When a seed is planted in the ground, germination happens. Germination is the process in which a seed changes from a state of dormancy (just a seed) to a growing, living plant.
A seed contains a tiny plant embryo as well as all of the nutrients an emerging plant needs to begin its growth cycle. In order for the plant embryo to become a plant, key environmental factors must be present when the seed is planted.
Water, Air and Warmth
Water is one of the most important factors in seed germination. Water causes the seed pod to swell and eventually burst, which allows water to reach the plant embryo. Water is essential for cellular respiration, the metabolic process that gives a seedling energy until it can emerge from the soil and get sunlight.
Oxygen is also essential to cellular respiration, so it must be present in order for the seed to begin to grow under the soil. This is why it's important not to plant seeds too deep. If they can't get enough oxygen underground, they may be unable to grow.
Temperature is another important factor in seed germination. Though the ideal temperature for germination varies from plant to plant, most seeds need an environment that's between 60 and 75 degrees F in order to germinate.
Into the Light
Once the seed pod bursts and cellular respiration begins, the tiny seedling gains the energy it needs to push up through the soil. The emerging plant will begin to grow plant tissue, forming the beginning of a stem.
Most seedlings grow out of the seedling curled over, like a person touching her toes. As the seedling gains more plant tissue and grows more stem, it breaks free from the soil. After this point, germination is complete, and the seedling can begin photosynthesis in order to gain energy. This will cause the emerging seedling to straighten up into a tiny plant as it aims for the sun.