INTRODUCTION 



27 



covered, of deciding whether to modify the existing defini- 

 tions of groups or whether to create new groups. Too many 

 groups would be liable to obscure the underlying scheme of 

 the classification and too few might result in each group being 

 so wide in definition as to be useless. The scheme on v/hich 

 this book is based is substantially the same as that proposed 

 by Reimers in the 1954 edition of Engler's Syllabus der 

 Pflauzenfamilien^^ and has been chosen because it seems to 

 strike a balance in the number and the size of the groups 

 that it contains. (An asterisk is used throughout to indicate 

 fossil groups.) 



PTERIDOPHYTES 



A PSILOPHYTOPSIDA* 

 Psilophy tales* 



B PSILOTOPSIDA 

 Psilotales 



C LYCOPSIDA 



1 Protolepidodendrales* 



2 Lycopodiales 



3 Lepidodendrales* 



4 Isoetales 



5 Selaginellales 



D SPHENOPSIDA 



1 Hyeniales* 



2 Sphenophyllales* 



3 Calamitales* 



4 Equisetales 



E PTEROPSIDA 

 a Primofilices* 



1 Cladoxylales* 



2 Coenopteridales* 



b Eusporangiatae 



1 Marattiales 



2 Ophioglossales 



c Osmundidae 

 Osmundales 



d Leptosporangiatae 



1 Filicales 



2 Marsileales 



3 Salviniales 



