Photoperiodism, << `foh` toh PIHR ee uh dihz uhm, >> is the ability of plants and animals to detect the relative length of daily light and darkness in their environment. Seasonal changes in the length of the day affect such activities as bird migration and the reproductive behavior of plants and animals.
There are three main types of photoperiodic responses. Short-day responses occur when the period of light each day is shorter than a certain length of time. For example, in the fall, many birds migrate south in response to shortening days. Long-day responses occur when the period of light each day is longer than a certain length of time. In the spring, many plants flower in response to lengthening days. Day-neutral responses occur equally well during both long and short days. For example, some plants flower when they reach a certain size, regardless of the length of day.
