The plasmodium is a membrane-bound single cell containing multiple
nuclei. The plasmodium of some species may have a slime sheath, which appears
as a "slime track" left behind when the plasmodium migrates across a given substrate.
indicating that plasmodium has been there. Plasmodial color may be affected by
pH, temperature or ingested materials. Although there are 4 types of plasmodia,
the most common is the phaneroplasmodium (Greek phaneros = visible). This
type appears as a fan-shaped network of veins containing streaming protoplasm.
This type is common in the order Physarales. A second type, the aphanoplasmodium
(Greek aphanes = invisible is rarely observed in nature. The veins are
very thin and a slime sheath is lacking. Aphanoplasmodia are characteristic of
the order Stemonitales. Protoplasmodium (Greek protos = first) is the third
and probably the most primitive type. Always microscopic, this plasmodium has
no veins; exhibits slow, irregular protoplasmic streaming and produces a single,
very small fruiting body. Protoplasmodia are found in the Echinosteliales and
Liceales. A fourth type of plasmodium exhibits characteristics of both phaneroplasmodia
and aphanoplasmodia and occurs in the order Trichiales. Fruiting Bodies
There are four types of myxomycete fruiting bodies. The most
common type is the sporangium. The sporangium is actually
a small spore container which may be sessile or stalked, with wide variations
in color and shape. Sporangia usually occur in groups, since they form from
separate portions of the same plasmodium. Fruiting Body Structure Fruiting bodies
are usually composed of 6 parts: hypothallus, stalk, columella, peridium, capillitium
and spores. In some fruiting bodies a pseudocolumella or a pseudocapillitium
may be present. Not all of these components are present in all fruiting body
types. The hypothallus is a plasmodial remnant forming the base for
one or more fruiting bodies. The hypothallus connects the stalk or stipe to
the substrate. It may be dull or brightly colored, thin and delicate or coarse.
It is not always in evidence. In some instances, the hypothallus may be composed
of calcium carbonate. In the case of the transparent type, the hypothallus may
be proteinaceous in composition. Stalk The stalk or stipe is an important identification characteristic.
The stalk may vary in length and color and texture. In some species the stalk
is opaque, while in others it is translucent. The stalk may also be coated with
lime or filled with granular or sporelike structures.
Columella and Pseudocolumella The columella appears as an extension of the stalk into the
spore mass, although it may not resemble the stalk. In a sessile fruiting body,
the columella may be an area on the inside of the peridium where it contacts
the substrate or appears as a dome-shaped structure. A pseudocolumella (pseudo=false)
is a columella that does not attach to the stalk. The pseudocolumella is found
only in the order Physarales, existing as a lime mass within the spore mass.
Capillitial elements may be attached to the columella or pseudocolumella.
The peridium is a covering enclosing the spore mass. It may
or may not be evident in a mature fruiting body. In some species the peridium
persists as a calyculus, a cup-like structure holding the bottom of the spore
mass. The presence or absence of the calyculus may be used as a diagnositic
feature along with the manner in which the fruiting body opens. The peridium
may split open along lines of dehiscence, as a pre-formed lid, or in an irregular
pattern. In an aethalium, the relatively thick covering over the spore mass
is referred to as a cortex rather than a peridium.
Capillitium and Pseudocapillitium The capillitium consists of threadlike elements within the
spore mass of a fruiting body. Many species of myxomycetes have a capillitium,
either as a single connected network, or as many free elements called elaters.
Capillitial elements may be smooth, sculptured or spiny or they may appear to
consist of several interwoven strands. Some members of the Physarales have limy
capillitial elements (a badhamioid capillitium), while others have limeless
tubules connecting to lime nodes (a physaroid capillitium). The capillitia elements
are separate from the spores within the spore mass and are not connected to
them. Some elements may be elastic, allowing for expansion when the peridium
opens, while other types are hygroscopic and capable of dispersing spores by
a twisting motion. A pseudocapillitium is present in some aethalia and pseudoaethalia
producing species. Pseudocapillitial elements are highly variable in size and
shape, and may appear as bristles, threads or perforated plates. Spores Spores range in size from almost 5 to 15 micrometers. Nearly
all of them appear to be round and most are ornamented to some degree. In fact,
entirely smooth spores may not exist. Spore ornamentation can be reticulate
(covered by a network of ridges), echinate (spiny), verrucose (warted), or asperulate
(finely warted). Spore shape and size are very important in identification.
Spores can be classified as either dark (found in the Stemonitales and Physarales)
or light to brightly colored (all of the other orders). Dark
spores include the colors black, violet, brown, and purplish brown. Brightly
colored spores may be red, yellow, orange, white, pale gray, pink, light or
rusty brown. In some species of Badhamia and Dianema corticatum,
the spores appear clustered into "spore balls".
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The second type is the aethalium, a cushion-shaped,
sessile structure. Aethalia are presumed to be masses of completely fused sporangia
and are relatively large, sometimes exceeding several centimeters in size.
The third type is the pseudoaethalium (false aethalium).
This fruiting body is composed of sporangia closely crowded together. Pseudoaethalia
are usually sessile, although a few may be stalked.
The fourth type of fruiting body is called a plasmodiocarp.
Usually sessile, plasmodiocarps take the form of the plasmodial veins from which
they were derived.
Examples of capillitium and pseudocapillitium