Figure 18–23. Photoperiod demonstration on Poinsettia 'Prestige Red'. Short Day exposure (right): More than 12 hours of darkness in a 24-hour period. Long Day exposure (left): Less than 8 hours of darkness in a 24-hour period2.
poinsettia on left has green leaves, poinsettia on right has red leaves

Diane Mays  CC BY 2.0