Up On The Roof

Standing around on Regent Road (as you do) I caught sight of what appear to be sacks arranged on top of a roof behind Studio 24 on Calton Road. There are 5 neat rows of 9 bags and they have an assortment of vegetation growing out of them.

I'm full of questions. Why are they there? What's in the bags? Are they for holding the roof down? What's the significance of the black and white colours? Is it a coded message? Has someone forgotten they're there? Or is it a badly-maintained roof garden?

Does anyone know?