FERNS, OR FIL´ICES.
The general appearance of plants belonging to the class of Ferns is so
well known that it need scarcely be described, especially since the
introduction of the glass plant-cases, by means of which the air can be
kept so damp that ferns are now grown in the very heart of our cities.
Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or
fronds (_frons_, a leaf) as the leaf-like organs of the lower plants are
called, arising in tufts from the stems, give them the elegant
appearance we are called upon to admire, whenever they are met with. The
brownish spots or stripes seen upon the back or under surface of the
fronds, and consisting of the fructification, form also a simple
character by which they may generally be distinguished; although in a
few of them the fructification is placed upon a distinct stalk. The stem
or rhi´zome (ῥἱξωμα, a root) of a fern is mostly situated just beneath
or at the surface of the ground, and is commonly mistaken for the real
root, which is buried in the earth. It is brownish outside, and covered
with scurfy scales or _ramen´ta_ (_ramentum_, a shaving). These scales
are interesting microscopic objects, from the distinctness with which
they exhibit the cellular network.
A section of the rhizome exhibits the fibro-vascular tissue arranged
differently from that in the stems of either Exogens or Endogens. It
forms curiously curved longitudinal plates, a very abundant component of
which is the scalar´iform (_scala_, a ladder) duct (Pl. II. fig. 34).
The walls of the scalariform ducts are angular, and the secondary
deposit is arranged in the form of transverse bars, somewhat resembling
the steps of a ladder; which structure is best seen in a transverse or
slightly oblique section. The fronds are usually cleft nearly down to
the main vein or midrib (fig. 9), or pinnatifíd (_pinna_, a feather,
_findo_, to cleave); sometimes the segments are similarly cleft, so that
the fronds are bipinnatifid. The manner in which the veins usually
branch is also peculiar, each branch separating from the point of
division at an acute angle with the original direction of the vein (fig.
20), so as to be forked. It is also worthy of notice, that the young
frond is rolled up into a flat spiral, or is cir´cinate (_circino_, to
go round), before it opens.
We will now examine one or two species more minutely.
POLYPODIACEÆ. _Polypódium vulga´re_ (Pl. II. fig. 9, one-third of the
natural size), a member of this family, is very common on old trunks of
trees, on banks, &c. The frond is deeply pinnatifid, the segments being
oblong, blunt (obtuse), scalloped (crenate) at the edges (PI. II. fig.
10), and becoming gradually shorter towards the apex of the frond.
On the back of the fronds are the little orange-coloured groups of
capsules (Pl. II. figs. 9 & 10); these are called _sor´i_ (σορὀς, a
heap). The capsules or _thécæ_ (θἡκη, a case), a magnified group of
which is represented in fig. 11, consist each of an aggregation of
cells, fixed to a stalk (fig. 12); and along the back of the capsule is
a close row of thicker cells, forming an elastic ring, the _an´nulus_.
When the seed-like bodies or spores (σπορἁ, a seed) are ripe, the
annulus becomes straightened from its elastic power, and tears the
capsule open, so that the spores are set free and scattered.
_Aspid´ium filix mas_ (Pl. II. fig. 16, reduced to one third or fourth
of the natural size), is the most common British fern. In this the
fronds are pinnate, _i. e._ the segments (pinnæ) corresponding to those
of _Polypodium_ are cleft entirely to the main stalk, the pinnæ (fig.
17) being pinnatifid; and the segments of the pinnæ or the pinnules
(fig. 18) are oblong, obtuse, and saw-edged (serrate). In all botanical
works the fronds are incorrectly said to be bipinnate.
The sori are brown, kidney-shaped (fig. 18), and differ from those of
_Polypodium_ in being covered by a thin membrane (_indúsium_), which is
fixed to the frond at the notch.
_Scolopen´drium vulgáre_ (Pl. II. fig. 19), the Hart’s-tongue Fern, is
common in hedges and on moist banks. Its fronds are simple or undivided,
strap-shaped, heart-shaped at the base, and narrowed to a point at the
apex. The sori (fig. 20) are brown, narrow, longish (linear), and
transverse or slightly oblique. The indusium is cleft down the middle,
so as to form a longitudinal fissure or suture.
_Reproduction._--The spores of the ferns (Pl. II. fig. 12 _a_) resemble
in appearance the seeds of flowering plants on a small scale. They are
usually brown, covered on the surface with little tubercles or other
markings, and when kept on a slide in a moist atmosphere, as over a
saucer of water covered with a bell-glass, they germinate. When this
takes place, one or more short, brownish, hair-like radicles emerge from
one part of the surface (Pl. II. fig. 13), and a process containing
chlorophyll issues from an adjacent spot. As growth proceeds, the latter
by cell-division gives rise to a flattened two-lobed leafy
cotyledon-like body or _prothal´lium_ (πρὀ, before, θαλλὀς, leaf), with
numerous rootlets springing from the base of the lobes. The prothallium
is of a peculiar dull-green colour, different from that of the young
frond which is subsequently formed. This arises from the absence of
stomata and intercellular passages containing air; for the air in these
passages of leaves and petals contributes greatly to the production of
the brightness of their colours. Moreover, the cells of the prothallium
resemble those of the parenchyma of a leaf, the epidermis with its
wavy-margined cells being absent.
When the prothallium has attained its full development, minute scattered
protrusions from its cells occur on the margin or under surface,
resembling short and blunt hairs; and each of these becomes partitioned
off to form a new cell, within which a number of crowded smaller cells
are produced. These organs are called _antherid´ia_ (anther, and εἶδος,
resemblance); and within each of the crowded smaller cells is contained
a very minute, colourless, coiled fibre, furnished with still finer
filaments, called _cil´ia_ (_cilium_, an eyelash); the ciliated fibres
being termed _spermatozo´a_ (σπἑρμα, seed, ξῶον, animal). At a later
period, other organs are found also on the back of the prothallium.
These are larger than the antheridia, and are composed of several cells,
arranged around a central canal which leads to an embryo-cell situated
at its base (Pl. II. fig. 2). These organs are the _archegónia_ (ἁρχἠ,
beginning, λὁνος, offspring). When the antheridia are ripe, they
discharge the spermatozoa, which are enabled to swim about by means of
their cilia in water (rain), and entering the canal, reach the
embryo-cell, which thus becomes fertilized. When fertilized, the
embryo-cells produce the little fronds which afterwards grow into the
mature plants.
Hence the spores of ferns differ strikingly from the seeds of the higher
plants in not containing the embryo radicle and cotyledons already
formed, these being produced during or after germination; also in the
fertilizing organs, viz. the antheridia or representatives of the
anthers, and the archegonia or the representatives of the pistils, being
produced from the cells of the prothallium.
The more minute of these structures are too difficult of observation and
preparation for any one unaccustomed to microscopic manipulation, so
that they have not been figured in detail; the figures given will,
however, serve to guide the observer in their recognition.
_Preservation._--The ferns may be easily preserved in the entire state,
by laying them flat between sheets of coarse unsized paper, and
subjecting them to moderate pressure in a screw-press; the paper should
be changed, or dried before a fire every two or three days, and the
pressure repeated until the specimens become dry and rigid. They may
then be mounted on sheets of paper, being fastened either with thread
passed round the stalk or portions of the frond with a needle, and tied
in a knot behind, or with strips of paper gummed at the ends.
The minute structures may be preserved either in the dry state or in
glycerine.