INTRODUCTION TO THE PROBLEM 



FiLiCALES (Ferns): about 150 genera and 6000* species grouped into orders and 

 families, of which the Ophioglossaceae, Hymenophyllaceae, Osmundaceae, Polypo- 

 diaceae and Marsiliaceae occur in Britain. 



Some idea of the taxonomic and phyletic complexity of the ferns may be obtained 

 by a glance at Fig. 10 reproduced from Bower (1935). It is obvious that the ferns alone 

 could easily provide material for cytological study for more than a lifetime, and one of 

 the consequences of this book may perhaps be to suggest that such a study would be 



Davalho/ds 



Dickson laceoe 



^ Pteroids 



^ Osmundacece 



% 



P/ag/o^yriaceoe Gymnogrammoids 



Cyadieacece 



B/echnoids 



•s 



\ 



\ 



Dryoptero/ds 



~ Dipf-eroids 



Fig. 10. Phylogeny of the ferns, redrawn after Bower (1935). 



worth pursuing. The little that is presented here in the first few chapters may, how- 

 ever, also supply a wholesome warning. The ground must be conquered step by step 

 with the utmost labour, and a rapid attack on all the intricacies of phylogeny cannot be 

 hoped for. It will therefore not be attempted at this stage except incidentally and 

 in small details, and attention will primarily be directed to those larger general questions 

 for which the smaller but perhaps equally ancient or older living groups are as important 

 as the ferns. 



* These numbers are taken from Verdoorn's Manual of Pteridolog}', in which Christensen (1906) is 

 quoted. Since then Christensen (Supplement 1934) emended these figures to 213 genera and 9387 

 species. Other estimates will be found in Copeland (1947). 



24 



