1. The TRUE FERNS, FILICES, have one kind of spore which generally
developes monœcious prothallia, relatively large and green. The
sporangia are most frequently situated in groups (_sori_), which are
often covered but not enclosed by an _indusium_.
2. WATER FERNS, HYDROPTERIDÆ, have microsporangia with many (4 × 16)
microspores, and _macrosporangia, each with one macrospore_. The
prothallium is small, and projects but slightly from the germinating
spore. The sporangia are situated in groups (_sori_), which are either
enclosed by an indusium, or enveloped in a portion of a leaf, to form
“fruits” termed _sporocarps_.
The old name for the Hydropterideæ, “Rhizocarpeæ,” _i.e._ the
“root-fruited,” originated from the erroneous supposition that
the sporocarps were borne on the roots.
Sub-Class 1. =Filices= (=the True Ferns=).
Of the eight orders (with about 4,000 species) comprised in this
sub-class, the Polypodiaceæ is the largest (having about 2,800 species)
and the most familiar; for this reason it will be taken as typical.
=The sexual generation.= When the spore germinates, the external
covering (exospore) is ruptured, as in the Mosses. The internal
cell-wall (endospore) grows out as a filament, which soon divides and
gives rise to the prothallium, a flat, cellular expansion resembling
the thallus of a Liverwort. In its fully developed state the
prothallium is generally heart-shaped, dark green, and provided with
root-hairs, and it attains a diameter of about one centimetre (Fig.
205). It is formed of one layer of cells, except along the central
line near the anterior depression, where it becomes several layers
of cells in thickness, forming the “cushion,” on the lower side of
which the archegonia are developed. The antheridia are first formed;
they are thus found on the oldest parts of the prothallium, on its
edge, or among the root-hairs. The archegonia are developed later,
and are therefore found near the apex. Several tropical Ferns have
prothallia[18] deviating from this typical form; _Trichomanes_
(Order _Hymenophyllaceæ_) has filamentous, branched prothallia,
which resemble the protonema of a Moss. Others, again, have
strap-shaped prothallia, which resemble the thallus of certain
Liverworts.
[Illustration: FIG. 205.--Prothallium (_p p_) of Maiden hair
(_Adiantum capillus veneris_) with a young plant attached: _b_
first leaf; _w′_ primary root; _w″_ adventitious roots; _h h_
root-hairs of the prothallium (× abt. 30).]
[Illustration: FIG. 206.--Antheridia of Maiden-hair (× 550).
_A_ Unripe; _B_ ripe, but unopened; _C_ open and ejecting the
spermatozoids (_s_). Those which have been last ejected are still
lying enclosed in their mother-cells, the others are coiled up
and drag with them the cytoplasmic remains (_b_); _f_ cells of
the prothallium.]
The ARCHEGONIA have been already mentioned (p. 199, Fig. 201). The
ANTHERIDIA are hemispherical or slightly conical bodies (Fig. 206).
They consist, as in the Mosses, of a wall formed by one layer of
cells, which encloses a number of spermatozoid-mother-cells (_A_
and _B_). The antheridia when ripe absorb water, and are ruptured,
and the spirally-coiled spermatozoids liberated (Fig. 206 _S_).
The spermatozoids have been observed to pass down the neck of the
archegonium, and to fuse with the oosphere.
=The asexual generation.= The first leaf, the “cotyledon,” of the
embryo developed from the oospore (Figs. 202, 205) is always small,
and has a very simple shape. The leaves which occur later become more
perfect, stage by stage, until the permanent form of leaf has been
attained.--The STEM is most frequently a subterranean or a semi-aerial
rhizome; it is only in the tropical, palm-like Tree-Ferns, that the
stem raises itself high in the air and resembles that of a tree,
with leaf-scars or with the remains of leaves attached (Figs. 207,
203); in certain species the stem is encased in a thick mat of aerial
roots (_Dicksonia antarctica_). When the rhizome is horizontal the
internodes are frequently elongated, and the leaves are arranged in two
rows, as in _Polypodium vulgare_ and in the Bracken-Fern (_Pteridium
aquilinum_), etc.; it is also generally _dorsiventral_, having a dorsal
side on which the leaves are situated, and a ventral side, different
from the former, on which the roots are borne. When the stem ascends
in an oblique direction, or is nearly vertical, its internodes are
extremely short, and the leaves are arranged in a spiral line with a
complicated phyllotaxis, _e.g._ in _Athyrium filix-fœmina_, _Aspidium
filix-mas_, etc. The BRANCHING upon the whole is extremely slight, and
is generally confined to the petiole (_e.g. Aspid. filix-mas_), or
to the stem near the insertion of the leaves. Several species normally
form buds on different parts of the lamina. The buds which are formed
on the stem are not confined to the leaf-axil as in the higher plants.
The Tree-Ferns, generally, do not branch at all.
The VASCULAR BUNDLES are _concentric_, with the wood surrounded by
the soft bast. In transverse section they are seen as circles or
irregularly-shaped figures (Fig. 203), the name of “King Charles and
the Oak” (Bracken-Fern) having originated from the appearance which the
bundles present in oblique section. In _Osmunda_ they are collateral
and resemble those of the Flowering-plants. Round each individual
bundle is often a sheath of thick-walled, hard, brown, sclerenchymatous
cells, which act as a mechanical tissue; similar strands are also found
in other parts of the stem.
[Illustration: FIG. 207.--Various Ferns (1, 2, 3, 4).]
The LEAVES in nearly all species are only foliage-leaves, borne in a
spiral. They have an apical growth which continues for a long time,
and some require several years for their complete development. In the
buds they are rolled up (_circinate_); not only the midrib, but also
all the lateral veins, and even the terminal portions of a leaf are
sometimes rolled up together, the tissues of the leaf being already
fully developed and only waiting to expand. The leaves are often
excessively divided and compound, with pinnate branches, and have
an epidermis with stomata and a well-developed system of venation.
Stipules are only found in _Marattiaceæ_ and _Ophioglossaceæ_.
Very often peculiar hairs or scales (_paleæ_, _ramenta_), dry, brown,
flat and broad, are found on stem and leaf.
The SPORANGIA are small, round capsules, which, in a very large number
of Ferns, are formed on the back, but more rarely on the edge of the
ordinary foliage-leaves. It is very seldom that there is any difference
in form between the barren foliage-leaves and the fertile leaves, as is
found for example in _Blechnum spicant_ or _Struthiopteris_; or that
the fertile part of the leaf is differently constructed from the barren
portion of the same leaf, as in the Royal-Fern (_Osmunda_). In such
instances the mesophyll of the fertile parts is poorly developed.
The sporangia in the _Polypodiaceæ_ are lens-shaped, with long stalk
(Fig. 211 _D_): their wall consists of one cell-layer on which a single
row of cells, passing vertically over the top (that is along the edge
of the sporangium), is developed into the “ring” (annulus). The cells
of the annulus are very much thickened on the inner and side walls,
and are yellowish-brown. The thickened cells, however, do not entirely
encircle the sporangium, and on one side, near the stalk, they pass
over into large, flat, thin-walled cells. These form a weak point in
the wall, and it is here that the sporangium is opened diagonally by
the elongation of the annulus. The sporangium of the Polypodiaceæ
opens as it dries. The cells of the annulus are very hygroscopic, and
in straightening, the annulus bends back with a jerk, thus ejecting
the spores to considerable distances. The cells of the annulus absorb
water with great readiness. [The sporangium arises as a single
epidermal cell, from which a basal stalk-cell is cut off. Three oblique
cell-walls, intersecting near the base, are next formed in the upper
cell, and a fourth between these and parallel to the free surface; an
inner tetrahedral cell enclosed by four others is thus formed, the
outer cells become the wall of the sporangium, while the inner cell,
by a series of walls, parallel to its sides, cuts off a layer of
cells which eventually form the tapetum, the remaining central cell
constituting the archesporium.]
The SPORES are either oblong and bilateral, or they are tetrahedric
with curved sides, depending upon the way in which the tetrad division
has taken place.
The sporangia are almost always situated on the nerves and gathered
into groups, _sori_, which differ in form in the various genera. The
sori, in many genera, may be covered by a scale-like structure, the
_indusium_ (Figs. 211 _B_, 212).
In the majority of cases, each sorus is situated on a small papilla
(_placenta_, or _receptacle_), which is supplied by a small vascular
bundle. Between the sporangia, hairs (_paraphyses_) are often situated,
which spring either from the placenta or from the stalks of the
sporangia.
=Systematic Division.= The Ferns may be divided into two groups,
characterized by the structure and development of the sporangia.
The sporangia in the EUSPORANGIATÆ take their origin from a group
of epidermal cells, and their walls are formed by several layers of
cells. The archesporium is the (not tetrahedric) hypodermal terminal
cell of the axial row of cells which give rise to the sporangium.
In the LEPTOSPORANGIATÆ the sporangia are developed from single
epidermal cells, and their walls are uni-layered. The archesporium is a
central, often tetrahedric cell, from which sixteen spore-mother-cells
are developed.[19] It is difficult to say which form is the oldest
(according to Prantl, those which have the sori on the nerve-endings);
however, the Eusporangiatæ would seem to have made their appearance
long before the others, and also well defined Marattiaceæ and
Ophioglossaceæ occur in the Kulm and Coal period, before the true
Polypodiaceæ.
About 4,000 species of Ferns are now existing, and they are found
especially in tropical and sub-tropical forests.