Building Bodies from Slime: Plasmodia
Slime moulds fall into two basic types: cellular, for example Dictyostelium, and plasmodial, e.g. Physarum, which is
modeled above. Both are extraordinary classes of organisms! The cellular slime moulds are amoebae which can
come together, when needs must, to form a basic multicellular organism! This makes them of great scientific interest
in studying multicellularity and the evolution of multicellular life-forms and cell communication. Plasmodial
slime-moulds are extraordinary for being amongst the largest living 'cells'. The plasmodial slime-moulds are also
known as
myxomycetes and are the subject of this article.

The Plasmodium

The pictures above show a computer Pov-Ray model of a slime-mould plasmodium. These creatures form a sheet
of mobile slime, only a couple of millimetres thick at most but up to one metre in more diameter. The remarkable
things is, that despite their large size, these creatures are single cells! They resemble single giant animal cells, but
are protoctistans, as is the single-celled
amoeba which they resemble. Amoebae are normally one millimetre across
at most, but the plasmodium slime-mould is in essence a giant amoeba. The plasmodium actually begins its life as a
tiny amoeba, but grows and grows! Like an
animal cell, the plasmodium consists of cytoplasm enclosed by a 'skin'
called the
plasma-membrane. Whereas a typical animal cell contains one nucleus, in contrast, a plasmodium
contains many thousands of nuclei (as required to maintain its large size). The plasmodium is able to slowly crawl
along the surface, at about one centimetre per hour, in which case it assumes a fan-shape, with the broad margin
leading as the front end, though it can easily change shape and start moving in another direction. In essence,
plasmodial slime-moulds are giant, terrestrial and multinucleate amoebae. Note that although a plasmodium is
essentially a multinucleate cell, many biologists do not consider them 'cells' on the basis that a cell has a single
nucleus (although many classical cells normally have multiple nuclei anyway, such as muscle fibres and osteoclasts).
A better definition of cell is as a single protoplasmic unit, bounded by a cell membrane, in which case plasmodia are
indeed multinucleate cells.

To facilitate this movement, the plasmodium forms a network of vessels to transport its cytoplasm from one end to
the other. Plasmodia vary in colour, depending upon species, though most are either transparent or bright yellow or
white, some are reddish, green or even blue in colour. Although they occur in most habitats, they are most likely
encountered crawling in the leaf litter and inside rotting logs in damp woods. They feed by smothering their food and
absorbing it directly (a process called
phagocytosis). They engulf bacteria, fungi, and decaying organic matter and
can home-in on potential food by responding to its odour. I have heard stories of people keeping small plasmodia in
petri dishes, and entertaining people by placing food atone end of the dish, and then watching the slim slowly crawl
over to eat the food!

I once found two translucent plasmodia (almost totally transparent) crawling across a felled log in a damp
Devonshire woodland. Each was about the size of a human hand. Pressing one of the veins, it was surprisingly rigid,
but squeezing it showed the cytoplasm inside squirt along the vein in either direction from the point of pressure.
Cytoplasm ordinarily moves back and forth along these veins in oscillations lasting about one minute, with cytoplasm
streaming along at up to 1.3 millimetres per second. Superimposed on these oscillatory movements is a general net
movement of cytoplasm toward the leading edge.

The diagram below shows the structure of a small plasmodial slime-mould.
Above: the structure of a plasmodial slime mould (redrawn from Fleischer and
Wohlfarth-Bottermann, 1975). CFC, circular fibrils in cross-section; CFL, circular
fibrils in longitudinal section; ECC, ectoplasm in cross-section; ECL, ectoplasm in
longitudinal section; ENC, endoplasm in cross-section; ENL, endoplasm in
longitudinal section; PI, plasma-membrane invagination;PL, plasma-membrane
(plasmalemma) and PS, pseudopod.
First, it should be noted, that in a plasmodium of at least moderate size, the network of vessels are much more
complex than depicted either in the computer model or in the diagram above - many finer vessels branch and
ramify to form a lattice-like network, rather like the veins in a leaf, especially toward the leading edge where the
finer veins are denser.

Notice that each vein consists of two concentric cylinders inside the plasma-membrane sheath (PL) - the outer
ectoplasm and the inner endoplasm. The ectoplasm is stiff and gelatinous and contains longitudinal and circular
fibrils (and also radial fibrils which are not shown) which are composed principally of rods of a protein called
actin.
Actin is a component of the cell skeleton and is capable of exerting tension and ordering water in the cytoplasm to
form a stiff gel, such as the ectoplasm. The endoplasm is free of these stiffening actin rods and much more watery.
The actin fibrils in the ectoplasm contract in sequence, squeezing the fluid endoplasm along the veins, rather like
squeezing toothpaste along a tube. This transports cytoplasm from the rear of the creature to the advancing edge.
When the time comes, ectoplasm can be mobilised by simply dissolving the actin cytoskeleton and turning the
ectoplasm into endoplasm. Actin has the remarkable property of being able to assemble rods (columns or struts) or
dissolving into fluid, as and when required! The pseudopods (literally 'false feet') absorb any food on the substrate
as the slime crawls along and are also assembled and disassembled as required.

Plasmodia have some remarkable properties. If you cut one into several smaller pieces, no matter, each fragment
will continue to move and crawl as an individual organism, but if it encounters another piece of its former self, then
the two will fuse back together again! If, however, a foreign plasmodium (differing genetically) is encountered, then
the bigger will usually devour the smaller!

Finally, when the plasmodium has fed sufficiently, it will crawl out from hiding and ascend a tall structure (if one is
available) stop moving and then transform into a
spore-producing structure. This is when plasmodia are most
often seen - as foam-like material on grass or the trunks of trees. In this non-motile state, the fan-like shape with its
veins disappears and the whole resembles a foamy mass which easily fragments on touch, rather like shaving
foam. (It is also cool to the touch and has a subtle slightly minty odour in my opinion, an odour which is
unmistakable). I once saw a specimen about 30 cm long and 5 cm wide streaked vertically along the outside of an
old oak trunk, about 2 metres from the ground. This state is modelled in the computer graphics below.
Above: the plasmodium is preparing to spore and transforming into a sporing mass called an aethelium. This is the
form that most people see slime moulds in, and they usually mistake them for foam that some one has sprayed!
This is the type of structure formed by the large
Fuligo and Physarum polycephalum species. Actually, many
species do not form these structures, but instead form one or more rounded nodules or clusters of tiny stalked
sporangia, very similar to those produced by
cellular slime moulds but usually occurring in groups rather than
singly. These stalked structures are easily seen on rotting logs, but are missed by most people because of their
small size - they are slimy and usually brightly coloured and much smaller than toadstools. An example of the
stalkless (sessile) type that nevertheless has distinct capsules (
sporangia) is shown below. The stalkless types
are often larger than the stalked sporangia, with each capsule often several centimetres across in some species.
A cluster of stalkless slime mould
sporangia.
Eventually, these structures, either the foam-like mass or discrete sporangia capsules, will harden and turn darker
and eventually the whole organism turns into a mass of dark powder enclosed by a dry skin. This 'skin' cracks and
the spores escape, to be carried away by wind or rain. This powder is a mass of
spores have a characteristic
minty odour (never sniff spores of any slime mould or fungus if you are asthmatic!). If they find a suitable place in
rich soil, then the spores will germinate, releasing a single microscopic amoeboid cell, to continue the cycle over
again.

The advantage of this complex life-cycle is that it better enables the creature to find food when food is scarce,
since the large plasmodium can cover more ground than a microscopic amoeba, and it also enable the creatures
to crawl high up tree trunks and the such, to better disperse their spore over long distances.

Overview of the life-cycle

The sporing bodies become brittle and dry structures which disintegrate to release spores. The parent plasmodium
is
diploid (possessing two sets of chromosomes, 2n) and spores are produced by meiosis (a reduction division)
and so each is
haploid (possessing only one set of chromosomes, n). Each spore is a single haploid cell, with a
single nucleus, encased in a resistant spore-coat or 'shell' (test). The spores are easily spread by wind, rain,
animals and vibrations. In suitable conditions, the spores germinate - the tough spore coats break-open and the
single amoeboid cell escapes. When immersed in water each amoeba can develop two flagella used for swimming,
with one tinsel flagellum leading and one smooth flagellum trailing. If conditions dry again then the cell loses its
flagella and returns to amoeboid crawling. If conditions become unfavourable then each amoeba can encyst by
secreting a protective shell around itself and becoming dormant; 'hatching' again when conditions become suitable.

If two compatible amoebae meet then they may fuse together, in a sexual process, forming a single diploid amoeba
or zygote. Compatible haploid amoebae may derive from spores produced from the same parent plasmodial
(homothallic strains) or they have to be derived from different parents, ensuring cross-fertilisation (heterothallic
strains). This diploid amoeba grows as it feeds on bacteria and other micro-organisms, eventually becoming a
plasmodium by repeated nuclear mitosis / division (though plasmodia of the same genetic type will fuse and this
possibly contributes to plasmodial growth). Interestingly the division of the many diploid nuclei, of which there may
be millions, is synchronised. In some species each haploid spore gives rise to a haploid plasmodium and two
compatible plasmodia mate by fusing into a diploid plasmodium. If starved then the plasmodium may develop into a
dormant sclerotium: consisting of a mass of dormant multinucleate spherules. If l;ight is also present then it may
develop into a mass of fruiting bodies and sporulate.

When the plasmodium is preparing to sporulate and has found a suitable elevated position, then it loses the
vein-like structure and develops bumps or papillae over its surface. Each papilla may elongate into a stalk (inside
which granules deposit to give the stalk strength) by drawing up slime from the surrounding plasmodium, and is
capped by a tiny spherical, discoid or elongated
sporangium containing the spores and capillitial threads.
These threads aid spore dispersal by twisting about as they dry. Other species simply transform into one or a
group of often large stalkless sporangia, or else the plasmodium transforms into a foam-like mass (
aethelium)
which directly transforms into a dry powdery mass of spores, as in
Fuligo. The surface of the aethelium is covered
in blebs and the aethelium is quite thick as protoplasm is drawn up into a mound. Some species form a
plasmodiocarp which keeps the form of the original plasmodium, with its netlike veins, but is immotile like all the
sporing forms. In some forms many sporangia may fuse together into a compound structure which is sometimes
known as a
pseudoaethelium when it resembles an aethelium. Generally, the sporing structure, regardless of
type, is covered in a thin 'skin' or peridium which dries and breaks apart to release the spores.

Haploid amoebae may develop a flagellum and develop into a flagellated stage, which may also transform back
into a non-flagellated amoeba. These flagellated forms may also germinate directly from a spore and may also
fuse to form a non-flagellated diploid zygote. In short, the haploid amoebae can alternate freely between
flagellated and non-flagellated forms, according no doubt to certain stimuli. Like many amoebae, they may also
encyst, surrounding themselves in resistant shells and entering a dormant period and then emerging from the cyst
again when conditions are suitable.

Field Observations

Is it any wonder that for a long time biologists could not decide what to classify plasmodial slime-moulds as -
sometimes they were included with the fungi, sometimes with animals and finally (or not?) with the protoctistans -
which includes all sorts of creatures that do not neatly fit anywhere else!  

Plasmodia are food for some creatures - certain insects, like some fly species, make a living out of laying eggs in
plasmodia, the maggots then eat part or all of the plasmodium before pupating. I once saw a batch of maggots eat
part of a large plasmodium that had settled down to spore (the same one on the oak tree mentioned above) before
pupating as strange spiny pupae which emerged into what I never saw. They did not destroy the whole slime
mould, however, which went on to produce millions of spores! I believe that a certain species is eaten in Mexico as
a delicacy (?) but they are generally not edible. Years in which slime moulds grow especially well have been known
to create public scares as people report seeing strange pulsating blobs,
mistaken for aliens from outer space!!
sporangia
plasmodium
Above: a plasmodium (transforming into an aethelium) about one foot in length has formed a foam-like streak
on the bark of this old oak tree (the tree is estimated to be about 400 years in age) - much of its is covered by
the ivy. [Unfortunately, this is an old photo from an old camera that wouldn't let me get any closer without
losing focus.] This plasmodium is stationary, having left the fan-like crawling stage (the plasmodium proper)
and begun to transform into a sporing stage or aethelium. This is probably a species of
Fuligo (probably
Fuligo septica). Over the next few days the mass dried, and beneath the hardened crust was a mass of
powder with a minty odour (do not sniff spores if you suffer from asthma!). Some plasmodia can dry reversibly
into a dormant stage (sclerotium) that comes back to life on application of water, but I think this one was in the
terminal fruiting stage. Slime moulds have specialised predators, and some sort of fly laid its eggs in this one,
the hatchling maggots devoured part of the slime before pupating, several days later, as spiny pupal cases
from which adult flies emerged (I never got to see the adults). However, most of the slime mass remained, so
this creature did its job in producing plenty of spores. They are very hard to destroy by mechanical means -
no matter in to how many parts they are cut, broken or crushed, the parts go on living and can rejoin if the
plasmodium is still in its motile stage!
Above: a cross-section through a plasmodial strand
(vein). Bundles of actin filaments in the outer
(gelatinous) attached to the plasma membrane at
membrane invaginations (clefts) contract (probably with
the aid of mysosin) ectoplasm contract, narrowing the
vein and squeezing the endoplasm along it. Endoplasm
may move at speeds of 1 mm/s in this fashion. Actin
and myosin form bundles called microfilaments or
microfibrils. In animal muscle these bundles (called
myofilaments) bring about contraction of the muscle.
This mass of spores is all that remains of a sporing mass probably
belonging to
Reticularia lycoperdon. A few days earlier, the
plasmosdium had turned into a white hemi-spherical mass (about
the size of a fist in this case) attached to this fallen log (the white
'skin' or peridium enclosing the developing mass of spores). This
browns as it ripens and turns into a mass of spores. Unfortunately,
when I returned a few days later the impressive sporing body had
already disintegrated! The slightest tap to the log would send a
cloud of spores shooting into the air. The moral of the tale is - when
walking in the countryside always carry a camera, because you
never know what you may find!
This is probably Trichia deciphens.  In many slime moulds, such as
this, the plasmodium transforms into a mass of small sporangia,
which are often born on short stalks (as in this case). The whole
mass (of which half is visible here) covered about two hand-spans
of this rotting ash log. The sporangia at first appear pink on short
white stalks (can you see one that is still pink?) but by the next day
they mature to black. This entire mass could be from a single
plasmodium.
More field observations:
Above: Lycogala, probably Lycogala terrestre, which forms small,
spehrical, puffball-like sporing bodies, pink or orange, developing
from an orange, peach or cream-coloured plasmodium. Becomes
pale or grey at maturity and breaks down into a powdery pink spore
mass. Each sporing body is about 0.5 to 1.5 cm in diameter.
A Pov-Ray model of a plasmodium.
A Pov-Ray model of a plasmodium.
AQ Pov-Ray model of a slime-mould plasmodium.
Trichia slime mold
Trichia slime mold
Above and below: Trichia sporulating on a rotting ash log. Photos courtesy of Chris
Pearce (copyright).
Article updated:
4 July 2015
11 Aug 2016