296 MORPHOLOG y. 



large size which many of them attain. They occur chiefly in tropical moun- 

 tainous 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 Parker iacece. There is a single species in this family (Cera- 

 topteris thalictroides), abundant in the tropics and extending into Florida. 

 It is aquatic. 



Family Polypodiacece. This family includes the larger number of living 

 ferns and many genera and species are found in North America. Exam- 

 ples, 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 Salmniacea. There are two genera, Salvinia and Azolla. 

 Family Marsiliacea. Two genera, Marsilia and Pilularia. In this family 

 the sporangia are enclosed in a sporocarp, which forms a pod-like structure. 

 CLASS EQUISETINE2E.* 



604. Order Equisetales. The single order contains a single family, 

 Equisetaceae, 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 LYCOPODILNEJE.f 



605. Order Lycopodiales. The first two families of this order include 

 the homosporous Lycopodiineae, while the Selaginellaceae are heterosporous. 



Family Lycopodiacea. There are two genera. Lycopodium (club 

 moss) includes many species, most of them tropical, but a number in tem- 

 perate and subarctic regions. The gametophyte of many species is tuber- 

 ous, lacks chlorophyll, and in some there lives an endophytic fungus. Phyl- 

 loglossum with one species is found in Australia. 



Family Psilotacece. There are two genera. Psilotum chiefly in the 

 tropics has one species (P. triquetrum) in the region of Florida. 



Family Selaginellacece. These include the little club mosses, with one 

 genus, Selaginella (see Chapter XXX). 



CLASS ISOETINEJE. 



606. Order Isoetales, with one family Isoetaceae and one genus Isoetes 

 (see Chapter XXXI). There are about fifty species, with about sixteen in 

 the United States. 



* As class Equisetales in Engler and Prantl. 

 f As class Lycopodiales in Engler and Prantl. 



