How does the depth at which a seed is planted affect its germintation?

some seed (garden ones and flowers) .some 1/4 to 1/2 inch. a lot of small flower seed say do not cover as they need light to germinate. some like petunia take a coupe of weeks or longer with dark periods but germinate much faster if they are under lights 24 hrs a day. Even the ones planted with out being covered set good roots as often the roots grow faster then top growth does. I have some now with 2 inch roots and about 1/2 inch top growth