Write a short note on Photoperiodism.

(N/A) Photoperiodism is the physiological response of plants to the relative lengths of light and dark periods.
$1$. Some plants require a periodic exposure to light to induce flowering. These plants are able to measure the duration of exposure to light.
$2$. Based on the requirement of light duration relative to a critical duration,plants are categorized as:
- Long Day Plants: Require light exposure for a period exceeding a well-defined critical duration.
- Short Day Plants: Must be exposed to light for a period less than the critical duration.
- Day-neutral Plants: There is no correlation between the duration of light exposure and the induction of flowering.
$3$. The critical duration varies for different plants. It is now known that the duration of the dark period is equally important as the light period.
$4$. Flowering in certain plants depends on a combination of light and dark exposures and their relative durations.
$5$. While shoot apices modify themselves into flowering apices,they cannot perceive photoperiods themselves. The site of perception of light/dark duration is the leaves.
$6$. It is hypothesized that a hormonal substance is responsible for flowering,which migrates from the leaves to the shoot apices to induce flowering when the plant is exposed to the necessary inductive photoperiod.