THE PTERIDOPHYTES 297 



of petroleum." ^ Hard coal is carbon, with some ash-forming 

 substances, while soft coal contains ash-forming substances and 

 volatile matter which produces smoke when the coal is burned. 



The amount of plant material required to form an ordinary- 

 bed of soft coal 10 feet in thickness is estimated to be about 

 1500 feet in thickness. There may be several layers of coal 

 separated by layers of rock. When we recall that the United 

 States has several hundred thousand square miles of coal 

 lands, we obtain some notion of the enormous amount of 

 plant growth necessary to form this deposit. It has been esti- 

 mated that the body of a tree, which when dry weighs 11,000 

 pounds, contains 5500 pounds of carbon. A piece of ordinary 

 soft coal 10 feet high and 1 foot square might weigh as much 

 as 8750 pounds. The pteridophyte plant body is not nearly 

 so solid as our woody trees, thus necessitating- greater bulk 

 to secure a corresponding amount of carbon. 



274. Summary of pteridophytes. The true ferns are widely 

 distributed and successful plants. The possession of fibro- 

 vascular tissue enables these plants to assume a position of 

 prominence and to expose chlorophyll to the light in greater 

 abundance than is done by bryophytes. Well-organized woody- 

 stems and elaborate leaf structures are further suggestions of 

 increased ability to do the work of plants. 



Both generations of the true ferns bear chlorophyll. The 

 gametophyte is simpler than that of bryophytes, but has all 

 the structures requisite to enable it to live for a time in an 

 independent way. The embryo sporophyte originates from 

 the oospore within the gametophyte tissue. The sporophyte 

 is the prominent (often perennial) generation of the plant. 

 It is very much more highly differentiated than the same 

 generation in the bryophytes. Its complex sporangia, formed 

 upon specialized areas of the leaf, produce and distribute 

 spores in very large numbers. In some species special spo- 

 rangium-bearing leaves (sporophylls) are produced. 



1 Jordan, David Starr, Science Sketches, essay upon "The Story of a 

 Stone.' 



