FIG. 31.25 Control of flowering by daylength. Short-day plants flower only when hours of daylight are below a certain number. Long-day plants flower only when hours of daylight are above a certain number.

Photo sources: (top left) Darlyne A. Murawski/Getty Images; (bottom left) Liz Van Steenburgh/Shutterstock; (top right) Dr. Jeremy Burgess/Science Source; (bottom right) Biosphoto/Dr. Jeremy Burgess/Science Photo Library.