I. Introduction

These organisms - the slime molds - have been considered as animals, plants, and as protists. They are probably best regarded as fungi, though a good case can be made for calling them protists.

II. Labyrinthulomycetes - Net Slime Molds

This is a small, poorly known group. One form, Labyrinthula macrocystis is thought to cause a serious disease of eelgrass (Zostera marina), which had a serious impact on brant, redhead, flounders, cod and scallop.

III. Myxomycetes - Plasmodial Slime Molds

A. Habitat
    The plasmodial stage is found on rotting logs, leaf litter, or in moist, shaded soil. The plasmodium may reach several centimeters in diameter. Some forms are brightly colored, though slime molds in general are rather inconspicuous members of the woodland scene.
B. Importance
    Some slime molds are serious plant pests. One of the best examples is the cabbage clubroot disease, caused by Plasmodiophora brassicae, an endophytic parasite. In this disease, zoospores enter the epidermal cells of the seedling, where they cause abnormal growth forms, which stunt, and even kill the plant. Of the secondary bacterial infections can lead to further problems.
    On the positive side, slime molds are significant decomposers, and thus contribute to the maintenance of nutrient cycling.
C. Life cycle of Physarum, a typical plasmodial slime mold.
1. Plasmodium.
    The vegetative state is the multinucleate, pigmented, naked protoplast known as the plasmodium. This yellow protoplast can reach a size of more than 30 cm.
    As it grows over its substrate, it ingests bacteria, etc., and also absorbs soluble food directly into the mass. It can be seen to consist of vein-like regions where rapid flow of cytoplasm can be seen. The flow is erratic, sometimes slowing, stopping, or reversing.
    As long as environmental conditions are favorable, the plasmodium will continue to increase in size, and add nuclei by more or less simultaneous mitotic divisions of the nuclei.
    When temperature or moisture are reduced below acceptable levels, the plasmodium hardens into a sclerotium. This dormant stage is composed of irregular hardened masses of macrocysts, each containing a number of nuclei. The sclerotia are viable for a period of years, and often serve as the over-wintering stage.
2. Sporulation
    When the food supply of a plasmodium is exhausted or diminished, the slime mold moves to an exposed portion of the substrate, and begins the formation of sporangia. An additional requirement for the induction of sporulation is the presence of light - preferably in the blue/violet portion of the spectrum. In Physarum, the sporangia are dichotomously lobed irregular structures, but other species have beautifully formed delicate sporangia. (See Crowder, National Geographic 1926, p 421)
    Typically a sporangium consists of an outer wall, which may be either delicate or firm. It is termed the peridium. Inside, one often finds a delicate network of hair-like structures known as the capillitium, among which one finds the spores.
    The stipe or stalk of the sporangium arises from a thin membranous structure known as the hypothallus.
    Sporangial initiation is indicated by protoplasmic aggregation at some of the higher and drier portions of the substrate. Within this aggregation, very fine vacuoles are formed, within which there is an accumulation of non-living matter. This form the capillitium. The nuclei within the sporangium undergo meiosis, with only one meiospore usually surviving.
3. Swarm cells and amoebae
    The meiospores, as they germinate may produce from one to several naked protoplasts, which may or may not be flagellated. The habitat is the determiner of this. If germination is in an aquatic medium, the spore will form flagellated swarm cells. On the other hand, if the conditions are drier, then ameboid myxamoebae will ensue. There seems to be no significant difference between these two forms, other than the presence of flagellae. The flagellated forms seem to all be biflagellated, with unequal flagellae. They are also capable of ameboid movement.
    Should either of these forms encounter unfavorable conditions, they can encyst, and later, upon excystment, produce one swarm cell or myxamoeba, which will feed by means of pseudopodia at the posterior end, and multiply by mitosis and cytokinesis.
    Eventually pairs of cells will undergo synagamy, and produce a zygote, which, if formed by flagellated cells, will eventually loosed the flagellae. The zygote is the beginning of a new plasmodium.
(diagram of Physarum life cycle)

A. Habitat
    These slime molds differ from the plasmodial slime molds in that their vegetative phase consists of uninucleate amoebae, which, because of their size, are rarely observed.
    They seem to be most abundant in moist, rich soils, especially in manured fields. The group is not well known, but is best exemplified by Dictyostelium.
B. Life cycle of Dictyostelium.
1. Amoebae
    These vegetative cells are very similar to the protistan Amoeba. They feed in a similar manner, on bacteria and yeasts, They reproduce by cytokinesis. There is active debate as to whether or not sexual fusion of amoebae occurs.
2. Aggregation
    When a certain density of amoebae has developed, the cells begin to converge toward centers of aggregation, in response to secretion by some or all members of a substance termed acrasin. This substance has been shown to be cyclic AMP. It appears that some cells act as founder cells, beginning the secretion, then others, as they aggregate, also secrete the acrasin, enhancing the aggregation process.
3. Pseudoplasmodium
    The aggregation of the individual cells results in the formation of a miniature slug or pseudoplasmodium, whose length is about 1 mm. This slug continues as a multicellular structure, with the individual cells retaining their identity, even though intimately associated together. (They can be separated by shaking the slug in water.) The slug migrates for a period of hours to days, depending on the moisture in the environment. Eventually the sorocarp stage is initiated.
4. Sorocarp
    When migration ceases, the pseudoplasmodium condenses somewhat. And begins to form a sorocarp, or spore-producing body. The sorocarp is composed of two regions - the stalk-like sorophore, and at the tip, a cluster of spores in a structure termed the sorus. Each amoeba in the sorus becomes a walled spore. Each spore will eventually germinate to form a single amoeba.