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Atrichum is in the moss family Polytrichaceae, which also contains the genus Polytrichum. This genus is similar to Atrichum in that they both have lamellae (aka. moss snorkels) on their leaves. The difference is that the lamellae in Polytrichum cover the entire surface of the leaf, whereas in Atrichum there are fewer lamellae per leaf (around 1-9) and they are located only on the midrib. The photo to the right is of a cross-section through a leaf that has four lamellae on the thickened midrib.
These lamellae are composed of a single file of green photosynthetic cells. They function by sticking out above the film of water that covers the plant to provide a dry surface for gas exchange. These lamellae can be seen with a hand lens as lines running along the surface of the leaf. The distribution of these snorkels are unique to Atrichum and are a good feature to use to recognize the genus.