48 AN INTRODUCTION TO THE STUDY OF FOSSILS 



(3) The sensitive fern (Onoclea sensibilis). — Now living 

 only in eastern North America and eastern Asia, but present 

 throughout the Northern Hemisphere during the Pliocene 

 (Figs. II, D, 12). 



1. Discuss two characteristics of pteridophytes which show 

 them to be more highly organized plants than bryophytes. 



2. Give briefly the life history of the common Christmas fern. 



3. Tell how the Pteridophyta are sub-divided, giving both 

 scientific and common names. 



4. With what ferns are you familiar? Have you ever seen 

 a fern prothallus ? Which is the more commonly seen stage 

 of the fern, — the reproductive or the vegetative ? 



5. When, in geological time, did ferns appear? 



6. Name two fossil examples. 



7. Give an example of a fern which has existed from the 

 Tertiary to the present. Has it changed much in appearance 

 since the Tertiary ? 



ORDER b, EQUISETALES 



Living horsetails are low plants with simple or branching stems 

 which are strongly furrowed longitudinally and are divided 

 into sections by joints. The leaves are small papery scales, 

 arranged around the stem in a circle at each node. In the Car- 

 boniferous, however, representatives of this order were large 

 forest trees and bore large leaves. 



These fossil remains, moreover, show that horsetails were 

 formerly one of the most abundantly represented groups of 

 plants, though at present the order survives in only one genus, 

 Equisetum. The Equisetales have existed from the Devonian 

 to the present. Apparently the last of the Calamites group, — 

 the Paleozoic horsetails, died out at the close of the Paleo- 

 zoic ; but in the Mesozoic the equisetes were represented by 

 forms which were probably intermediate between the Paleo- 

 zoic and the living horsetails. Equisetites was a very large 

 horsetail of the Triassic with a stem eight inches in diameter 

 and with over a hundred leaves in a whorl. Other fossil ex- 

 amples are : — 



