A transmissible signal that transmits stimulus for flowering from the leaves to apex of the plant in species where flowering is a response to photoperiod (see photoperiodism). Such a transmissible signal has long been hypothesized, and was termed ‘florigen’ in 1936; however, its identity in some species has only recently been revealed. For example, in the model plant thale cress (Arabidopsis thaliana) the florigen is FT protein, encoded by the FT (FLOWERING LOCUS T) gene, expression of which is regulated by CO protein in response to night length. FT protein is synthesized in phloem companion cells and translocates via phloem from the leaf to the apical meristem. Here it binds to FD protein (encoded by the FLOWERING LOCUS D gene), and the resultant FD-FT complex acts as a transcription factor, activating genes (i.e. LEAFY and APETALA1) that prompt the apical meristem to switch from producing leaves to making flowers.