Polytrichopsida have leaves with sets of parallel lamellae, flaps of chloroplast-containing cells that look like the fins on a heat sink. These carry out photosynthesis and may help to conserve moisture by partially enclosing the gas exchange surfaces. The Polytrichopsida differ from other mosses in other details of their development and anatomy too, and can also become larger than most other mosses, with e.g., "Polytrichum commune" forming cushions up to high. The tallest land moss, a member of the Polytrichidae is probably "Dawsonia superba", a native to New Zealand and other parts of Australasia.