Editing Plant physiology (section)

===Photoperiodism===
[[Image:Poinsettia 2.jpg|thumb|The [[poinsettia]] is a short-day plant, requiring two months of long nights prior to blooming.]]
{{main|Photoperiodism}}

Many [[flowering plant]]s use the pigment phytochrome to sense seasonal changes in [[day]] length, which they take as signals to flower. This sensitivity to day length is termed [[photoperiodism]]. Broadly speaking, flowering plants can be classified as long day plants, short day plants, or day neutral plants, depending on their particular response to changes in day length.  Long day plants require a certain minimum length of daylight to start flowering, so these plants flower in the spring or summer. Conversely, short day plants flower when the length of daylight falls below a certain critical level. Day neutral plants do not initiate flowering based on photoperiodism, though some may use temperature sensitivity ([[vernalization]]) instead.

Although a short day plant cannot flower during the long days of summer, it is not actually the period of light exposure that limits flowering.  Rather, a short day plant requires a minimal length of uninterrupted darkness in each 24-hour period (a short daylength) before floral development can begin.  It has been determined experimentally that a short day plant (long night) does not flower if a flash of phytochrome activating light is used on the plant during the night.

Plants make use of the phytochrome system to sense day length or photoperiod.  This fact is utilized by [[florist]]s and [[greenhouse]] gardeners to control and even induce flowering out of season, such as the [[poinsettia]] (''Euphorbia pulcherrima'').