THE BRACKEN FERN ill 



grow in a cone-like group as do the spore-bearing leaves of 

 the horse-tails. 



138. Fossil Ferns. — The ferns and some of their relatives, 

 having hard, resistant structures, have been preserved in 

 large numbers in the rocks whose material was deposited in 

 past ages. This is not true of most of the plants simpler than 

 the ferns, whose tissues are soft and have been destroyed, 



Fig. 63. — Plants of the ages when the coal beds were being formed. 

 In the foreground, on the left, relatives of the club-mosses; just back of 

 these, tree-like forms related to the horse-tails; on the right, tree ferns. 

 After Stevens. 



except for occasional fragments, by the pressure, heat, and 

 chemical changes to which the rock layers have been sub- 

 jected. For this reason, much more is known about the past 

 history of the ferns than about that of mosses, for example, 

 or that of most of the algae or fungi. Our great beds of coal 

 were formed at a time when ferns and their relatives made up 

 a large part of the vegetation of the earth, and so it is espe- 

 cially in the coal measures that rich material is found for the 

 study of fossil ferns (Fig. 63). Among the ferns of the coal- 

 forming period were many true ferns evidently related to 



