 CONSTANS

CONSTANS is an Arabidopsis thaliana transcriptional regulator involved in photoperiodism. Its daily expression pattern and light-dependent protein regulation help activate FLOWERING LOCUS T when internal timing coincides with suitable illumination.

The useful comparison is not merely long days versus short days: changing the phase of light relative to the regulator's availability tests the coincidence mechanism. This explains how a plant can use a daily light pattern as seasonal information.^[ref]