The hard protective coating on some seeds was designed by nature to protect the seed, but in many cases this protective coating actually inhibits the germination of the seed because water and air can not penetrate the hard coating. Many of these seeds actually require a double dormancy period before they will germinate. In other words, the seeds must lay on the ground completely dormant for one full growing season, and then germinate the following growing season. During the first season the only thing that is taking place is the outer coating is being softened by the elements. Once the outer coating is softened water and air can penetrate and germination can begin.
Timing is critical.
Once the protective coating is softened, and the seeds begin to receive sufficient amounts of oxygen and water to begin germination, the plant will start to grow. However, if this takes place at the wrong time of the year, the young seedling will be destroyed by the intense summer sun, or the freezing temperatures of winter. That's why, of the millions of seeds produced by landscape plants, so few actually germinate and survive to become adult plants. As a gardener you can control when your seeds will germinate by initiating the pre-treatment and stratification at just the right time. You can actually fool some seeds into germinating much quicker by creating the necessary environmental conditions to soften the outer coating and initiate germination sooner.