The incorporation of the cyanobacterium Nostoc by the Zygomycete Geosiphon is the only known example of an extant endosymbiosis between a fungus and a cyanobacterium. It is regarded as a model for the origin of plants. The fungus is also closely associated with mosses with which it can co-exist in a symiotic, mycorrhiza-like relationship, as established by recent research. In addition, its endosymbiotic partner Nostoc lives symbiotically within the thallus of mosses in this biocenosis. This video illustrates for the first time the incorporation of the cyanobacterium and its further development into a bladder. This comprehensive and high-quality educational aid was realized by means of time-lapse cinemicrography as well as electron microscopy and images of the natural habitats. An introductory sequence explains the history of discovery of the fungus. This video is designed for research and education of university level.