ON GENERA AND SPECIES. 25 
E and description of Ferns in’ the Berlin Gardens, 
entitled * Hortus Berlinensis;" the second a work entitled 
i |" Analecte Botanics;" as also a continuation of Schkuhr's 
“Filices.” He also published many articles on Ferns in 
the “ Linnea,” 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.—0N 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 jor 
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 — 
ntain two to four each; while the old Linnean genera ` 
half of the whole number. enumerated by Sprengel. Bo 
characters. of their respective genera, as founded on 
'olypodium, Pieris, and Asplenium, and the Aspidium of — 
Swartz collectively contain 648 species, being nearly one ` - 
rh this great number of species agree in the techni- . : 
