4 
INTRODUCTION. 
perfect plants, but rather of a flat band, like a riband rolled spirally on a cylinder. 
Mixed with these vessels, which are real tracheae, are tubular perforated ducts. 
The whole, thus compounded of the two sorts, is sometimes collected into a close 
bundle, but more generally into a cylindrical sinuous ring, either hollow or filled 
with cellular tissue, and surrounded with a dark membrane. The number of these 
fascicles bears considerable relation to the size of the frond ; thus in Pteris aquiliua, 
there are eight or ten; in Aspidium aculeatum, five; in Polypodium vulgare, 
three ; while in the minuter species there is but one, which then occupies the 
centre of the rachis. When several bundles are present, no general rule can be 
given for their position (though constant in the same species), so varied are they 
in shape, size, and distance from each other. It is thought that the depression so 
often visible on one side of the rachis is occasioned by the absence of vessels on 
that part. 
The cellular substance appears to have no tendency to arrange itself in strata, 
nor do the vessels increase in number as the plant increases in age. The stems, 
therefore, contain no real wood ; the nearest approach to it being the hardened 
cuticle and the ducts themselves. They increase very little in diameter, but 
grow longitudinally throughout their whole length. 
The Frond is in its leafy part thin, veiny, and green. The veins do not extend 
longitudinally through the leaf in any species, as in the Monocotyledones, but 
diverge in a forked form (dicliotomously divided), from the base of the leaf, or 
from the midrib ; differing, however, from those in dicotyledonous plants in not 
containing woody fibre, and in being uniform in size throughout all their ramifica- 
tions, and therefore more properly called nerves than veins. The divisions of the 
frond are for the most part constant in the same species, but varied in their size 
and number by external circumstances ; the primary causes of w hich are super- 
abundance or deficiency of nutriment, while temporary heat or moisture, exposure, 
shelter, or season of the year, occasion other but less striking irregularities. 
Even these causes have but little effect over numerous kinds, and very seldom in 
any case do they occasion so great an alteration of ordinary characters, as to throw' 
doubt upon the species. (See Cistopteris fragilis.) The Ferns are several years 
before they come to maturity, until which their essential characters are not always 
obvious. Thus young plants of Aspidium Filix-mas very much resemble Woodsia 
ilvensis ; they arc first pinnatifid, then pinnate, afterwards when perfect nearly 
doubly pinnate. Also when a Fern has its barren fronds different from those 
which are fertile, the latter are more contracted, as if the sap which expanded the 
leaves of the one w r as employed in nourishing the fruit of the other. 
The Vernation. — The circinate vernation, or curling up of the unexpanded 
frond, which prevails in all the dorsal Ferns, is almost peculiar to this tribe and 
one of their allies, being found in only two other orders; namely, the Palimc and 
Cycadeae. If the frond be simple, so is the vernation, resembling a flat spiral 
spring; but when the frond is subdivided, the vernation becomes equally com- 
pound, the larger divisions first opening, and by degrees the branches, pinnae, 
and lobes. 
