So, what you see here is a very young seedling that has sprouted in a piece of Roman glass. The technique for making this kind of piece involves folding over the lip to make a nice, soft and even rim. It usually traps air all along the edge, under the rim. What happened here?
After a very long time - perhaps 2 thousand years - a little seed got caught against one edge of the glass. When it sprouted after our recent rain storms, the root grew into the little tunnel of trapped air, which had filled with dirt. When I spied the chunk of Roman glass and picked it up, the seedling came with it. In the photo you can't really see that the dirt is INSIDE the glass, not on the outside, unless you look carefully at the bottom left of the chunk, where the roots are hanging out.
This is a very cool find - and I've put it on wet cotton in my window, until I find an appropriate way to pot it. I'm wondering if the growing root will eventually crack the rim open, or if the little thorn bush will be a dwarf, since its root growth will be severely limited.