What is photoperiodism. How are plants classified based on photoperiodism?

Explanation

The physiological change on flowering due to relative length of light and darkness (photoperiod) is called photoperiodism. The term photoperiodism was coined by Garner and Allard.
The photoperiod required to induce flowering is called critical day lenght. Maryland mammoth (tobacco variety) requires 12 hours of light and cocklebur (Xanthium pensylvanicum) requires 15.05 hours of light for flowering.
Classification of plants based on photoperiodism:
Depending upon the photoperiodic responses plants are of the following types:
1. Long day plants: The plants that require long critical day lenght for flowering are called long day plants or short night plants.
Eg: Pea, Barley and Oats.
2. Short long day plants: There are long day plants but should be exposed to short day lenghts during early period of growth for flowering.
Eg: Wheat and Rye.
3. Short day plants: The plants that require a short critical day lenght for flowering are called short day plants or long night plants.
Eg: Tabacco, Cocklebur, Soybean, Rice and Chrysanthemum.
4. Long short - day plants: These are actually short - day plants but they have to be exposed to long days during their early periods of growth for flowering Eg: Some species of Broyophyllum and Night jasmine.
5. Intermediate day plants: These require a photo period between long day and short day for flowering. Eg: Sugarcane and Coleus.
6. Day neutral plants: There are a number of plants which can flower in all possible photo periods. They are also called photo neutrals or indeterminate plants. Eg: Potato, Rhododendron, Tomato and Cotton.