No, it doesn't start from the tips. First thing I checked. Which leads me to the second question: What I mean about the soil "being new" is that I went out and bought a bag of ordinary (but excellent quality used it before) potting soil, and added no nutrients because it should contain enough to start with. I've fed them nothing (unless you count my little desperate attempt last week where I sprayed them with a light nutrient mixture to see if it would make a difference). About the stems, they aren't black. They have strong purple lines, which I guess indicates MG deficiency. I've never grown this strain (AK47) before, so I don't know it's peculiarities. I'm continuing the Mg solution treatment and so far there is little change, for the better or the worse. Any other ideas or insights?

And on a different note, why do you suggest I leave out the vermiculite?

Thanks for your time.