Family Cyatheaceæ.—These are known as the tree ferns, because of the large size which many of them attain. They occur chiefly in tropical mountainous regions, many of them palm-like and imposing because of the large trunks and leaves. Dicksonia, Cyathea, Cibotium, Alsophila, are some of the most conspicuous genera.
Family Parkeriaceæ.—There is a single species in this family (Ceratopteris thalictroides), abundant in the tropics and extending into Florida. It is aquatic.
Family Polypodiaceæ.—This family includes the larger number of living ferns and many genera and species are found in North America. Examples, Polypodium, Pteridium (= Pteris), Adiantum, etc.
603. Order Hydropterales (or Salviniales).—The members of this order are peculiar, aquatic ferns, some floating on the water (Azolla, Salvinia), while others are anchored to the soil by roots (Marsilia, Pilularia). They are known as water ferns. The sporangia are of two kinds, one containing large spores (macrospores) and the other small spores (microspores). They are therefore heterosporous ferns.
Family Salviniaceæ.—There are two genera, Salvinia and Azolla.
Family Marsiliaceæ.—Two genera, Marsilia and Pilularia. In this family the sporangia are enclosed in a sporocarp, which forms a pod-like structure.
CLASS EQUISETINEÆ.[35]
604. Order Equisetales.—The single order contains a single family, Equisetaceæ, among the living forms, and but a single genus, Equisetum. There are about twenty-four species, with fourteen in the United States (see [Chapter XXIX]).
CLASS LYCOPODIINEÆ.[36]
605. Order Lycopodiales.—The first two families of this order include the homosporous Lycopodiineæ, while the Selaginellaceæ are heterosporous.