342 THE PTERIDOPHYTES 



much thicker than the coal beds which they formed, for it has 

 been estimated that it took about five feet of plant remains to 

 make one foot of coal. 



It is interesting to think of the part which the pteridophyte 

 flora of the Carboniferous Age plays in the present life and 

 economic activities of the world, giving us a fuel whose carbon 

 was taken ages ago from the air, which was then much more 

 heavily charged with carbon dioxide than is the atmosphere of 

 to-day. 



THE ORIGIN AND EVOLUTION OF THE 

 PTERIDOPHYTES 



331. The origin of the pteridophytes. The pteridophytes 

 undoubtedly arose from a bryophyte ancestry, when the sporo- 

 phyte generation, in some forms having a structure capable of 

 doing chlorophyll work, developed a root system and vascular 

 tissues, and taking the land habit became independent of the 

 gametopliyte. This was one of the most important forward steps 

 in the evolution of the higher plants, for it gave the sporophyte 

 complete freedom to live and grow to its maximum size. It 

 marked a turning point in plant evolution, for after that the 

 sporophyte became the most complex and conspicuous phase of 

 the life history, and the gametopliyte grew less prominent, until 

 finally in the seed plants the sexual generation becomes actually 

 dependent or parasitic upon the asexual generation, a relation- 

 ship which is exactly the reverse of that between the gameto- 

 phyte and sporophyte in the liverworts and mosses. These very 

 important results in the evolution of plants are summarized in 

 Chapter xx IX, The Evolution of the Sporophyte and Degenera- 

 tion of the Gametophyte. 



There are no bryophytes that show clearly how the root 

 system arose, but we can easily understand that so complex a 

 sporophyte as that of Anthoceros (which has chlorophyll-bearing 

 tissues with stomata, and a long, indefinite period of growth) 

 would at once become an independent plant, if it could develop 



