~XCIV, FILICES. 555 
and almost dust-like, arranged either in clusters, called sori, on 
the under surface of the frond, and often covered, when young, 
with a thin membrane, called the zmdusium, or in little invo- 
lucres on the margin of the frond; sometimes rather larger, in 
spikes or panicles at the top of the frond, which has, lower 
down, either leafy branches or one leaf. These capsules open 
in various ways to discharge the minute, usually microscopical 
spores. 
A very large Order, ienin dant diffused over the whole surface of the 
globe, especially in moist climates, although some species may be found 
in the chinks of the hottest rocks. The elegance of their foliage has of 
rate years attracted as much interest in them, on the part of cultivators 
and amateurs, as has their fructification and germination on the part of 
the physiologist. It has long been known that they can be reproduced 
from their spores, but it has only lately been ascertained that these 
spores when sown develop minute, green, leafy expansions, called 
prothallt. On the prothallus are produced minute bodies, which have 
been compared to stamens and pistils, from whence the young Fern is 
subsequently developed. 
The limitation of genera and species in the Ferns has always been a 
matter of great difficulty, and of late years their splitting and changing 
has been carried to such a degree as to throw the whole nomenclature into 
a state of utter confusion. The best characters are taken from the form 
and arrangement of the sori and of their indusium; and some large 
genera, such as Adiantum, Asplenium, &c., are natural, and readily 
recognised ; but in Polypodiwm, Aspidiwm, Cystopteris, &c., there is 
nothing in habit to serve as a guide, and the indusium of the two latter 
genera is often so evanescent that it requires the most careful examina- 
tion of specimens, in exactly the proper state, to ascertain its existence. 
I have been induced, therefore, with a view to assist the beginner in 
the determination of the British species, to include in the following 
Table of Genera the species also of the most difficult ones, endeavouring 
to lead to them by more prominent characters, without reference to the 
more minute, although essential ones, which distinguish the genera. 
It must be recollected, however, that to determine Ferns they must be 
in fruit. It is hopeless to attempt to find out by books to what species 
a barren frond belongs ; and monstrous developments, and deformed 
fronds, now common in cultivation, and found occasionally wild, are 
here wholly passed over. 
Fructification in a terminal spike or panicle. The frond either leaf-like, or 
bearing a leaf in the lower part. 2 
1+ Fructification in a little cup or involucre at the edge of the frond . ‘ pt 
| Fructification on the back or under side of some or all the fronds . ' 5 
( Fronds twice pinnate, usually 2 or more feet high, the fructification forming a 
panicle at their extremity . . 938 OSMUNDA: 
2) Fronds stem-like, not 6 inches high, with a terminal spike orpanicle. 3 
Spike simple. Leafentire . : , . 1, OPHIOGLOSSUM. 
Spike branched into a panicle. Leaf pinnate 2. BOTRYCHIUM. 
Fronds numerous, scarcely 2 inches high, pinnate, with few deeply-lobed seg- 
4 ments, Involucre ovate, 2-lobed ? 17. HYMENOPHYLLUM. 
Fronds 6 or 8 inches high, 9 or 3 times pinnate, with ‘crowded segments. Invo- 
lucre cup-shaped . : ~  « 16. TRICHOMANES. 
