Hans Kautsky's groundbreaking discovery(ies) in 1931, its scientific environment, and the ensuing developments.

In 1931, Hans Kautsky discovered not only chlorophyll (Chl) fluorescence induction ("Kautsky effect") in green leaves but also metastable excited oxygen, now known as singlet oxygen, which he showed to act as an intermediate in dye-sensitized photooxidations of organic substances in vitro ("Kautsky mechanism"). While at that time practically nothing was known about the primary reactions of photosynthesis, Kautsky firmly believed that "his" mechanism is also effective in the "Chl-sensitized" conversion of light energy into chemically fixed energy. This erroneous assumption complicated the interpretation of rapid Chl fluorescence induction kinetics, particularly those measured by his student Ulrich Franck in his 1941 dissertation, part of which indicated the existence of two excitonically separated light reactions. This historical note deals with the essence of Kautsky's two discoveries, the scientific environment under which they took place, and the question of why mainstream photosynthesis researchers have largely ignored the ensuing detailed experimental work of Ulrich Franck. The first commented English version of Kautsky and Hirsch (1931) is presented in the Appendix.