[A Reyt Tayty]

I sowed some spring cabbage outside in modules on Tuesday afternoon. They are showing this afternoon. Do they normally germinate that quickly?

[JayG]

The majority of crops we now grow in the UK originate from far warmer climates than we have here, so the answer is no, they don't usually germinate that quickly in this country outdoors, but I'm sure they won't be complaining! 

[DD.]

Mine only took a couple of days.

BUT - why to they refuse to germinate if you give them artificial heat to this degree?

[seaside]

DD, like many living organisms, I believe seeds have conditioning that include several senses, not just heat. They may even have a recorded time clock tucked away in their genes. The last thing they need is a false dawn. The same applies to when fruiting occurs, or tries to.
Nobody really knows for sure, but seeds ain't half clever. You can fool them a bit, but deep down they're not happy bunnies if something doesn't make sense. After all, it's their one and only chance to make something out of themselves. 
Doesn't stop us trying though.

[Sparkyrog]

Humidity ?