Nothing articulates autumn like comforting, warm, orange and red tones: sweet potatoes, carved pumpkins, fallen leaves, a night by the fireplace. But what is the chemistry behind autumn’ s colours? Why do some trees lose their leaves? In contrast to deciduous plants, some trees retain their leaves all year long. The Latin names for these trees usually consist of the word‘ sempervivens’ which means‘ always alive’. Most perennial trees grew in tropical climates, where cold seasons are shorter, or even non-existent. While perennial trees do drop their leaves, they at all times replace them instantly. Most trees in cold climates are deciduous because the energy cost of possessing leaves alive is far too high. Also, large leaves behave like sails on those wintery breezy days, putting the tree in risk of being blown over. Even so, some evergreens like conifers can last the cold, but they are an exception rather than the norm. How do trees‘ decide’ to ditch their leaves? The response to this question is hormones. When you hear the word hormones, you might think of sullen teenagers, the menopause or sexual attraction. But hormones are simply molecules that transport chemical messages in both animals and plants. When autumn arrives, chlorophyll levels decrease, a signal that activates the release of a series of hormones. For years, scientists thought that the main molecule that led to trees