I just saw this article about mosses living on glaciers in Iceland and the animals living inside them in the New York Times. These balls of moss (glacier mice) are unattached to the ground and can move around the glacier like tumbleweeds. I knew that mosses grow in extreme habitats and even live in Antarctica, but I had not heard about this type of moss growth form before. Pretty cool, moss tumbleweeds!
Coulson, S. and Midgley, N. 2012. The role of glacier mice in the invertebrate colonisation of glacial surfaces: the moss balls of the Falljökull, Iceland. Polar Biology
Other names for these glacier mice are moss balls and rolling moss. This growth form is advantageous for growing on substrates that are unstable like the surface of a glacier or sand dune.
For some more reading on this type of moss growth form -