The response of plants to periods of day/night is termed photoperiodism.
Some plants require the exposure to light for a period exceding a well-defined critical duration known as long day plants, whereas others must be exposed to light for a period less than this critical duration before the flowering is initiated in them, known as short day plants.
The plants in which there is no correlation between exposure to light duration and induction of flowering response; such plants are called day-neutral plants.
The site of perception of light/dark duration are the leaves.
There is a hormonal substance, which migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.