What is photoperiodism?

The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle. Photoperiodism is responsible for the distribution of many plants worldwide. For example, ragweed (a SDP) is not found in northern Maine because the plant flowers only when the day length is shorter than 14.5 hours. In northern Maine, days do not shorten to this length until August. This is so late in the growing season that the first frost arrives before the resulting seeds are mature enough to resist the low temperatures, and so the species cannot survive there. By contrast, spinach (a LDP) is not found in the tropics because there the days are never long enough to stimulate the flowering process.