W.W. Garner and others discovered that day length is critical for flowering induction. A Russian scientist found that leaves perceive the day length changes and produce a flowering hormone called florigen. Later, it was found that florigen is a protein encoded by the FT gene that moves through the phloem from leaves to the shoot apical meristem. Grafting experiments showed that florigen is graft-transmissible and can induce flowering between species and photoperiod types. The FT protein forms a complex with the FD transcription factor in the meristem to activate floral identity genes and induce flowering.