What are these sugar grain lumps on the underside of my plants' leaves? Are they insect eggs? They don't move, they look like cysts...

When I first saw these, I naturally assumed they were caused by insects. But, not necessarily.

Closer observation: You cant rub or clean them off. They are visible from the top when light shines through too, so it looks more like a deformity than a pest. Sometimes they look like white salt or sugar grains, sometimes clear bumps, and sometimes a bit brown.

There's a thing called "oedema" that plants can get, the main cause of which is overwatering, or rather, waterlogging. I certainly think that's it for me, because I chose badly with soil, I ended up with a sandy soil that holds water for days like wet cement. Unfortunately its too late, but I'll learn for next year.

Anyway - it's not too bad. I guess the cure is to water with less volume, don't soak the soil as much.

More info, including photos, here
http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=637