I'm trying to germinate basil for hydroponic purposes. When I put the seeds on wet foam, they get almost instantly covered with a white goo (within approx. 1 min). Sorry, I cannot get a picture.
The seed coat is hydrating. Perfectly normal, just not usually seen when germinating in soil. having played with "germinate on a wet paper towel and carefully plant with toothpick" method, I've seen it.
Here's a lovely poster (pdf) (of the science conference type) by Dongfang Zhou, Monica Ponder, Jacob Barney and Greg Welbaum of Virginia Tech - far more detail than I've ever gotten into, but the "less than a minute" was actually a good clue that we were not looking for, say, fungus.
The white substance is nothing but the seed coat that hydrates when immersed in water. I don't know why it does happen so.