It has been observed that some plants require a periodic exposure to light to induce flowering. It is also seen that such plants are able to measure the duration of exposure to light.
For example, some plants require the exposure to light for a period exceding a well defined critical duration, while others must be exposed to light for a period less than this critical duration before the flowering is initiated in them.
The former group of plants are called long day plants while the latter ones are termed short day plants. The critical duration is different for different plants.
There are many plants, however, where there is no such correlation between exposure to light duration and inducation of flowering response; such plants are called day-neutral plants.
It is now alsoknown that not only the duration of light period but that the duration of dark period is also of equal importance. Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations.
This response of plants to periods of day/night is termed photoperiodism. It is also interesting to note that while shoot apices modify themselves into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves cannot percieve photoperiods.
The site of perception of light/dark duration are the leaves. It has been hypothesised that there is a hormonal substance(s) that is responsible for flowering.
This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.