46 OUR NATIVE FERNS AND THEIR ALLIES. 



spent in one of the largest collections of growing ferns, added to 

 Presl's system, characters based on methods of growth. Fee, 

 who gave much attention to the ferns of the West Indies, 

 Mexico, and Brazil, also established numerous genera. To 

 these three writers and to Moore, who followed them, we are 

 indebted for a more liberal and more consistent conception of 

 fern genera. The modern tendency is toward this recognition 

 of a larger number of fern genera, depending on characters 

 drawn from venation and from habit of growth. Such unnatu- 

 ral aggregations of species as have hitherto been grouped to- 

 gether under the name Gymnograme* because of the fact that 

 the species all had elongated naked sori on the back of the leaf, 

 cannot be tolerated in a system that professes to be founded on 

 natural relationships. Natural genera must contain only species 

 that are more closely related to each other than they are to any 

 other species. 



1 2O. Families. Genera are grouped in families according 

 to the characters of the sporangium itself, its method of dehis- 

 cence, and especially its origin from the tissues of the leaf. 

 Eight families of ferns, if we include the eusporangiate Ophio- 

 glossacetz and the heterosporous Saltitniacea and Marsileacece, 

 are found in our flora. Besides these there are the Marat- 

 tiacece, Gleicheniacecz, and Matoniacece among the ferns of trop- 

 ical regions. Families of plants now have the uniform termina- 

 tion -acece. 



121. Orders. Families are grouped into orders based on 

 still wider characters. The plants with fern-like habit make up 

 the order FILICALES, though it is an open question if the 

 eusporangiate types and heterosporous types ought not to be 

 separated as distinct orders. The rush-like species forming the 

 single family Equisetacece constitute the order EQUISETALES, and 

 the club-moss types, isosporous and heterosporous, form the 

 order LYCOPODIALES. 



1 22. Principle of Classification. The true idea of clas- 

 sification is the grouping together of objects according to essen- 

 tial and fundamental resemblances. Every system is more or 

 less artificial, yet there is a continual approach toward the true 



* Cf. Hooker and Baker, Synopsis Filicium, pp. 376-390. 



