PREFACE. vii 
a sorous or heap, and usually consists of many membraneous 
capsules, thecse or sporangia, with or without an elastic 
ring. The ring is designed to aid, both in the bursting of 
the capsule, and in the dispersion of its contents. If a frond 
of the common Hartstongue be gathered on an open day in 
winter, and the under surface be suddenly exposed to the 
sun, and the result observed with a magnifier, the dis¬ 
persion of the spores, or seed-buds, may be seen to great 
advantage. The sori, or bundles of capsules, are sometimes 
elevated upon a flat or saucer-shaped receptacle; and some¬ 
times arranged round a stalk: rarely, the capsules lie soli¬ 
tarily, or in pairs, upon the frond itself. The sori, again, 
may be observed to be covered with a membrane, called an 
indusium, or involucre, or they are naked. From these cha¬ 
racters, and from the form, arrangement, absence, or presence 
of them, Ferns are divided into tribes, genera, and species. 
Species are the distinct kinds belonging to one genus; genera 
are the distinct forms belonging to one tribe. And species 
may be further divided into varieties; as the Hartstongue, 
Scolopendrium vulgare, has no less than sixty-five marked 
variations from the simple or normal form, catalogued by 
cultivators of the species.* 
Collectors of Ferns, whose knowledge of the primary charac¬ 
ters of a species has been confined to the study of a few fronds, 
perhaps of one only, upon a limited locality, are often tempted 
to assume unusual deviations from the primary characters, as 
proofs of a distinct species: and by this means, the names of 
Ferns have been greatly and inconveniently multiplied. There 
is no necessity to give a name to every variety of form: the 
* See Vol. Y. pp. 156, 157, Ferns Brit, and Exotic, by E. J. Lowe. 
