The resulting juice from diseased plants Beijerinck placed on the surface of a flat cup with a dense layer of agar (it is prepared from the extract of seaweed). Due to the high nutrient agar properties razvivayutcya microbes on its surface, forming colonies. Penetrate deep into the jelly-like substance that no microbe can not. A few days later Beijerinck took off the top layer of agar, which really grown colonies of microbes, and used to infect healthy plants middle and lower layers, where germs could not penetrate. Assumptions were confirmed: a substance which has got deep into the agar, caused a mosaic disease in infected leaves.