Plant life, just like human life, begins with an embryo. In fact, seeds have a lot in common with humans when it comes to origin of life. Seeds have embryos stored inside. These embryos are tiny replicas of the adult plant. They contain all the parts of an adult plant, and just as a human embryo needs care and nourishment to grow and mature, so does the embryo of a seed. In the plant kingdom, the growing of an embryo is called germination.
Anatomy of a Seed
Seeds wear coats just like humans. The coat protects the seed from extremes of temperature, injury and invasion from parasites. The embryo is safely stored inside the coat, awaiting the perfect conditions that will allow it to begin germinating -- sending out roots and shoots. Some seeds such as nasturtium have thicker, harder coats than others. These seeds need to pass through a process called scarification -- the altering of the hard seed coat to allow moisture to enter. Scarification can happen in several ways -- cold weather, being eaten and passed by an animal or pre-soaking.
The Process of Germination
The germination of a seed begins when the combination of water, oxygen and proper temperature is right. Dry soil might be the first sign that your seed is germinating. That's because the seed is absorbing the moisture through its coat to hydrate the cells of the embryo. Once the embryo has absorbed enough moisture, it synthesizes the energy needed to break through the seed coat and begin sending out roots. Shoots -- the stems and leaves of a plant -- form next.
Barriers to Germination
Circumstances sometimes are not ideal and your seed might have a hard time germinating. Over-watering can interfere with the oxygen level your seed needs to grow. Alternately, not watering enough can stunt it when it's unable to absorb enough moisture to break through the coat. If you plant your seed too deep, it might use up all its stored energy before it ever breaks through the surface. Large seeds tend to contain more stored food, called endosperm. You can plant large seeds deeper and still see good germination.
When a seed falls from a tree or plant in the fall, somehow it knows that now is not the right time to begin germinating -- winter is near, and the young seedling would not survive the cold temperatures. Nature has provided a way for the seed to survive the winter and begin germinating again in the spring when the conditions are more favorable. This is called dormancy. Seeds that are dormant are in a sort of sleep-state until the conditions are right for growing.