Stratification - Many dormant tree seeds need to be "after-ripened" before they can germinate. This is the most common cause of seeds failing to germinate. If the seed embyro produced by a tree is dormant, it must be stored at the proper temperature and in the presence of abundant supplies of moisture and air.
Stratification is the process of mixing the seed in a moist (not wet) medium like peat moss, sand or sawdust, then placed in a storage container and stored in an area where the temperature is controlled at a low enough level to "ripen" the seed. This storage is usually over a definate period of time at a specific temperature (around 40 degrees F.)