4 
INTRODUCTION. 
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 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. It is 
thought that the depression so often visible on one side of the rachis is occa- 
sioned 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 tho Monocotyle- 
dones, but diverge in a branched form (dicliotomously divided) from tho base 
of the leaf, or from the midrib, differing, however, from those in dicotyledonous 
plants in not containing woody fibre, and being uniform in size throughout all 
their ramifications. The divisions of the frond are for the most part constant 
in the same species, but varied in their size and number by external circum- 
stances, the primary causes of which arc superabundance or deficiency of 
nutriment, while temporary heat or moisture, exposure, shelter, or season of 
the year, occasion other but less striking irregularities. (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 
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 
was 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 Palma: 
and Cycadeae. If the frond be simple, so is the vernation, resembling a ilat 
spiral spring, but when the frond is subdivided the vernation becomes equally 
compound, the larger divisions first opening, and by degrees the branches, 
pinnae and lobes. 
