Figure 1.

The external coincidence model (Pittendrigh & Minis, 1964). The external coincidence model assumes a circadian rhythm in the abundance of a hypothetical substrate (S), which is converted into its active form S* by a light‐dependent enzymatic reaction. The S level (and hence S*) crosses a threshold and reaches its maximum at a certain time of the day, which is termed as the photo‐inducible phase (ϕ_i). The coincidence of ϕ_i with light under long days generates a sufficient amount of S* leading to an induction of long‐day response such as reproduction. In other words, the photo‐inducible phase (ϕ_i) lies in the light phase. Under short days, ϕ_i lies in darkness, which leads to short‐day responses such as diapause.