matings.
This fusion of two swarm cells or
myxamoebae is the start of the
plasmodium stage. The two haploid
nuclei soon fuse, forming a diploid
nucleus. This nucleus will soon divide
and the products will divide again and
again. At the same time, the
plasmodium will engulf bacteria which
it digests for energy and growth. It
manufactures more mitochondria and
more vacuoles and builds more
protoplasm which will be ftIled with the,
nuclei. In a matter of hours, the mass
will be of sufficient size to be seen with
the unaided eye, if conditions for
feeding and growth are at all
satisfactory.
Since the plasmodium is a naked
mass of protoplasm without even a
membrane surrounding it, it has no
definite form. One can stick a needle
into it and withdraw it without
disrupting anything; it is just like
sticking a needle into a bowl of jelly.
The plasmodium is more gelatinous-­
more slimy--than is water, so it does
have some semblance of integrity, but it
is impossible to describe its shape. The
best that one can do is to say that it
generally is somewhat fan-shaped as it
moves forward like a small, slow tidal
wave engulfing bacteria.
Bacteria are the plasmodium's main
food source, but it will also engulf other
particulate material, some of which it
will utilize. It will also absorb some
materials in solution. Those materials
which are not digested and utilized will
simply be left behind as waste as it
flows forward. Consequently, one can
actually see where a plasmodium has
been as it moves across a surface.
The plasmodium of the myxomycetes
is capable of producing an immediate
resistant state in response to
unfavorable environmental conditions.
This structure is called a sclerotium.
Sclerotia usually are dry and horny in
consistency and appearance. They are
usually full of irregularly-shaped
compartments internally. In response to
unfavorable conditions, the plasmodium
simply clumps, condenses, produces
something of a wall about itself, and
dries out. When conditions are again
sufficiently moist and appropriate, it
becomes an active, moving, feeding
plasmodium. Myxomycete sclerotia are
known to remain viable for a good
many months.
The Fruiting StlUcture
The approximately SOO species of
myxomycetes are differentiated on the
basis of the type of fruiting structures
they produce. They are grand and
diverse in form and color. Since their
purpose is to produce spores, they are
properly called sporangia, or "spore
vessels."
Though all can be called sporangia,
14
three basic forms of fruiting structures
are recognized. These are
p/asmodiocarps, aet!ta/ia, and sporangia.
Plasmodiocarps are so-named because
they take the form of the plasmodium.
The "carp" portion means "fruit," so it is
simply a plasmodium-like fruit, which
tends to be a system of tubes, or veins,
directly on the surface. The plasmodium
stopped flowing, humped-up a bit into
elongated, tubular clumps, produced a
wall about itself, and the protoplasm
within divided into spores. In effect, the
plasmodiocarp still has something of the
shape of the plasmodium (Figure 11).
FIG 11
FIG 12
The aethalium, also, is without
distinctive form, but it is more of a
cushion, or clump, and higher than is
the plasmodiocarp (Figure 12). It is
simply a matter of a mass of
protoplasm condensing into a rounded
cushion, producing a wall about itself,
with the protoplasm within dividing into
spores.
Sporangia are generally stalked
structures with the actual spore­
containing portion on the end of the
stalk. A few are sessile and, thereby,
rest directly on the substrate without
being raised into the air. Far more
species produce sporangia than any of
the other types of fruiting structures.
Since there are so many different types
of sporangia, it is difficult to generalize
about the forms which may be found.
And they come in a variety of colors.
Figure 13 illustrates some of the best­
known sporangia I shapes.
As has been indicated above, most
myxomycete fruiting structures are
stalked sporangia. A stalk, then, is a
part of the majority of the fruiting
structures. It is primarily composed of
dried slime produced by the
FIGJ3
15
protoplasmic mass as it raises itself up,
though some nuclei and cell organelles
may be left trapped behind and, thus,
become part of the stalk. Its purpose is
to raise the spore up where they can be
readily disseminated.
In addition to the stalk, there are
three other parts of the fruiting
structure worthy of mention. All of the
fruiting structures contain round spores.
Each spore contains a nucleus and a bit
of protoplasm; each is bounded by a
wall of dried slime. The spores are the
actual tough, resistant structures.
Most fruiting structure possess a
peridium, the name given to the dry,
tough, protective wall of dried slime
around the spore mass. In some, the
peridium is so evanescent that it is gone
by maturity, but it is quite persistent in
most species.
A third structure found in a
majority of the fruiting structures of the
myxomycetes is known as the
capillitium. The capillitium is a system
of threads--produced from dried slime­
-intermingled throughout the structure.
the capillitium is a part of the spore
mass, but has no connection to the
spores. Its purpose is to expand,
something like a spring, thereby
throwing the spores out into the
surrounding atmosphere. The
capillitium is a device to aid spore
dissemination. Capillitial threads often
are so distinctive that they can be used
as characteristics for identifying the
various species.
One further
aspect of the
characteristics of the fruiting structures
should be pointed out. About half of
them produce dark-colored spores; the
others produce light-colored spores.
Consequently, all the species can· be
readily divided into two groups on this
basis of them metabolizing lime,
calcium carbonate, so that it ends up in
their peridia and capillitia. This "lime
present" or "no lime present" character,
then, becomes another
useful
characteristic for dividing the dark­
spored species into two groups.
The Life Cycle
A spore germinates to produce one­
to-four haploid, flagellated swarm cells.
These feed, grow, and divide. They may
change to myxamoebae which, also,
feed, grow, and divide and, perhaps,
switch back to being swarm cells. Two
compatible swarm cells or myxamoebae
fuse to form a structure in which the
two haploid nuclei soon fuse to form a
diploid nucleus. This is the beginning of
the plasmodium.
The young plasmodium engulfs
bacteria and other particulate material
on which it feeds. It grows, its nuclei
and other cell organelles divide, and it
becomes a multinucleate mass of active,
naked protoplasm. This is the mature
plasmodium.
If conditions become unfavorable,
the plasmodium may respond in two
fashions. It may produce a sclerotium,
which is a dry, resistant mass of resting
protoplasm with no particular form.
Sclerotia are capable of resting for
many months before becoming active
plasmodia again.
Another thing that
adverse
conditions--or depletion of the food
supply--may stimulate in the
myxomycetes is the formation of fruiting
structures. Their purpose is to produce
spores. The spores are highly resistant,
with some of them, we know, lasting for
over 100 years. They are also the
disseminatable units, readily blown
about to new locations. If the habitat is
favorable and environmental conditions
are reasonable, the spore ultimately
germinates to release swarm cells and
we are back where we started.
Kansas School Naturalist
Emporia State University
1200 Commercial Street
Emporia, Kansas 66801-5087
Role in Nature and Importance to Man
There is little to be said here. In
spite of their profusion and world-wide
occurrence, the myxomycetes do not
seem to fill any special niches or
perform any particular functions. They
do play a role in control of bacterial
populations, as do the other two groups
of slime organisms. They are simply
common, regular components of the
magnificent natural system which is our
Planet Earth.
Nor are they of any great
importance directly to man and his
enterprises. One of the myxomycetes,
PhysaJUm polycephalum, is readily
available from biological supply houses
and is easily grown in the teaching
laboratory so that students can have a
good look at what protoplasm is all
about. Some people experience allergic
reactions when they breathe in
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myxomycete spores which, of course,
are in profusion in the atmosphere.
Sometimes, people, who especially value
well-tended, beautifully-groomed lawns,
get quite excited when some of the
myxomycetes produce sporangia on the
blades of grass in those lawns. Such can
be eye-catching and disconcerting
patches of grayish discoloration in the
green lawn. But it is totally superficial;
there is no damage and the sporangia
can be washed off easily with a garden
hose.
Except for such generally
insignificant intrusions into the human
scene, the myxomycetes are of no direct
importance to us. They go their way,
minding their own business, while
making the world a slightly more
beautiful and interesting place. Who can
ask for more from any group of
organisms, slime or otherwise?