I004 PRINCIPLES Ol-^ STRATIGRAPHY 



may become enclosed in the finer lutaceous sediments. Spores 

 referred to rhizocarps (Protosalvinia) are found in abundance in 

 the black Devonic shales of North America, and it has been held 

 that the black color and petroliferous character of these shales 

 are wholly due to the spores of these plants. If the spores are, 

 indeed, those of rhizocarps, they would be an indication of the 

 formation of the black shales in the estuaries of rivers, since 

 these plants are found only in fresh water. 



Ferns are most abundant in the tropics, where they develop 

 an extraordinary wealth of form, and vary in their dimensions 

 from small moss-like plants to trees. They are especially character- 

 istic of humid forests. 



Equisetincr, or horse-tails, are represented by the living Equise- 

 tum and by the extinct Calamites, which latter often grew into 

 large trees. Equisetuni to-day grows in low moist groinid and in 

 the sand and gravel of railroad embankments, and along road sides. 

 Much silica is present in the epidermis of the plants, giving to it a 

 rough, harsh feel. 



Lycopodiacecc, or club-mosses. These are- to-day represented 

 by small prostrate plants found mainly in the deeper woods. In 

 late Palseozoic time, however, they were represented by forest trees 

 (Lepidodendron, Sigillaria, etc.) A few aquatic lycopods exist 

 (quill-worts, Isoetae). These plants grow either partly or com- 

 pletely submerged, and in general resemble the smaller club-mosses. 



Both Equisetae and Lycopodiace?e are characteristic of the tropic 

 and temperate zones, the Lycopodiacese being more prominent in 

 the tropics. 



Spermatophyta. 



By far the greater mass of spermatophytes or phanerogamous 

 plants are terrestrial in habitat, though a not inconsiderable number 

 live in fresh water. 'Certain members of the pondweed family 

 (Potamogetonaceje) and the frog's-bit family (Hydrocharitaceae), 

 comprising about twenty-five species in all, have become wholly 

 adapted to a marine benthonic habit and are known as sea-grasses. 

 The pondweed family is represented on the Atlantic coast by the 

 eel-grass (Zostera marina L.) and the ditch-grass {Ruppia mari- 

 fima L.), both of which are extremely common, and both of which 

 are concerned in the gradual choking of the marshes. A number 

 of species are partly marine, as the marsh-grass (Spartina) growing 

 within the limit of tide-water. The mangrove, as already noted, 

 is partly adapted to a marine habitat, all of it but the leaves being 

 periodically submerged. 



