And remember, just because you don't see much growth above ground doesn't mean they aren't growing. Often the are making amazing roots below the soil line. A clear container allows you to see the root development.
My wintersown tomatoes, these photos taken in April 2011. Outside in Ohio in April, they tend to be at cool temperatures. I can't say at that temp they grow, because they are at the mercy of Mother Nature. You can see how small the plants look, growing in their 16 oz. cups
But you can see the roots through the cups, already reaching the bottom of the 16 oz. cup. By the time they're planted out in May, they're still tiny above ground (as in my above post), but they have huge roots relative to their small size
Karen
