The Maryland Coastal Plain, where the Tawes Garden is located, is definitely not noted for rocks, so one of the Garden's most distinctive features is a number of large boulders placed singly or in clusters at various points. They were brought in from the far rockier terrain of Western Maryland when the Garden was created as a part of its "Maryland in Miniature" theme, and now their differing characteristics provide focal points in the landscape.
For instance, the boulder in the photo below is serving as a natural substrate for some very strange looking green "flowers"--which not only are not flowers; they're not even plants. Both the round shapes and the green wash over the rock surface are instead produced by lichens, complex organisms consisting of fungi and algae, both non-plants, in a symbiotic relationship.
Lichens are everywhere in the Garden, as they are worldwide, even in the harshest habitats and on every kind of surface. They grow not just on rocks but on trees, soil, and desert dunes--as well as on roof shingles, wooden structures (like our benches, where we wish they didn't!), tombstones, old glass bottles, even plastic. They are so tough they can even survive in space. These particular lichens have it easy on a shaded boulder in a quiet park, as they go about their infinitely slow work of weathering the solid rock into soil. They won't mind if you stop to admire their ghostly green "petals", or even gently feel their rough surfaces, so nearly a part of the rock itself, yet full of life.