Peter Michaelis (28 May 1900 in Munich – 3 August 1975 in Cologne) was a German plant geneticist who focused most of his research on cytoplasm inheritance and segregation. Most of Michaelis work was carried out during the period from the 1940s to 1970s at the Max Planck Institute for Plant Breeding Research in Köln/Vogelsang, where he was a group leader. Education and career Michaelis was born in Munich as the son of the portrait painter Oskar Michaelis. He studied at the University of Munich where he received his PhD under the supervision of Karl von Goebel in 1923. He worked as an assistant to Otto Renner at the University of Jena after graduation. In 1927, Michaelis moved to Stuttgart to work for Richard Harder and started lecturing at the University of Stuttgart. In 1933, upon Harder's departure for University of Göttingen, Michaelis was hired by Erwin Baur to join the Kaiser Wilhelm Institute for Breeding Research in Müncheberg/Mark and continued under the direction of Erwin's successor Wilhelm Rudorf. The institute was later renamed the Max Planck Institute for Plant Breeding Research, where Michaelis continued his research until his retirement in 1968. Research Michaelis worked primarily on the inheritance and segregation of cytoplasmic mutations in Epilobium, a genus related to Oenothera. He calculated in 1955/1956 the statistical probability of the segregation of two different types of plastids under different conditions. With further research Michaelis concluded that segregation of a mutant plastid could be the only cause of the observed variegation in the species. After this Michaelis examined thirty cases of plastid variegation where no mixed cells were found. He proposed that mutant and non-mutant plastids were still segregating in early mitotic divisions. He suggested two possible ways the plastid could influence one another: (1) The mutant plastid might manufacture a substance that damaged the normal plastids during the maturation process. (2) The normal plastid might manufacture a substance that would allow the mutant form to develop normally. Theory predicted that the two types could be distinguished clearly by an examination of the distribution of mutant and normal cells. Michaelis found clear examples of both types of mutation in his material and published his work in 1956. Michaelis firmly believed in complex unity in the cytoplasm, but he rejected simplistic notions such as the plasma gene or that the cause of cytoplasmic inheritance was solely due to plastids and mitochondria. His