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Slime molds were once placed in the kingdom Fungi, although authorities now place them in the kingdom Protoctista. Most fungi are composed of threadlike filaments of eukaryotic cells called hyphae, collectively referred to as a mycelium. Slime molds are composed of an acellular mass of naked protoplasm with no cell walls in its vegetative state. Like many slime molds, the cells of this species typically aggregate to form a plasmodium, a multinucleate mass of undifferentiated cells that may move in an ameboid-like fashion during the search for nutrients. Fuligo septica's plasmodium may be anywhere from white to yellow-gray, typically 2.5–20 cm in diameter, and 1–3 cm thick. The plasmodium eventually transforms into a sponge-like aethalium, analogous to the spore-bearing fruiting body of a mushroom, which then degrades, darkening in color, and releases its dark-colored spores. Fuligo septica produces the largest aethalium of any slime mold. The spores have a two-layered wall, with a dense outer layer with spines, and a fibrous inner layer. During germination, the outer layer splits to create an opening, and more elastic inner layer ruptures later as protoplasm emerges. A remnant of the inner layer may be persistent and adhere to the protoplast after it has emerged from the spore. A peroxidase enzyme present in the inner cell wall plays a role in germination.
A common species with a worldwide distribution, it is often found on bark mulch in urban areas after heavy rain or excessive watering. Fuligo septica grows on rotten wood and plant debris, but can also grow on the leaves and stems of living plants.
Spotted near Tongerensche Beek, Veluwe, Holland. (sources: see reference)