[AS.] An order of plants belonging to the class of acrogens. They are usually found in moist soil, sometimes they grow as parasites on trees, and in the tropics reach so large a size as to be called tree ferns. The brake or bracken, polypody, asplenium, maidenhair, are all well-known ferns. They bear clustered cells filled with spores, which germinate and form minute growths, on which are borne the true organs of reproduction. These spores are generally produced in rows on the back of the leaves, or fronds, as they are usually called ; or sometimes on a central branch or spike. There are more than two thousand kinds of ferns in the world.