Flasker I think no one has mentioned the most important difference--osmolarity. If you think about the water in planting medium, it is relatively pure, maybe maxing out at 200 ppm tds. In contrast, the tc medium might have 30 g sugar, 6 g agar, and lets say 1.5 g salts. That water contains 37,500 ppm tds. So for the purposes of osmosis, these environments are very different. My own opinion is that this is the most likely explanation for differential results between soil and tc.
Mikewilder,
You may be right!!! Osmolarity is too high so seeds get less water -> germinate longer. I use 10 g sugar, 4 g agar and 1 g salts but it's still too high than ~ 0 ppm tds (distilled water). Seeds germinate well but take longer. It happened to me 3 times. I will try next time in vitro with coconut coir and distilled water only or with very diluted salts. However, some other seeds germinate very fast in normal media so it may depend on what kind of seeds. Anyway, Thanks !!! You're ingenious!!! Play with TC Mike ?