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As plants moved onto land, they encountered environments with distinct seasons. In temperate regions, temperatures vary from summer to winter; in most tropical habitats, wet seasons alternate with dry ones. Thus, plants evolved mechanisms to control the timing of developmental transitions such as the production of flowers and other reproductive organs, the germination of spores or seeds, or the formation of overwintering buds. To accomplish this, plants need reliable information that tells them what season it is. While temperature tends to be unreliable—think of unusually warm or cold summers—day length is a dependable source of information about the seasons that works everywhere except close to the equator.