Complex sentence: The open areas of the bog are covered with cotton grass, cross-leaved heath and at least six species of sphagnum mosses, the last of which are especially important, as their remains accumulate to form peat, the substrate of the bog.
Split into simpler sentences: The open areas of the bog are covered with cotton grass, cross-leaved heath and at least six species of sphagnum moss. The sphagnum mosses are especially important as the remains of dead plants accumulate to form peat, the substrate of the bog.