Order 1. =Polypodiaceæ.= Sporangia on the lower side of the leaves,
_stalked_ and provided with a _vertical_, incomplete annulus; dehiscing
by a transverse cleft (Fig. 211 _D_).--The genera are distinguished by
the form of the indusium and the position of the sori, etc.
1. The sporangia cover the entire lower surface of the leaf (Tropical
America and Asia). _Acrostichum_, _Platycerium._
2. Sori without indusia, circular or oval. _Polypodium_ (Fig. 211 _A_).
The leaves are most frequently situated in two rows on the dorsal
side of the creeping rhizome, and fall off leaving a smooth scar
behind.--_P. vulgare_, common in woods, on stones. (_Phegopteris_ also
has no indusium; see page 214).
3. The sporangia are situated in continuous lines just inside the
margin of the leaf.--_Pteris_[20]: the sporangia form a continuous
line along the entire margin of the leaf (Fig. 211 _C_), which bends
over and covers the sporangia, forming a “false-indusium.” _Pteridium_
has linear sori situated on a marginal vascular bundle, covered by two
linear basal indusia, of which the outer is bent over like the edge of
a leaf.--_P. aquilinum_ (Bracken) has a wide-spreading rhizome with
large alternate leaves, placed on opposite sides, at some distance
apart. Only one leaf is developed from each branch every year.
[Illustration: FIG. 211.--Portions of leaves with sori. _A_
_Polypodium_. _B Aspidium_. _C Pteridium_. _D_ A sporangium
of one of the Polypodiaceæ: _r_ the annulus; _s_ spores.]
_Adiantum_ (Maiden-hair): sori on the underside of small
portions of the edge of the leaf, which are bent over (false
indusium). _Cryptogramme_ (_Allosorus_), _Cheilanthes_.
4. The sori are oval or linear, situated on one side of the vascular
bundle.--_Asplenium_ (Fig. 212 _A_): sori linear; indusium with one of
its edges attached at the external side. _A. ruta muraria_ (Wall-Rue);
_A. septentrionale_; _A. trichomanes_.--_Athyrium_: sori linear or
curved; _A. filix-fœmina_ (Lady-Fern).--_Scolopendrium_ (Fig. 212 _B_):
sori as in _Asplenium_, but situated in pairs across the lanceolate,
entire leaves. Each sorus is covered on the external side by an
indusium, whose free edges are parallel and approach each other. _S.
vulgare_ (Hart’s-tongue).--~_Blechnum_ (_B. spicant_, Hard Fern; the
fertile leaves differ from the barren, the pinnæ being narrower, while
the underside is almost entirely covered with sori, and hence they
are of a much darker brownish hue than the barren ones).--_Ceterach_:
indusium rudimentary or absent.~
5. Sori circular and covered by a shield-like, or reniform
indusium.--_Aspidium_ (Fig. 211 _B_); the leaves wither away and
leave no scar upon the root-stock. _A. filix-mas_ (Male-Fern); _A.
spinulosum_.--_Phegopteris_ has no indusium, the withered bases of the
leaf-stalks are persistent; _P. dryopteris_ and _P. polypodioides_.
6. The indusium is situated below the sori, and has the shape of a
one-sided scale (_Cystopteris_, _Struthiopteris_), or of a cup or
cupule, which in _Woodsia_ is sometimes fimbriate (Fig. 212 _C_, _D_).
[Illustration: FIG. 212.--_A Asplenium_.
_B Scolopendrium_. _C Woodsia_; _D_
single sorus of the same. _E Cyathea_: the sporangia
have fallen off in the upper sori. (All magnified.)]
7. The sori are situated on the margin of the leaf, and at the end of a
vascular bundle. Indusium, semi-cupular. _Davallia._ Principally
tropical species. 1 in S. Europe.
This order is the greatest, comprising about 2,800 species, the
majority being perennial plants. A few are large, and known as
Tree-Ferns.
As plants in conservatories and rooms the following are
cultivated: species of _Gymnogramme_ (tropical America),
_Lomaria_, _Nephrolepis_, _Pteris_ (_P. serrulata_, _cretica_).
Officinal. _Aspidium filix-mas_, rhizome and the withered
petioles.--Species of _Alsophila_ and _Cibotium_ give Penghawar
Djambi. The rhizome of _Pteridium aquilinum_, var. _esculentum_,
contains so much starch that it is used as food.
The other orders of true Ferns deviate from the Polypodiaceæ,
especially in the formation of the annulus, the bursting of the
sporangium and its mode of attachment and development, and in
the differences in the formation of the prothallium, etc. The
principal are:--
Order 2. =Hymenophyllaceæ.= To this order belong the lowest and most
Moss-like Ferns; the leaves, with the exception of the veins, are
most frequently formed of _only one layer of cells_, and consequently
stomata are wanting; the formation of the prothallium also somewhat
resembles the Mosses. Sori marginal, on the _extremities of the
vascular bundles_, and surrounded by a _cupular indusium_. The
sporangia are sessile, with equatorial annulus. _Hymenophyllum_ (_H.
tunbridgense_, European). _Trichomanes_ (_T. speciosum_, European).
Species about 200, which live especially on rocks and trees in damp and
shady tropical forests. Some have no roots.
Order 3. =Cyatheaceæ.= Annulus _complete_ and oblique. To this order
belong, principally, the tree-like Ferns with palm-like habit. The
number of species is about 200, they are all tropical and form forests
in some regions of Australia. _Cibotium_ and _Dicksonia_ have marginal
sori, with cupular, basal indusium. (The stem of _D. antarctica_ is
covered with aerial roots.) _Alsophila_ (without indusium); _Cyathea_
with cupular, inferior indusium (Fig. 212 _E_).
[Illustration: FIG. 213.--_Gleichenia_: _A_ part of a leaf with
sori; _B_ a single sorus.]
Order 4. =Gleicheniaceæ.= Sporangia with equatorial annulus, and
longitudinal dehiscence, most frequently groups of 3–4 in sori without
indusium (Fig. 213). _Gleichenia_: the apical growth of the leaves
continues for a long time.
Order 5. =Schizæaceæ.= Annulus apical. To this order belongs _Aneimia_,
which is so commonly cultivated in conservatories. The two lowest pinnæ
are metamorphosed, having no leaf parenchyma and being covered with
sporangia. _Schizæa. Mohria. Lygodium_, a climber, whose leaves
have unlimited growth and attain a length of several metres. About 70
species. Tropical.
Order 6. =Osmundaceæ.= The sporangia have at the apex a lateral group
of strongly thickened cells, which gradually pass over into the
ordinary cells. The sporangia open by a longitudinal cleft. Indusium
wanting. _Osmunda_ bears the sporangia upon peculiar, branched
pinnæ, without parenchyma (the uppermost in the leaf). _O. regalis_
(Royal-Fern): European.
[Illustration: FIG. 214.--_Salvinia natans_: _A_ microsporangium
with germinating microspores and protruding prothallia (_s_);
_B_ a prothallium with the bicellular antheridium (_s_) growing
out of the microsporangium; _C_ the two cells of the antheridium
have opened by transverse clefts; beneath is seen the microspores
enclosed by the hardened mucilage; _D_ spermatozoids still
enclosed in the mother-cells.]
[Illustration: FIG. 215.--_Salvinia natans. A_, _B_ Female
prothallia, _f-f_, protruding from the macrospore which is still
enclosed in the macrosporangium; _œ_ archegonia. _C_ An embryo (×
16) still in connection with the spore (_s_): _a_ the scutiform
leaf; _b-e_ the subsequent foliage-leaves, of which _b_ and _c_
stand singly, _d-e-v_ in a whorl; _v_ the submerged-leaf; _f-f_
wing-like lobes of the prothallium: _m_ the foot.]
Sub-Class 2. =Hydropterideæ= (formerly Rhizocarpeæ), =Water Ferns=.
The following further characteristics must be added to those given on
page 205:--
=Sexual generation.= The MICROSPORES produce an extremely rudimentary
prothallium, formed of only a single cell, and having also a very much
reduced bicellular antheridium with a small number of spermatozoid
mother-cells in each cell (in _Salvinia_ 4, in _Marsilia_ and
_Pilularia_ 16). In _Salvinia_ the microspores remain embedded in a
hard mucilaginous mass (at first frothy) which fills up the cavity of
the sporangium. The prothallium must therefore grow out through this
slime and also through the wall of the sporangium (Fig. 214), and it
thus terminates in a relatively long cell.
In _Marsilia_ the microspores are set free from the microsporangium,
and the prothallia, with the antheridia, remain in them until the
spermatozoids are liberated. The latter are spirally-twisted threads.
The MACROSPORES, on germination, give rise to a very reduced
prothallium, which in _Salvinia_ bears 3 archegonia; but, if these
are not fertilised, the prothallium may continue to grow and become a
fairly large, green body with several archegonia (Fig. 215 _A_, _B_).
In _Marsilia_ the prothallium is still more reduced, it is enclosed
in the macrospore, and only bears one archegonium. The archegonia are
similar in structure to those of the Ferns, but are smaller, and sunk
more deeply in the tissue of the prothallium.
[Illustration: FIG. 216.--_Salvinia natans. A_ An archegonium,
unripe, seen in longitudinal section: _h_ the neck-cells; _k_ the
neck-canal-cells; _c_ the central cell. _B_ An open archegonium
of which the neck-cells have separated off. _C_ An open, old
archegonium seen from the top.]
=The asexual generation= is developed from the fertilised egg-cell.
It is a dorsiventral, horizontal shoot. In _Salvinia_ it bears at
first a shield-like leaf, the scutiform leaf (Fig. 215 _C_, _a_),
which is succeeded by the ordinary foliage-leaves. The young plants of
_Marsilia_, likewise, have less perfect leaves in the very early stage.
The formation of the sporangium is the same as in the Leptosporangiate
Ferns. (The 16 spore-mother-cells originate from one central,
tetrahedric archesporium.)
The Hydropterideæ are divided into 2 orders, the chief differences
between them being found in the asexual generation.
[Illustration: FIG. 217.--_Salvinia natans_ (natural size): _A_
seen from above, floating on the water; _B_ a portion seen from
the side in its natural position in the water.]
[Illustration: FIG. 218.--Sori of _Salvinia_ in longitudinal
section: _h_ microsporangia; _m_ macrosporangia. (× 10.)]
Order 1. =Salviniaceæ.= This order more nearly approaches the true
Ferns, especially so on account of the form of the indusium. Only one
species is found in Europe, _Salvinia natans_ (Fig. 217). This is a
small, floating, annual, aquatic plant, entirely _destitute of roots_.
The dorsiventral, horizontal stem bears two kinds of leaves, which
are arranged in whorls of three. Two of these which turn upwards are
oval, entire, “_aerial foliage-leaves_” (Fig. 217 B, _b^2_-_b^3_);
the third, the “_water-leaf_” (_b^1_) is submerged and divided into a
number of hair-like segments, similar to the submerged leaves in many
aquatic plants, for instance, Water-buttercup (see also Fig. 215 _C_).
The whorls of leaves alternate with each other; there are thus 4 rows
of dorsally-placed aerial leaves, and two rows of ventrally-placed
submerged leaves. The sporangia are situated in sori, each sorus
being borne on a small column (receptacle or placenta) and enveloped
by a _cupular_, but _entirely closed indusium_ (Fig. 218). _The sori
are situated on the submerged leaves_ (Fig. 217 _B_, _s-s_) _and
are unisexual_, _i.e._ each sorus contains microsporangia only, or
macrosporangia.
_Azolla_ belongs to this order. It is a very small, floating,
tropical water-plant (America and East India), with horizontal,
root-bearing stem. The stem branches profusely by lateral buds,
and bears the two rows of leaves on its dorsal side, the roots
on the ventral side. Each leaf is bifid, and divided into an
upper dorsal, and a lower ventral portion. The upper segments
float on the surface of the water and are arranged like tiles
on a roof, each one overlapping its neighbour. In each floating
segment a large cavity is found, in which _Anabæna_ is always
present. The lower segments are submerged.
Order 2. =Marsiliaceæ.= The characteristic feature of this order,
and one not possessed by other Fern-like plants, is that the sori
(2–many) are enveloped _in leaf-segments_ which _close round them_
and form a “sporocarp,” just in the same manner as the carpels, in
the Angiospermous Flowering-plants, close round the ovules and form
ovaries. The sori contain both micro-and macrosporangia. When the
spores are ripe, the sporocarp opens in order to disperse the spores
(Fig. 220).
[Illustration: FIG. 219.--_Marsilia salvatrix_ (natural size):
_K_ terminal bud; _b_ leaves; _f_ sporocarps; _x_ point of
branching of petiole.]
The two genera (with 57 species, Temperate, Tropics) are land-and
marsh-plants, whose dorsiventral, creeping stem bears roots on the
under surface, and the leaves in two rows on the upper side (Figs.
219, 221). The leaves of _Marsilia_ are compound, and divided into
four small leaflets springing from the apex of the petiole (Fig.
219), and resemble the leaves of _Oxalis_. In the bud the leaves are
circinate (Fig. 219 _b_), and at night they exhibit the well-known
sleep-movements. The sporocarps are borne on the petioles of the
fertile leaves, near their bases (Fig. 219 _f_); they are oblong and
resemble small beans, the outer cells being hard and sclerenchymatous,
while the inner ones are divided into a number of loculi arranged in
two rows. On germination, water is absorbed, the two sides separate
slightly, as valves (Fig. 220 _A_), and a long vermiform mass of
gelatinous, parenchymatous cells (Fig. 220), swollen by the water,
emerges, bearing a large number of sori arranged pinnately. Each
sorus (_sr_) is covered by a thin indusium. (The thin covering may be
considered an indusium physiologically, though not morphologically).
[Illustration: FIG. 220.--_Marsilia salvatrix_: _A_ the
sporocarp commencing to germinate; _B_ a more advanced stage of
germination.]
[Illustration: FIG. 221.--_Pilularia globulifera_ (natural size):
_s_ sporocarps; _b_ leaves; _k_ the growing point; _r_ roots.]
_Marsilia quadrifolia_, in Europe. Many species are found in
Australia. The nutritious sporocarps of _M. salvatrix_ were
the means of saving the Burke expedition in the interior of
Australia, and hence this species has earned its specific name.
_Pilularia_ has linear leaves, without lamina. The sporocarps are
spheroid (Fig. 221), brown and hard, and situated near the base of the
leaves. They are 2–4 chambered and open by a corresponding number of
valves.