Introduction It’s a mysterious substance with a mysterious past: intrigue and imprisonment followed the first suggestion of its existence, and a lifetime of labour could not discover its identity. Mikhail Khristoforovich Chailakhyan, a biologist in the USSR during Stalin’s reign, argued in 1936 that there must be a hormone – called florigen – that causes plants to produce flowers. Florigen, and the evidence for its existence that Chailakhyan amassed, was an exciting contribution to the era’s understanding of plant development and attracted considerable scientific attention. What is surprising is that florigen was also a politically radical notion. Chailakhyan’s idea was…