Define photoperiodism, and discuss its relationship to flowering in certain plants.

Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light and dark periods.  On the basis of photoperiodism the plants are classified in two classes:
    • Short Day plants: A plant that requires a long period of darkness, is termed a "short day" (long night) plant. Short-day plants form flowers only when day length is less than about 12 hours. Many spring and fall flowering plants are short-day plants, including chrysanthemums, poinsettias, and Christmas cactus.
    • Long day plant: Long-day plants require less darkness to generate the reaction needed for production. These plants need about eight hours of darkness to start flowering. The long-day flowers that grow well in the home garden are lettuce (Lactuca sativa), spinach (Spinacia oleracea) and petunias (Petunia x hybrids).