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Probably since the beginning of civilisation, humans have observed that plants are seasonal organisms. Whether crop plant or native species, annual or perennial, herbaceous or woody, the most obvious manifestation is in time of flowering. The connection of periodic flowering — and subsequently fruit and seed development — with seasonal climates has also been surmised for centuries, but we now know which environmental factors are largely responsible for regulating time of flowering. In this chapter, we focus on the most critical signals, photoperiod, temperature and water. Other signals enable plants to attune themselves for optimum de-velop-ment at other stages of the life cycle: directional stimuli such as light, gravity and touch, as well as dramatic cues for stopping and starting life, namely fire and drought. Many of these strategies will be highlighted further in Part IV. The chapter concludes with an exploration of how photoreceptors function.