When ready to sprout, or germinate, a seed starts to absorb water. It swells up. The embryo sends hormones (called gibberellins) to the seed coat to signal proteins to start making other molecules that the new plant will need to live and grow.
The stored food begins to break down, providing energy to the embryo for growing. Soon a tiny root peeks through one end of the seed. A shoot rises in the opposite direction.
Just as Sleeping Beauty needed a kiss from a prince before she could wake up, some seeds need a cue from the outside world so they sprout when the world is good for growing, rather than in midwinter or during drought.
Hot and cold. Some seeds must be exposed to cold before they germinate. Wild rice seeds, for example, usually need time in cold water. Seeds of false rue anemone, a woodland wildflower, need summer's heat to sprout.
All wet. Chemicals called inhibitors send the embryo of some seeds a message not to grow. When exposed to water, the inhibitors leach away (the way flavor disappears from bubble gum after you've been chewing it awhile) so the seed can sprout.
Right light. Some seeds need a period of darkness. Others need light to sprout.
Rough stuff. Seeds with tough coverings might need roughing up—such as tumbling downstream and bumping into rocks, or freezing and thawing, to crack.
Fungus helpers. To sprout and grow, orchid seeds need certain fungi to supply water and nutrients.