CLASSIFICATION AND NOMENCLATURE. 45 



shape, method of discharging its spores, and the existence, char- 

 acter, or absence of a ring. The true ferns contain with us 

 three well-marked sub-orders; the Gleicheniacece are mainly 

 tropical ferns. 



121. Orders. Ferns were formerly classed in a single 

 order, but in accordance with later researches they are sepa- 

 rated into three distinct orders, FiLiCES, MARATTIACEVE, and 

 OPHIOGLOSSACE/'E, which are distinguished by the method in 

 which the sporangia are developed, by the character of their 

 spore development, and by other minor characteristics. Two 

 of the orders are well represented in our flora; the MARAT- 

 TIACE^ are mostly confined to tropical regions. The horse- 

 tails (Equz'selum) form a distinct order, the EQUISETACE^E. The 

 club-mosses of the genera Lycopodium and Pstlotum, with two 

 genera not found in America, form the order LYCOPODIACE^E. 

 Selaginella and Isoetes each form an order which takes its name 

 from the single genus. Marsilia and Pilularia form the MAR- 

 SI Li ACE^E, while Azolla and Salvinia form the order SALVINI- 



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 

 natural system, which is the ultimatum of scientific classifica- 

 tion. The study of life-histories will continually clear up points 

 of relationship before unknown, and it will be long before the 

 classification will become fixed and constant. Every new study 

 contributes to this end. 



LITERATURE. 



The references to original writings would include all the 

 botanists who have named or classified ferns since the time of 

 Linnseus (1707-1778). Among the more prominent of these we 

 may mention Swartz (1760-1818), Willdenow (1765-1812), Presl 

 (1791-1849), Mettenius (1823-1866), Hooker (1785-1865), Fee 

 (1789-1874), Milde (1824-1871), Al. Braun (1805-1875), and J. G. 

 Baker ( - ). The following work gives a good review 

 of the various systems : 



SMITH (John). Historia Filicum. London, 1875. (Mac- 

 ro illan Co.) 



