The studies on flowering in its modern day sense began from the finding of photoperiodism by Garner and Allard in 1920. In many plants, day length determines the growth mode. This phenomenon was named photoperiodism. The response to day length was later found also in dormancy and dormancy break, bulb formation and some other phenomena. The nature responding to day length is now called photoperiodism. The responses to day length are called photoperiodic responses. The flowering regulated through photoperiodism is called photoperiodic flowering to distinguish it from the flowering regulated autonomously or by other factors such as low temperature and other stresses.
　　Plants which flower under day lengths shorter than a certain length are short-day plants. Those which flowers under day lengths longer than a certain length are long-day plants. A certain day length (critical day length) here is the length peculiar to each plant species. The critical day length differs with the plant species. Pharbitis nil is a short-day plant because it flowers under day lengths shorter than 15 hours. Sinapis alba is a long-day plant because it flowers under day lengths longer than 12 hours. Under a day length of 14 hours, both P. nil and S. alba flower. Plants which flower only under an appropriate day lengths are absolute short- or long-day plants. Those which can flower even under inappropriate day lengths are obligate short- or long-day plants.
　　Photoperiodic flowering can be considered as an adaptation to seasonal changes. Day length is the same in the same area, and changes constantly and certainly. Plants belonging to the same plant species inhabiting the same area can flower simultaneously by responding to the chanege in day length. Thus, they can produce offspring with genetic diversity through interindividual pollination.
　　The plants whose flowering is not influenced by day length are called day-neutral plants. There are also short-long day plants, long-short day plants, intermediate plants, and amphibian plants. The regulating mechanism and biological meaning of these special types remain unknown.