A seed is a capsule containing information (in the form of DNA) and a limited supply of nutrients (usually starches and/or lipids). Once a seed decides to try to grow, it has a very limited time window in which it can survive on its stored nutrients before needing energy from the sun. So how does a seed lay in wait for so long and then spring into action when the time is right? The exact process depends largely on the species of plant.
Some plants fill niches that occur only intermittently in their environment, for instance immediately after a forest fire. Seeds of these plants will lie dormant for long periods of time until heat from a fire breaks down the seed coat enough to allow water to penetrate the seed.
Plants that live in dense forests, such as trees, often must be exposed to bright light in order to germinate. Light indicates to the seed that there is an opening in the forest canopy so it stands a chance at growing tall before the canopy closes up again and shades out any opportunity for photosynthesis near the ground.
The range of conditions necessary for individual species of plants to germinate is vast and usually indicates a lot about the niche that the species fills in its native environment. One constant across species however, is the need for water to initiate germination. Water penetrates the seed coat in a process known as imbibition. Once water has entered the seed, it activates hydrolytic enzymes that begin breaking down the stored nutrients, setting off cellular respiration and growth. From this point, the clock is ticking for the young plant. It must use its stored nutrients to get a root deep enough to take in water and hold itself in place and a shoot high enough to begin photosynthesis before running out of stored energy. As farmers, we try to set the stage for a large proportion of seeds to make it past this critical phase and eventually produce food!