How are wall studs attached to the sole plate?

How are wall studs attached to the sole plate at the bottom of the wall? It looks awkward, the only way I can see is either with plates or by driving in nails at a 45 degree angle . From what I can see in constructions sites there arent too many plates down there .

It seems like a critical place in the frame , a couple of nails at 45 degrees doesnt seem right.