Physarum as a Radiation Test Object

One of the problems in experimental radiation has been to find a satisfactory test object which is on the one hand responsive to radiation and on the other hand sufficiently simply organized to make possible an analysis of the changes which occur. Physarum polycephalum partially fulfills this need. This slime mold is readily cultivated, macroscopically visible, rapidly growing, and continual streaming of its protoplasm gives an index of the direction in which it is growing as well as of the vigor of its growth in any given direction. In addition, we have found it to be sensitive to certain types of radiation.

Method. The Plasmodium of Physarum polycephalum was grown in glass refrigerator dishes on paper towels sprinkled with oatmeal. The towels were kept just above the bottom of the dishes by pieces of glass, and the bottom of the container was covered with tap water. If the Plasmodium is kept at a temperature below 20°C, there is little tendency to form fruit bodies; if the temperature falls below about 12°C, the cultures grow only slowly.

For testing, pieces of paper towel covered with Plasmodium were transferred to one end of a refrigerator dish in the bottom of which there was an agar medium which was thin, flat, and translucent. A sheet of paper ruled to square millimeters was placed under this dish so that at any time the extent of the growth of the plasmodium could be accurately recorded by tracing the frontal edge on a similarly lined piece of paper.

In some cases many of the indentations were left out of the tracing if they were not important as it took too much time to go into such detail with a rapidly shifting contour.