These are "Living Stones" and yes, they really are living plants. They come from a region of the world that gets about 2 inches of annual rainfall. The picture below shows one of them in bloom. For more info on these interesting plants, check out "The Living Stones, I presume" article at Colorado State University.
[Links: Tucson doesn't have much going on in the way of fall foliage colors, so I really enjoyed looking at Lynette's great pictures of fall foliage up in Portland, Oregon. Thanks Lynette!]