4 
INTRODUCTION. 
lower part. When cut transversely, it is seen to consist, first of an epidermis, then 
we find a hard, woody, green, brown, or black layer, the space within being filled 
with cellular tissue. Longitudinally through the tissue run bundles of vessels, 
most of which are true spiral vessels. 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 
by a layer of cells of dark brown colour. The number of these fascicles bears 
considerable relation to the size of the frond ; thus, in Pteris aquilina, 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. 
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 stemf, 
therefore, contain no real wood ; the nearest approach to it being the hardened 
epidermis and the ducts themselves. They increase very little in diameter, but 
grow longitudinally throughout their whole length. 
The Leaf or Frond is in its leafy part thin, veiny, and green. The veins or 
ribs do not extend longitudinally through the leaf in any species, as in the 
Monocotyledones, but diverge in a forked form (dichotomously divided), from the 
base of the leaf, or from the midrib ; differing, however, from those in Dicotyle- 
donous plants in not containing woody fibre, and in being uniform in size through- 
out all their ramifications. The plan of division of the leaf is for the most part 
constant in the same species, but varied in the size and number of parts by 
external circumstances ; the primary causes of which are superabundance 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 Cystopteris fragilis.) The larger (primary) divisions of the 
blade are called pinnae, the smaller pinnuke or pinnules. The Ferns are several 
years in coming to maturity, before which their essential characters are not always 
obvious. Thus young plants of Aspidium Filix-mas very much resemble Woodsia 
ilvensis ; they are 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 was employed in nourishing the fruit of the other. 
The Vernation. The circinate vernation, or curling up of the unexpanded 
leaves, which prevails in all the dorsal Ferns, is almost peculiar to this tribe and 
one of their allies, being found in only a few other orders ; especially the Cycadeic 
and the Droseraceae. If the leaf be simple, so is the vernation, resembling a flat 
spiral spring ; but when the leaf is subdivided, the vernation becomes equally 
