Cellulose is bound to azure dye and the cellulose/azure forms a thin layer above clear agar as in tube C. Cellulose is the ONLY source of carbon energy. The cellulose agar layer is inoculated with the test fungus. If no cellulase is produced the layer below stays clear as in tube C. If cellulase is produced then the bond between the cellulose and azure is broken, the dye is released and diffuses into the clear medium below which then turns blue. The speed of the reaction is proportional to the growth rate of the fungus on the cellulose based nutrient.