ON GENERA AND SPECIES. 25 



tion and description of Ferns in the Berlin Gardens, 

 entitled " Hortus Berlinensis ; " the second a work entitled 

 " Analectee Botanicae j " as also a continuation of Schkuhr's 

 "Filices." He also published many articles on Ferns in 

 the " Linnasa," in which many new species are described. 



In 1839 Professor Agardh, a Swedish botanist, published 

 a memoir on the genus Pteris, of which ninety-four species 

 are described. 



4. ON THE GENERA Of MODERN AUTHORS, AND REVIEW OF 

 THEIR SYSTEMS OF CLASSIFICATION. 



It would occupy too much space to enter into a review of 

 the many works and the different views entertained by 

 many of the above authors regarding the characters of 

 genera, especially as all matters of importance relating 

 to genera will be found noticed in their respective places. 



The chief characters adopted by many of the preceding 

 authors for defining genera consist in differences in the 

 form and arrangement of the sori, and in the different forms 

 of the indusium when present, and even some on the texture 

 and pubescens, and in the contraction of the fertile fronds. 



By the conjoint labour of authors the number of genera 

 have been considerably increased since the time of Swartz, 

 making the total number up to the time of Sprengel 

 sixty-five, several of which may be viewed as natural 

 genera. Eight are founded each on a single species, six 

 contain two to four each ; while the old Linnean genera 

 Polypodium, Pteris, and Asplenium, and the Aspidium of 

 Swartz collectively contain 648 species, being nearly one 

 half of the whole number enumerated by Sprengel. 

 Although this great number of species agree in the techni- 

 cal characters of their respective genera, as founded on 



