Seeds consist of encapsulated plant embryos residing in a highly resistant and metabolic inert state. This allows plants to travel in both space and time, securely holding growth until the appearance of favorable conditions allows them to germinate and develop into a young and fragile seedling. Germination is expected to be tightly controlled since seedlings may suddenly experience hazardous environmental conditions. Indeed, multiple levels of regulation have appeared during evolution to control germination in response to environmental cues.