96 



PLANTS AND MAN 



some of the seaweeds. They form matted growths on damp rocks 

 and stumps in shaded ravines and wet woods. Liverworts repre- 

 sent a type of plant body intermediate between the thallus spore 

 plants and the ferns. None of them is of economic importance. 



The phylum Pteridophyta includes less than half as many 

 species (about 6000) as are found in the Bryophyta. These are, 

 however, greater in size and more familiar, including the ferns, 

 club-mosses, horse-tails, ground-pines and scouring-rushes. In 

 prehistoric times they were more dominant than now, as proven 

 by the numerous fossil remains. During the late Paleozoic Era, 

 Pteridophytes formed vast forests, many species growing to the 



Fig. 66. — Liverworts are creeping or matted plants. 



size of trees. Today only a few species are tree-like, as exemplified 

 by the tropical tree ferns. Pteridophytes are characterized by a 

 highly complex sporophyte which is green and independent, 

 made up of true roots, stems and leaves; and an inconspicuous 

 gametophyte which is also green and independent but of the 

 thallus type of organization (see p. 30). 



One group of the Pteridophyta is the True Ferns, with about 

 5000 species (fig. 67). These are plants, for the most part, with 

 subterranean stems and large, often intricately subdivided, leaves. 

 Since the rather delicately constructed leaves are not suited for 

 exposure to sun and dry winds, most fern species live in damp 

 shaded habitats, being typically plants of the deep woods where 

 the air is continually moist. Some, as the Christmas fern, are 

 evergreen and keep their leaves amid winter snows. Others are 



