THE INTER-RELATIONS OF PLANTS AND MAN 121 



Peat moss {Sphagnum) (fig. 78) is particularly prone to this habit, 

 accumulations of old peat moss being exhumed, dried and then 

 used as fuel. 



During previous geologic eras vast swamps existed on many 

 continents, particularly in what is now eastern United States. 

 The dominant plants in these swamps were the Pteridophytes, 

 some of which grew to be trees (fig. 79). The leaves, stems and 

 spores of these ferns accumulated under the water, forming 



Fig. 79. — Prehistoric swamp forests included fern-relatives which grew to tree 



size. 



deposits of organic matter in which the potential energy of the 

 cellulose substances lay hidden. With the passage of millions of 

 years, layers of sedimentary rocks formed over the swamp 

 remains. The plant debris then became changed into bitumi- 

 nous coal, and this — if subjected to pressure while aging — became 

 anthracite. Today we are using the deposits as subterranean 

 storehouses of fossil energy which originally came from solar 

 radiation intercepted by chlorophyll and transformed into 

 plant tissues. Every time we get heat by burning a piece of 

 coal, we are releasing the sunlight which shone upon the earth in 



