Physarum polycephalum is a slime mold, a myxomycete. (Greek myxa = slime + myketes = mushroom, fungi)

The plasmodium or vegetative stage of Physarum is a mass of protoplasm without definite cell walls contained only a thin membrane. It does not have a specific size or shape. It may change from a globose mass to a sheet-like form spreading over a large area in a thin network. The plasmodium creeps over the surface of the substrate engulfing particles of food on the way.

Streaming of the protoplasm in the veins of a plasmodium is a fascinating process to watch under the microscope. Focus the low power objective on one of the veins to see the protoplasmic granules flowing in one direction at a rapid rate. A minute later the river of protoplasm wall slow down, come to a momentary stop and then begin to flow in the opposite direction.

Physarum feed on bacteria and in this culture it is growing on the bacteria living in the pieces of oatmeal.

Under certain drying conditions the plasmodium is converted to a hardened mass called sclerotium. In this condition, Physarum can ramain dormant for long time and then grow out into plasmodium again when moisture and food are favorable. In the laboratory sclerotia may be induced by drying plasmodia gradually in the dark.

The entire plasmodium is converted to fruiting bodies in the reproductive phase. Whatever happens in a plasmodium that causes it to fruit appears to be irreversible for when a plasmodium reaches the fruiting stage it cannot be induced to mimic growth as a plasmodium.

By frequent transfers, we can retain the active vegetative growth for a streaming protoplasm of plasmodium. Transfer a bit of paper supporting an active mass of plasmodium to a fresh moistened filter paper sprinkled with rolled oats. Paper towelling may be used instead of filter paper.