Spores typically multinucleate, more rarely uninucleate, spherical, 11-20.5 µ diam., with ring-like hilum where attached to the short stalk, forcibly discharged to a distance of up to 0.2 mm; stalks 4.3-8 µ long, becoming inflated with gas when submerged in water with spore attached; cysts produced, very variable in size and shape, mostly 12-30 x 13.5-50 µ; plasmodia becoming reticulate, eventually fragmenting into few to many multinucleate prespore cells that give rise directly to the fruiting bodies.

The new organism was isolated by placing small pieces of the natural substrate on the surface of a plate of a weakly nutrient medium such as hay infusion agar (5-7 g of hay per liter of water). The small fruiting bodies, closely resembling those of Cavostelium apophysatum, appear in about a week, usually on the agar surface near the substrate fragment. In at least one collection the new form was found to be associated with C. apophysatum, from which it may be distinguished by the empty crater-like stalks and discharged spores, each with a conspicuous hilum revealing where it was attached to the stalk (Fig. 8-11).
S. cavostelioides has very delicate spores that are easily damaged during transfer with a needle. It is most readily isolated by inverting the original plate with mature fruiting bodies over a fresh plate of hay infusion agar, onto which the spores are discharged. The spores may then be cut out on small blocks of agar and transferred to fresh plates of the same medium, along with a small amount of a suitable food organism. We have found most satisfactory for this purpose an unidentified cream-colored yeast (with or without a bacterium added) that appeared as a contaminant in our laboratory some years ago.
The new mycetozoan, like several others that we have recently described and still others that we shall describe shortly, has been isolated only from dead plant parts that were still attached to the plants and had not been in contact with the soil. The fact that this new group of mycetozoans is limited primarily to such substrates is believed to indicate a lack of tolerance for certain soil micro-organisms.
Almost immediately after their formation, the spores of S. cavostelioides are capable of germination. In fact, if they fail to be discharged, they may germinate while attached to their stalks within half an hour after they are formed (Fig. 3F).
Young plasmodia emerging from spores are typically inultinucleate, amoeboid, and quite thin. They produce filose pseudopodia. Very early they show a tendency to become reticulate and to anastomose with one another where crowded together (Fig. 3F). Plasmotomy, or the division of plasmodia, was also observed; the two separating protoplasts in later stages of division were connected by an ever-narrowing strand that eventually broke. If a suspension of the organism along with the food yeast or yeast and bacterium is poured onto the surface of a plate of hay infusion agar, the plasmodia spread out thinly and grow vigorously in broad advancing fronts (Fig. 1). They contain numerous contractile vacuoles. Behind the continuous advancing front of the plasmodium appears a reticulation of anastomosing strands (Fig. 1). The rapid and reversible flow of protoplascn over long distances, so characteristic of in myxomycete plasmodia, is not found here, but a slower ebb and flow of more limited scope occurs within the broad advancing front as well as in the anastomosing strands.
As the thin, delicate plasmodia slowly migrate over an agar surface containing a layer of yeast cells, very few of these cells seem to he ingested completely by the protoplast. At least, it is difficult to demonstrate any ingested cells in food vacuoles completely within the protoplast where they are free to move about as the protoplasm moves. Closer examination indicates that the plasmodium flows over the yeast cells with the latter becoming partially embedded in its undersurface, possibly without becoming completely surrounded by the protoplast. In addition, islands of yeast cells are often found within the bounds of a plasmodium without actually being included within the protoplast or covered by it. It is quite possible that some osmotrophic digestion occurs under these circumstances. However, the numerous empty yeast cells left in the wake of the advancing plasmodium attest to the efficiency of the digestive process. This is not to say that complete ingestion may not also occur, because in some of the thicker plasmodia food vacuoles with ingested yeast cells entirely within the protoplast have been found, though not abundantly.
The plasmodia may assume a variety of forms. Often they penetrate the agar surface either partially or completely. In the latter case they become irregularly branched and very narrow. At times the main part of the plasmodium remains on the agar surface and sends down into the agar from much of its undersurface narrow, branched, filiform extensions. These structures may also develop primarily from the front margin of the plasmodium, where they frequently anastomose with each other. They resemble similar structures referred to by protozoologists as rhizopodia and reticulopodia and will be so referred to hereafter. They have been found in two new species of Schizoplasmodium to be described shortly (Olive and Stoianovitch, 1966) and, therefore, probably represent a generic characteristic.
A few cultures that were started from single spores produced fruiting bodies within 9-11 days. However, sporulation may be induced within a single day by transferring plasmodia to a non-nutritive environment. For example, when a small block of agar with plasmodia was cut out of a plate culture and transferred to a drop of water in a van Tieghem cell at 9:30 AM, fruiting bodies began to develop by mid-afternoon of the same day (Fig. 3). The onset of sporulation may be recognized fairly early by the tendency of the plasmodium to become lumpy in appearance (Fig. 3A). This is followed by the rounding out of a number of masses, depending upon the size of the plasmodium, which we are calling the prespore cells. These are at first connected to each other by protoplasmic strands that become narrower and snap in two as the masses round up (Fig. 3B-D). The prespore cells contain 1-11 nuclei, the great majority being multinucleate.
Almost as soon as the prespore cells have separated from each other, they begin to develop into fruiting bodies (Fig. 3D, E). In fact, the entire process of sporulation requires a remarkably short time. The stages shown in Fig. 3A-E from lumpy plasmodium to mature fruiting bodies extended over a period of only 1 hr, 35 min. This rapid development is probably facilitated by the fact that nuclear divisions are not involved in the process, at least from the time that prespore cell formation begins. The spores were found to contain from 1-13 nuclei, the great majority being multinucleate (Fig. 22). This is approximately the same range observed in the prespore cells and indicates that no nuclear divisions are involved in spore formation.
Larger plasmodia may sporulate in one area while remaining vegetative in another (Fig. 2). In smaller plasmodia sporulation proceeds at a fairly uniform rate throughout. The smallest plasmodia may produce only two or three prespore cells.
As in Cavostelium and Protostelium, the nuclei are of a common type, each having a single centrally located nucleolus. This was clearly demonstrated in squashed preparations of plasmodia, prespore cells, and spores observed under a Zeiss phase microscope (Fig. 15-22).
The process of sporulation (Fig. 4-13, 24-30) is very similar to that already described for Cavostelium apophysatum (Olive, 1964b), although some additional details have been noted. The prespore cell soon after its formation becomes hat-shaped as the protoplast begins to hump up in the center, the protoplasm finally collecting entirely in the central hump. An outer membrane, or sheath, has developed over the protoplast in the meantime, and it is the collapsed basal part of this membrane from which the protoplast has withdrawn that remains as the supporting disc at the base of the stalk. The basal disc is generally not seen in aqueous mounts, apparently because it is evanescent. Now there develops within the hemispherical protoplast a basal core of finely granular cytoplasm, which appears to be primarily responsible for the development of the inner stalk membrane and is termed the steliogen. When the fruiting body is mature the steliogen becomes converted into the apophysis subtending the spore and delimited from it by a cross wall. The outer membrane, a pliable and protective structure, covers the entire fruiting body throughout its development (Fig. 12, 30) and is a major component of the stalk, as many of the figures show. Further details and illustrations of these stages of stalk development will be presented in our forthcoming description of two new species of Schizoplasmodium (Olive and Stoianovitch, 1966).
Viewed laterally along cuts in the agar culture, the fruiting bodies typically show narrow stalks (Fig. 4, 5), but when the fruiting bodies are submerged in water, the stalks become inflated with a gas as in Cavostelium (Fig. 6, 7, 23-26, 28-30). The gas, which is presumed to be CO₂ evolved from the protoplast, accumulates between spore wall and outer membrane (Fig. 12, 30). When fruiting bodies with spores attached are mounted in water, the gas is forced into the area between the two membranes of the stalk, thus inflating the stalk, as many of the figures show. In these aqueous mounts, the apophysis is sometimes detached from the spore (Fig. 29, 30).
Under proper conditions, probably related to satisfactory humidity, the gas trapped within the fruiting body distends the outer membrane at some point into a bubble, typically at one side of the spore (Fig. 8). The bubble enlarges over a period of usually 1½ to 10 min, and when it has reached a size varying anywhere from half the diameter of the spore to slightly greater than the spore diameter, it bursts and the spore simultaneously disappears from the top of the stalk. It is this explosion which is primarily responsible for spore discharge. Sometimes the spores only fall off near the base of the stalk, but more often they are discharged to distances of up to 200 µ. In plates that have been opened they may be found as far away as 500 µ from the stalk, but in such cases discharge is probably abetted by air currents.
Variations in the bubble mechanism have been observed. Occasionally, the bubble bursts without discharging the spore. In one case, a bubble appeared three times at the same spot on the spore, bursting each time without discharging the spore. Apparently, such spores become too firmly attached to the stalk to be discharged normally. At other times the bubble develops but fails to burst, remaining "frozen" indefinitely in this static condition. Under prolonged high humidity the entire outer membrane around the spore may enlarge from accumulated gas, but discharge of such spores has not been observed. Sometimes, in the absence of bubble formation, a fruiting body is jarred over onto the agar surface with spore attached. The cause of this is unknown, but it might result from a sudden shifting of gas within the fruiting body.
The discharged spore of S. cavostelioides has a conspicuous ring-like hilum at its base where it was joined to the stalk and apophysis (Fig. 9-11, 27). The spores in some ways resemble discharged sporangia of Conidiobolus. The stalks, when viewed from above, look like miniature volcanoes with central craters. The broad wrinkled bases (Fig. 8, 10-13) are thin and, in side views of undisturbed fruiting bodies, generally not apparent (Fig. 4, 5).
After the spore has germinated, a distinct spore wall remains (Fig. 13). When the empty spore cases are examined in aqueous mounts, some of them show a persistent layer of gas between outer membrane and inner spore wall, thus demonstrating the persistence of both wall layers. When tested with chloriodide of zinc, the spore walls and stalks gave a faintly positive reaction for cellulose.
Under conditions unfavorable for sporulatinn, particularly in older cultures, cysts are formed abundantly. These may develop directly from smaller plasmodia that round up or hump up on the agar surface, or they may develop by segmentation of plasmodia of all sizes. Often segmentation resembles prespore cell formation, but the small multinucleate masses encyst instead of sporulating. The cysts are highly variable in size and shape, ranging in outline from spherical or oblong to markedly irregular (Fig. 14). As the cultures age the cyst protoplast may re-encyst repeatedly so that the protoplast becomes surrounded by a succession of old cyst walls. Upon germination the cysts give rise to plasmodia, leaving the empty cyst walls behind. The walls of older cysts give a decidedly positive reaction for cellulose.