Sphagnum is a genus of between 151 and 350 species of mosses commonly called peat moss, due to its prevalence in wet habitats where it contributes to the formation of peat bogs and mires. Sphagnum accumulations can store water, since both living and dead plants can hold large quantities of water inside their cells; some species can hold up to 20 times their dry weight in water. The empty cells help retain water in drier conditions. Hence, as sphagnum moss grows, it can slowly spread into drier conditions, a form larger peatlands, both raised bogs and blanket bogs. These peat accumulations then provide habitat for a wide array of peatland plants, including sedges, ericaceous shrubs, as well as orchids and carnivorous plants. Sphagnum and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. In additional, bogs, like all wetlands, develop anaerobic soil conditions, which produces slower anaerobic decay rather than aerobic microbial action. Peat moss can also acidify its surroundings by taking up cations such as calcium and magnesium and releasing hydrogen ions. Under the right conditions, peat can accumulate to a depth of many meters. Different species of sphagnum have different tolerance limits for flooding and pH, so any one peatland may have a number of different Sphagnum species.