I ran across a great chart recently illustrating the difference in practical planting times and optimal planting times for vegetables.
Vegetables are quite domesticated, needless to say, and far from their native origins. They've been selected for rapid and uniform germination (among other traits such as big leaves or large tasty fruits).
But the interaction between soil temperature and seed germination is fascinating. Many of our warm season crops will germinate rapidly and uniformly at temperatures that most of our soils will never achieve, even in a warm climate like ours (in the Southeastern U.S.). But germination will occur, more slowly, at lower soil temperatures and this is what this chart illustrates so nicely.
It was included in a Gardener's Tip about When is it warm enough to plant? from the Gardener's Supply Company, an informative online/catalog retailer based in Vermont with lots of great information on their website.