Although slime molds have properties that resemble fungi, many scientists classify the organisms with the protists because of their protozoalike qualities. Slime molds may be plasmodial slime molds, which consist of a single, flat, very large cell with many nuclei; or they may be “cellular” slime molds, amoebalike cells that live independently and unite with other cellular slime molds to form a single, large, flat cell with many nuclei.
Though scientists are uncertain how slime molds evolved, the organisms show a hint of cellular cooperation that is characteristic of more complex, multicellular organisms. Cellular slime molds normally move about like amoebas. The organisms exist as a mass of cytoplasm with diploid nuclei. This mass is a plasmodium. Cytoplasmic streaming can be seen within the plasmodium. Slime molds are excellent research tools because they are large and easy to cultivate.
As long as a food supply is adequate and other environmental conditions are optimal, a slime mold grows indefinitely in its plasmodial stage. However, if conditions become harsh, the plasmodium can join with other plasmodia to form a large, sluglike mass. This giant plasmodium can transform itself into a spore-bearing structure similar to a fungus. A stalk arises from the plasmodium, and nuclei within the stalk cells divide by meiosis. Knobs called sporangia develop at the end of the stalk and fill with haploid spores. The spores are shed and removed by the wind when the stalk dries. Eventually, the spores germinate into flagellated swarm cells. Swarm cells function as gametes and fuse to form a diploid zygote, which divides by mitosis and forms a new plasmodium. The plasmodium represents the new generation of slime mold.