PAST HISTORIES OF PLANT FAMILIES 115 



Measure period the ferns must have been the dominant 

 class, and it is often spoken of even yet as the " Age of 

 Ferns". _ From the rocks of the same age, preserved 

 with their microscopical structure perfect, were stems 

 which were called Lyginodendron. In the coal balls 

 associated with these stems (which were the commonest 

 of the stems so preserved) were also roots, petioles, and 



Fig. 77- — Sphenofteris Leaf Impression , the fernlike loliage of Lyginodendrun 



leaflets, but they were isolated, like the most of the 

 fragments in a coal ball, and to each was given its name, 

 with no thought of the various fragments having any 

 connection with each other. Gradually, however, various 

 fragments from the coal balls had been recognized as 

 belonging together ; one specimen of a petiole attached 

 to a stem sufficed to prove that all the scattered petioles 

 of the same type belonged also to that kind of stem, and 

 when leaves were found attached to an isolated fragment 

 of the petiole, the chain of proof was complete that the 



