Some seeds have specific inhibitors that prevent them from germinating without some type of help.
Many desert plants, as well as some plants from more temperate regions, have special hard coatings or hulls that prevent them from germinating. These hard coverings will not be affected by water or gas. In fact, the seed will not germinate until the covering is changed physically, by breaking it, scratching it, or altering it in some other way to allow the seed to germinate.
This process is called scarification.
Seeds are scarified naturally by freezing, which cracks the outer shell, or by microbial activity dissolving a portion of the hard shell. Some seeds are adapted to the extent that they need to pass through the digestive tract of various animals in order to be scarified.
