Every time I plant Morning Glory seeds I'm amazed they still come up. I guess I shouldn't be. The packet instructions say to nick them with a file and let them soak overnight. The seeds are sort-of hard-shelled to keep them around through rough times. They are harder-shelled than things like sunflowers.
A number of trees and wildflowers need special treatment to be able to sprout. Lupine and Bittermelon are also difficult but not as bad as some. There are conifers who's cones don't open unless there's been a forest fire. Evolution comes up with some clever answers to the problem of survival.