_Family Schizæceæ._—These are chiefly tropical, but two species are found in eastern North America, Schizæa pusilla and Lygodium palmatum, the latter a climbing fern. _Family Hymenophyllaceæ._—These are known as the filmy ferns because of their thin, delicate leaves. They grow only in damp or wet regions, mostly in the tropics, but a few species occur in the southern United States. _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. _Family Lycopodiaceæ._—There are two genera. Lycopodium (club moss) includes many species, most of them tropical, but a number in temperate and subarctic regions. The gametophyte of many species is tuberous, lacks chlorophyll, and in some there lives an endophytic fungus. Phylloglossum with one species is found in Australia. _Family Psilotaceæ._—There are two genera. Psilotum chiefly in the tropics has one species (P. triquetrum) in the region of Florida. _Family Selaginellaceæ._—These include the little club mosses, with one genus, Selaginella (see Chapter XXX). CLASS ISOETINEÆ. =606. Order Isoetales=, with one family Isoetaceæ and one genus Isoetes (see Chapter XXXI). There are about fifty species, with about sixteen in the United States. FOOTNOTES: [34] As class Filicales in Engler and Prantl. [35] As class Equisetales in Engler and Prantl.