Plants and animals must be able to respond to the ever-changing conditions of their environment. The photoperiod is the ratio between daytime versus night. When a critical amount of light is available, organisms often respond by carrying out life processes key to their survival.
One way plants respond to a specific amount of light and length of day is by growing. The correct amount of light over a period of time can lead to sexual reproduction in flowering plants usually during warmer months. Then, as days grow shorter plants slow down their rate of photosynthesis preparing for the time when less light is available. They may store excess chemical energy in stems and roots or even become dormant as an adaptation to survive through the short days of winter.
Different plants are adapted to different photoperiods. Those deemed short day plants (SDP) flower in the autumn months as days become shorter while long day plants (LDP) flower in the summer months when the days are longer. In order to initiate flowering, plants require the correct ratio of light to dark periods depending on the amount of light they are adapted to in the latitude where they live.
For organisms living at the equator, the amount of light versus darkness is basically equal over the course of a 24 hour period. As latitude increases, the amount of light changes with the seasons due to the tilt of the Earth on its axis as it revolves around the Sun. Plants and animals need to respond to the changing seasons in order to perform important life cycle processes. For example, many animals respond to the longer days when the temperatures are warmer by reproducing. Their sex organs may become more active and enlarged and produce gametes at that time. This may also coincide with plants becoming active and producing pollen and ova within their flowers. Some of the animals affected by longer days and warmer temperatures might have co-evolved along with plants in the same habitat, to aid the plants in their pollination while getting the benefit of food from the plants.