● It has been observed that some plants require a `color{violet}"periodic exposure to light"` to `color{violet}"induce flowering"`.

● It is also seen that such plants are able to measure the `color{violet}"duration of exposure to light"`.

● For example, some plants require the `color{violet}"exposure to light"` for a period exceEding a well `color{Brown}"defined critical duration"`, while others must be exposed to light for a `color{violet}"period less than"` this critical duration before the `color{violet}"flowering is initiated"` in them.

● The former group of plants are called `color{Brown}"long day plants"` while the latter ones are termed `color{Brown}"short day plants"`.

● The `color{violet}"critical duration"` is different for different plants.

● There are many plants, however, where there is `color{violet}"no such correlation"` between exposure to `color{violet}"light duration and inducation"` of flowering response; such plants are called `color{Brown}"day-neutral plants"`

● It is now also known that not only the `color{violet}"duration of light period"` but that the `color{violet}"duration of dark period"` is also of equal importance.

● Hence, it can be said that `color{violet}"flowering in certain plants"` depends not only on a `color{violet}"combination of light and dark"` exposures but also their relative durations.

● This `color{violet}"response of plants"` to periods of day/night is termed `color{Brown}"photoperiodism."`

● It is also interesting to note that while `color{violet}"shoot apices modify themselves"` into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves `color{violet}"cannot perceive photoperiods"`.

● The `color{violet}"site of perception"` of light/dark duration are the `color{violet}"leaves"`.

● It has been hypothesised that there is a `color{violet}"hormonal substance(s)"` that is responsible for flowering.

● This hormonal substance migrates from `color{violet}"leaves to shoot apices"` for inducing flowering only when the plants are exposed to the `color{violet}"necessary inductive photoperiod"`.

● There are plants for which flowering is either `color{violet}"quantitatively or qualitatively dependent"` on exposure to `color{violet}"low temperature"`.

● This phenomenon is termed `color{Brown}"vernalisation"`.

● It prevents `color{violet}"precocious reproductive development"` late in the growing season, and enables the plant to have `color{violet}"sufficient time"` to reach maturity.

● Vernalisation refers specially to the `color{violet}"promotion of flowering"` by a period of `color{violet}"low temperature"`.

● Some important food plants, `color{violet}"wheat, barley, rye"` have two kinds of varieties: `color{Brown}"winter and spring varieties"`. The ‘spring’ variety are normally `color{violet}"planted in the spring"` and come to flower and produce grain before the end of the growing season.

● `color{violet}"Winter varieties"`, however, if planted in spring would normally `color{violet}"fail to flower "`or produce mature grain within a span of a flowering season.

● Hence, they are `color{violet}"planted in autumn"`.

● They `color{violet}"germinate, and over winter"` come out as small seedlings, resume growth in the spring, and are `color{violet}"harvested usually around"` `color{violet}"mid-summer"`.

● Another example of vernalisation is seen in `color{violet}"biennial plants"`.

● Biennials are `color{violet}"monocarpic plants"` that normally `color{violet}"flower and die"` in the second season.

● `color{violet}"Sugar beet, cabbages"`, `color{violet}"carrots"` are some of the common biennials.

● Subjecting the growing of a `color{violet}"biennial plant to a cold treatment"` stimulates a subsequent `color{violet}"photoperiodic flowering response"`.