238 HEREDITY AND EVOLUTION IN PLANTS 



According to this view, vascular plants appear at the 

 beginning of the fossil record as two distinct series, the 

 Lycopsida and Pteropsida. The Lycopsida, like the 

 modern Lycopodiales, are characterized by the possession 

 of small leaves (a primitive character), and by few spor- 

 angia on the upper surface of the leaves. The Pteropsida, 

 by contrast, like the modern Filicales, are in general, dis- 

 tinguished by large leaves, having the numerous sporangia 

 on the lower surface. The two groups also have well- 

 marked anatomical differences. The Lycopsida reached 

 their greatest development in the Paleozoic period, and 

 now appear to be on their way to extinction. The Pterop- 

 sida, on the other hand, although possessing many repre- 

 sentatives in former geological ages, still maintain their 

 full vigor, and are considered by this school of paleo- 

 botanists to be in the direct ancestral line of our modern 

 vascular plants, substantially as indicated in Fig. 1 1 1 . l 



(b) Greater precaution in drawing conclusions from the 

 few known facts has led still other students of fossil plants 

 to refrain from endeavoring to connect up the ancestral 

 lines, claiming that while they may converge, indicating a 

 common ancestry of the known forms in the geologic past, 

 on the other hand they may not unite, or at least may not 

 all converge toward the same ancestral type. In other 

 words, it is suggested that fossil and modern plants had a 

 poly genetic origin from the stage of primitive protoplasm. 

 Such views are illustrated in Table V (p. 240). 



It is seen from this diagram that our modern ferns have 

 a long ancestral history, extending from the present back 



1 Scott restricts the name Lycopsida to the Lycopodiales, and proposes 

 a third group, Sphenopsida, incuding the Equisetales, Pseudoborniales, 

 Sphenophyllales, and Psilotales. Wieland has recently adduced reasons 

 for using the term Hemicycadales. vs. Bennettitales. (Cf. foot-note, 

 p. 211.) 



