What is Poinsettia?
[[File:Euphorbia pulcherrima-1.jpg|thumbnail|Poinsettia]] [[File:Poinsettia varieties.JPG|thumb|Poinsettia varieties on sale in England]] '''Poinsettia''' (''Euphorbia pulcherrima'') is a [[red]] and [[green]] [[plant]] popular at [[Christmas]] time. It grows in [[Mexico]] and [[Central America]] and is an important export. It belongs to the order [[Malpighiales]]. [[Image:Poinsettia 2.jpg|thumb|The poinsettia is a short-day plant. It needs two months of long nights before blooming]] 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]]. Short day plants flower when the length of daylight falls below a certain critical level. 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. Plants make use of the phytochrome system to sense day length or photoperiod. This fact is used by [[florist]]s and [[greenhouse]] gardeners to control and induce flowering out of season, such as with ''Poinsettia''. {{-}} {{biology-stub}} [[Category:Euphorbiaceae]]