Plants have a wide variety of flowering strategies involving what time of year they will flower and, consequently, reproduce. In many plants, flowering is dependent on the duration of day and night; this is called photoperiodism.
All flowering plants have been placed in one of three categories with respect to photoperiodism: short-day plants, long-day plants, and day-neutral plants. Despite their names, however, scientists have discovered that it is the uninterrupted length of night rather than length of day that is the most important factor in determining when and whether plants will bloom. Short- day plants, then, begin to bloom when the hours of darkness in a 24-hour period rise above a critical level, as when days shorten in the autumn. These plants include poinsettias, chrysanthemums, goldenrod, and asters. Long-day plants begin to flower when the duration of night sinks below a critical level, as when days lengthen in the spring and summer. Spinach, lettuce, and most grains are long-day plants. Finally, many plants are day-neutral, which means that the onset of flowering is not controlled by photoperiod at all. These plants, which are independent both of night length and day length, include tomatoes, sunflowers, dandelions, rice, and corn.
It is hypothesized that photoperiodism is controlled by a flowering hormone called florigen, although such a hormone has never been identified. Scientists do know, however, that photoperiod (length of night and day) is detected by leaves, which signal buds to open when nighttime hours reach an appropriate duration. Phytochrome is the pigment in leaves that allows them to determine the duration of nighttime darkness.