Not sure if this is it, but acording to our extension service web site, these might be "adventitious roots". Here is what it says about them:
"Tomato plants, particularly heirloom varieties, and tomatillos may produce small growths with short, fleshy, bristles. These are adventitious roots or aerial roots which would grow into normal roots if placed in contact with the soil. The degree to which a gardener will see this is determined by cultivar, weather conditions and culture."
If you go to their web page, click on plant diagnosics, vegtables, stems, you can see a picture or two and see if that's what you have.