Some seeds need to be cold for months at a time before they will grow. In nature, winter weather would provide enough cold. It is called stratification . Depending on the kind of seed, it is cold stored for weeks or months before it is planted in soil and given normal temperature and watering. There are even some seeds that need to be warm for a while and then cold a second time.
Some seeds have a very tough outer layer or seed coat.
In nature, these seeds lie in the soil for a long time before they weather enough to sprout. Some seeds have their seed coats weakened by the stomach acids of birds or other animals which eat the fruit and pass the seeds on to the soil when they go to the bathroom. Seeds with tough seed coats can be scarified by scratching them with sandpaper, a file, or a knife, or by treating them with acid. Once the seed coat is weakened the seed can grow. For more on the topic of overcoming seed dormancy, click here .