Poinsettia
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.
Many flowering plants 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 florists and greenhouse gardeners to control and induce flowering out of season, such as with Poinsettia.
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Poinsettia Media
A full-grown specimen of E. pulcherrima
Scientific illustration of E. pulcherrima, ca. 1880
Newspaper headline from the Honolulu Star-Bulletin (1913) wrongly alleging that poinsettia is deadly
Euphorbia pulcherrima in Viherlandia
A male leopard lacewing butterfly landing on a poinsettia
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An inflorescence of the plant
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A hedge consisting primarily of poinsettias growing along a road in Barlovento, La Palma.
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Poinsettias in a church in the Philippines
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Poinsettia in Kolkata, West Bengal, India