FILICES, OR FERNS.
OF all the spore-bearing families, the Ferns are the most universally
known. They may easily be recognized by the coiling inwards of their
young leaves in spring previous to expansion, and by the arrangement of
the fruit on their undersides when expanded. The Ferns are exceedingly
numerous both in genera and species, and vary from low herbaceous plants
of an inch high, to trees with upright trunks forty or fifty feet or
more in height, bearing a graceful coronet of leaves at their extremity.
The tree ferns come to the utmost perfection in the warm, moist islands
of the tropical oceans. Their boreal limit is about the thirty-seventh
parallel of north latitude, but, on account of the vast extent of ocean
in the southern hemisphere, they reach the fortieth or fiftieth parallel
of south latitude, the more copious evaporation, and the consequently
moister air and soil, being specially congenial to the fern tribe. For
that reason a most luxuriant fern vegetation prevails in Juan Fernandez,
Western Chili, and New Zealand. In the latter there are one hundred and
twenty species, some of which are subarborescent, while others form tree
ferns of considerable altitude. Shade is not absolutely requisite to
ferns, for many of them grow luxuriantly when exposed to the sun,
provided the soil be damp.
The range of the non-arborescent ferns is very extensive. According to
Dr. J. D. Hooker, twenty-one species have been found in Fuegia and the
Falkland Islands, and one grows in matted tufts in Kerguelen’s Land. In
the north the Cytopteris fragilis has been found in the seventieth
parallel of latitude at Minto Inlet, and both it and Polystichum
Lonchitis have been gathered at Disco, on the west side of Greenland,
and Aspidium fragrans on the eastern side.
On account of the extremes of heat and cold in North America there are
only fifty species in all that enormous extent of country; while in
Britain we are indebted to the humidity of our climate for a rich
vegetation of thirty-six species, which adorn our woodlands, our valleys
and mountains in wild profusion. One-half of our ferns are also native
in the Himalaya mountains, where multitudes of British plants are
indigenous. The fern floras of Great Britain and New Zealand are the
richest in species of their respective latitudes, and have several
species in common. The ferns of Tasmania are, with few exceptions,
identical with those of New Zealand; and the occurrence of the rather
common Australian and New Zealand Gymnogramma rutæfolia in the Pyrenees,
and no where else in the whole world, so far as is known, is a
remarkable fact in the distribution of plants.[70]
Whatever the size of a fern may be, its spores (fig. 51 A) are
microscopic. They are produced within the sporangium by cell division,
and are therefore free and variously shaped. They consist of a grumous
mass enclosed in a double coat; and when the spore begins to grow, it
sends out from the cell wall of its inner coat a white tubular
projection or root fibre (fig. 51 B), which passes through the cell wall
of its outer coat. This root sucks up liquid till it expands the inner
coat sufficiently to burst open the outer one, and then it begins to
increase by the subdivision of its cells, till the primary green leaf or
marchantioid prothallus (fig. 51 D) is formed. This prothallus lies flat
on the ground, and is furnished on the under-side with fibrous roots to
fix it, and supply it with food.
[Illustration: Fig. 51. Development of spores of Pteris serrulata:—A,
spore; B, C, early stages of development; D, the prothallus with radical
fibres (_a_, _b_) and antheridia (_h_, _h_).]
Two sets of organs are subsequently developed on the under-side of this
prothallus; one of these is a stalked cell called an antheridium, and is
situated near the roots; the other is an archegonium containing a germ
cell, which is sunk in the cellular tissue. In each of the antheridia,
which are numerous, a cell is formed, which becomes filled with a mass
of mucilage mixed with a number of free cells, containing a flat
ribbon-shaped filament, or spermatozoid, coiled in a spiral manner,
which, as soon as set free by the rupture of the cell, revolves rapidly
by means of several long cilia placed close to the large end. Fig. 52
shows the globular antheridium and the spermatozoids of Pteris
serrulata. The archegonia are fewer in number, and contain the germ
cell, represented in fig. 53, as viewed from above, and sidewise; it is
placed at a little distance from the antheridia, and after being
fertilized by the active filaments or spermatozoids, and matured, it
contains the primordial cell of the young fern, which soon sends forth
leaves rolled up and curled inwards previous to expansion, and a root,
which being sufficient to feed the plant, the flat pro-embryo, which was
the first stage of development, perishes. Thus in the family of ferns
there are two distinct periods of growth, and one only of fertilization.
In flowering plants, fertilization and its products are the final result
of vegetation, and the maturation of the fructification is frequently
followed by death. In the ferns, on the contrary, fertilization precedes
the development of the plant, which, if perennial, continues to bear
fertilized fruit year after year.
[Illustration: Fig. 52. Antheridium and Spermatozoids of Pteris
serrulata:—A, projection of one of the cells of the prothallus showing
the antheridial cell (_b_); B, antheridium fully developed, containing
sperm-cells, each enclosing a spermatozoid; C, one of the spermatozoids
magnified, showing the cilia.]
[Illustration: Fig. 53. Archegonium of Pteris serrulata:—A, as seen from
above; B, side view, showing at A the cavity containing the germ cell;
at B, the walls of the archegonium made up of four layers of cells; and
at C, the spermatozoids within the cavity.]
It is evident that there is an essential difference between the
archegonium of the Marchantia, and that of the ferns. The archegonium of
the Marchantia merely produces a sporangium, while that of the ferns
produces a new plant, and is therefore, in some sense, analogous to the
seed of one of the higher vascular plants, though there is no affinity
between the two.
The roots of ferns, like those of forest trees, are delicate fibres,
which descend either from a woody stem called a rhizome, from _rhiza_, a
root, or from a caudex, so named from the Latin, _caudex_, a trunk. Thus
the stems of ferns are of two distinct types. The rhizome generally
grows horizontally, and creeps along the surface of the soil or rock, or
tree trunk to which it is affixed, but it is sometimes subterranean. In
this, the growing point is in advance of the fronds, which appear at
intervals along the exposed side or sides, and after they have reached
maturity drop off, leaving a clean scar or cicatrix. The caudex varies
from the size of a small wire-like thread to the size of a tree trunk.
It is sometimes elongated, as in the superfine wiry thread-like stems of
Hymenophyllum, the scandent ivy-like rooting stems of Stenochlæna, and
the subterranean, horizontal, widely-creeping stems of Pteris aquilina,
all of which develop single fronds at intervals, but fronds which adhere
permanently to the stem, and which do not fall off, leaving a scar, as
do the fronds borne by the rhizome. The more frequent form of caudex,
however, is that of a short stocky stem producing fronds on all sides
from its apex: very frequently this is a globose or oblong mass growing
erect, yet scarcely reaching above the surface of the soil; but
sometimes it is more lengthened, showing a more or less elevated stem,
or turning sideways, and taking a decumbent position, the young fronds,
however, always rising from its apex. It is on the latter plan, by a
continuous erect and upward growth, that the trunks of tree ferns, which
rise sometimes sixty or eighty feet high, are formed. The common Lady
Fern, Athyrium Filix-fœmina, furnishes an example of the ordinary
herbaceous caudex; as also does the Male Fern, Lastrea Filix-mas. In the
Hart’s-tongue, or Scolopendrium, the caudex is generally compact, and
increases by the formation of new crowns or centres of growth around the
older one, till the whole becomes an almost spherical mass of
considerable size.
The stipes or leaf-stalk of a fern is often of considerable length, and
in its upper part, called the rachis, which bears the leafy portion, is
commonly more or less branched, as well as furrowed on its upper
surface. The fronds, or parts analogous to leaves, are sometimes simple,
as in the Hart’s-tongue, the rachis of which has a symmetrical limb or
wing on each side, so as to form a long, even-margined, narrow,
tongue-shaped frond. If the limb on each side of the rachis be deeply
divided, as in the Polypodium vulgare, the frond is said to be
wing-cleft, or pinnatifid, and the divisions are called segments or
lobes. In most ferns, however, the frond is once, twice, or three times
winged or pinnate, and in such cases the first divisions are called
pinnæ, and the secondary or subsequent ones when present, pinnulæ or
pinnules. The leafy portion, whether simple, pinnatifid, or pinnate, is
always traversed by veins arranged on some definite plan, a most
important circumstance, since the sori or fructification of the ferns is
always produced in connection with the veins.
The fertile fronds, in certain groups, differ in form from the sterile,
generally by the greater or less contraction of their parts. In most
ferns the full-grown fronds are flat, that is, with all their parts
lying in one plane; but, during their vernation, that is, when they
first rise from the stem, they are circinate or curled inwards, like a
crosier.
When a fern acquires a considerable stem, as in Tree Ferns, it consists
of a central or medullary part, consisting of cellular tissue, and an
external or cortical portion, formed of the consolidated bases of the
fronds, in which may be seen, on cutting a section of a trunk, an
irregular zone, formed of fibro-vascular bundles, scalariform ducts, and
woody fibre. Prolongations from this zone pass into the leaf-stalks, and
thence into the midrib of the leaf, whence they spread into its lateral
branches, and ultimately appear in the leafy parts in the form of veins.
It is the arrangement of these bundles of coloured woody tissue in the
cellular tissue of the leaf-stalks of the herbaceous ferns, which give
rise to those peculiar figures on their transverse sections, such as a
star, the letter T, and the heraldic spread eagle, from which latter the
common Brake Fern (Pteris) takes its botanical specific name of
aquilina. Fig. 54 shows an oblique section of the footstalk of a fern
leaf with its bundles of scalariform ducts, as determined by Dr.
Carpenter’s microscopic observations.
[Illustration: Fig. 54. Section of footstalk of Fern frond showing
scalariform ducts.]
The fructification of the ferns is arranged with the most perfect
symmetry, usually on the under surface of the leaf, but sometimes at the
margin, and assuming a great variety of forms and positions. It consists
of sori, that is, groups of nearly globular spore cases or sporangia,
sometimes sessile on the frond, sometimes with a footstalk. They are
always situated on a vein or its branches, or at the extremity of the
veins on the margin of the frond. In fact the sporangia originate in the
cellular tissue immediately in contact with a vein, beneath the
epidermis or skin of the leaf, which is forced up as the sporangia
increase in size, in the form of a whitish membrane, which constitutes
the indusium, or protecting cover of the sori. While the fruit is
advancing to maturity, the indusium separates partly or wholly from the
surrounding skin or epiderm, and subsequently either shrivels or falls
off altogether. In some few species the opening is in the centre of the
indusium, and then it surrounds the sori like a cup; in other ferns, the
skin from both surfaces of the leaf extends beyond the margin, includes
the sori between it, and fulfils the office of an indusium.
[Illustration: Fig. 55. Pinnule of Polypodium bearing sori.]
The sori, as already noted, take a variety of forms, and are variously
situated. Some are round and dot-like, some are oblong and straight,
some are hippocrepiform or horse-shoe-shaped, while some are continuous
in a line-like band. The indusium, when present, takes more or less
exactly the form of the sorus. These peculiarities are so well marked
that they are taken advantage of in the discrimination of genera.
The sporangia or spore-cases are for the most part of globular form, and
are nearly or quite surrounded by a strong elastic ring, which in some
cases is continued so as to form a stalk. When the spores are ripe, this
ring, by its elastic force, tears open the sporangia, and gives free
egress to their contents. The ring assumes various forms. In one large
group it passes vertically up the back of the sporangium, and is
continued to a point called the stoma, where the horizontal bursting
takes place. This form is seen in fig. 56, _a_, _b_. In other groups it
is, though vertical, somewhat oblique, as in fig. 56 _c_. Sometimes,
though more rarely, it is transverse and complete; in which case the
rupture is vertical, as in fig. 56 _d_. In a few cases it is apical,
fig. 56 _e_; and in a few others it is obsolete, fig. 56 _f_. These are
the true Ferns. In one or two small groups, sometimes called
Pseudo-Ferns, the ring is altogether wanting.
[Illustration: Fig. 56. Sporangia of Polypodiaceous Ferns:—_a_, _b_,
Polypodiaceæ; _c_, Cyatheineæ; _d_, Gleichenineæ; _e_, Schizæineæ; _f_,
Osmundineæ.]
The systematic arrangement of the Ferns is chiefly founded on
peculiarities of the sori and sporangia, characters which are quite
intelligible by the aid of a good magnifying lens, these spore-cases
being very pretty opaque objects under the microscope. Thus some of the
primary divisions are founded on the presence or absence of the ring or
annulus on the spore-case. Another series of divisions are founded
mainly on the nature of the ring in those cases when it is present; and,
for the rest, the form and position of the sori come in as
discriminating characters. In this way the main groups are marked out,
but, in the case of the genera, still further recourse is had to the
sori and its covering, and by some modern authors supplementary
characters derived from the venation are brought into use. The following
is the arrangement adopted by Mr. Moore,[71] and which agrees in its
general features with that of most modern observers.
The two leading groups are the _Annulate_ and the _Exannulate_ Ferns,
the first being much the larger division, and consisting of the order
Polypodiaceæ, which comprises the True Ferns, while the second includes
the two orders Marattiaceæ and Ophioglossaceæ, which are some times
called Pseudo-Ferns.
The POLYPODIACEÆ, distinguished by the ring or annulus, which more or
less completely girts the sporangia, offer so much variety of
structure that it becomes necessary to subdivide them; and for this
purpose characters derived from the form, number, or position of the
sporangia, or the structure or development of the ring, are chiefly
relied on. This gives several groups, _e.g._ Polypodineæ, the most
comprehensive of all, including some ten or twelve minor groups, in
which the sporangia are almost equally convex, and have a vertical and
nearly complete ring, and in which the dehiscence is transverse at a
part called the stoma, where the striæ of the ring are elongated, and
apparently weaker; Cyatheineæ, in which the spore-cases are sessile or
nearly so, seated on an elevated receptacle, with the nearly complete
ring more or less obliquely vertical, that is, vertical below, curving
laterally towards the top, and the dehiscence transverse; Matonineæ, a
single species only, in which the sporangia are sessile, bursting
horizontally, not vertically, the ring being broad, suboblique, and
nearly complete, and the dorsal sori oligocarpous, covered by
umbonato-hemispherical indusia, affixed by a central stalk;
Gleichenineæ, with the ring complete, transverse, either truly or
obliquely horizontal, the sporangia globose-pyriform, forming
oligocarpous sori, _i.e._ sori consisting of but few spore-cases (two
or four to ten or twelve), situated at the back of the frond, sessile
or nearly so, and bursting vertically, the fronds, moreover, being
rigid and opaque, and usually dichotomously-branched; Trichomanineæ,
with the ring resembling that of the Gleichenineæ, but the sporangia
lenticular, numerous, clustered on an exserted receptacle, which is a
prolongation of the vein beyond the ordinary margin of the frond, so
that the sori become extrorse marginal, or projected outwards as well
as opening outwardly, while the fronds are pellucid-membranaceous;
Schizæineæ, with the ring horizontal or transverse, situated quite at
the apex of the oval sporangia, which is, in consequence, said to be
radiate-striate at the apex; Ceratopteridineæ, one or two aquatic
species, the sporangia sometimes furnished with a very rudimentary
ring, reduced, as in Osmundineæ, to a few parallel striæ, sometimes
furnished with a very broad and more lengthened ring; and Osmundineæ,
with the spore-cases two-valved, bursting vertically at the apex, the
ring very rudimentary, reduced to a few parallel vertical striæ on one
side near the apex. In all but the last of these groups, the sporangia
are not valvate, and consequently, when they open for the liberation
of the spores, they burst partially or irregularly; but in the
Osmundineæ they split at the top in two equal divisions.[72]
A large portion of the Polypodineæ are either tropical or subtropical.
The genus Polypodium itself is one of the most extensive and diversified
genera of the group. It is chiefly distributed over the tropical regions
of the western hemisphere, but four species are indigenous in Britain,
and of these the Polypodium vulgare, or Common Polypody, is abundant
about the trunks of moss-grown trees, on banks, rocks, and old thatched
roofs. The young fronds appear in May, and rise from five or six to
twelve or eighteen inches in height. They are lanceolate, deeply
pinnatifid, with obtuse, linear, lanceolate, indistinctly serrated
segments. The genus is distinguished by its naked globular sori, which
in this species are regularly disposed in a line on each side of the mid
vein, half way between it and the margin of the leaf. When young, they
are of a yellow or bright orange colour, which changes to brown when
they are ripe. The rhizome, which branches in all directions, is at
first clothed with a skin densely covered with yellowish brown
membranaceous lanceolate scales, which at length fall or become
obliterated, leaving the surface nearly smooth. The Polypodium vulgare
has many varieties, several of which are well marked, especially that
called cambricum, which is twice pinnatifid. The plant is common in
temperate climates.
[Illustration: Fig. 57. Pinnule of Lastrea Filix-mas with sori.]
The Aspidieæ form an extensive and widely distributed group, embracing
several of our common British species. One of these is the genus
Lastrea, which has, for the most part, lanceolate fronds, bipinnate or
tripinnate in division, with linear lanceolate, and usually pinnatifid
pinnæ. The sori are nearly circular, seated upon the back of the veins,
and covered by a reniform indusium, which is attached by its sinus. In
Lastrea Filix-mas (fig. 57), one of our commonest ferns, the fronds
spring up in a rather spreading mass, from the extremity of a long scaly
caudex, and often present a vase-like tuft hollow in the centre. The
rachis, leafy through a third or a fourth of its length, is more or less
clothed with thin membranaceous scales, of a pale often brownish-golden
hue, a peculiarity common to the other Lastreæ. Lastrea Thelypteris, the
Marsh Fern, however, has them not, and differs also from most of its
congeners in having its sori submarginal. Its rhizome has a widely
creeping habit of growth. The Lastrea æmula has globular glands, sessile
over the whole under-surface of the fronds, which secrete a perfume like
new-made hay. From a similar cause, Lastrea rigida was botanically known
as Polypodium fragrans. This coumarine odour is possessed by several
other ferns, notably by Cheilanthes odora.
The Oleandra, which is a foreign genus, has simple fronds articulated
near to the rhizome, and its sori are placed on the tips of the parallel
veinlets. The Oleandra neriiformis, which grows on open spots, has an
erect rhizome, and assumes quite a shrubby habit, rising to from four to
six feet, and bearing at intervals whorls of fronds.
[Illustration: Fig. 58. Sorus and Indusium of Polystichum or Aspidium.]
In the genus Polystichum, the fronds spring in tufts from a short
slow-growing caudex. They are rigid, linear or lanceolate, and either
pinnate, bipinnate, or tripinnate. The upper terminations of the fronds
are sharp and spinous, while the pinnæ are auriculate at their base
above, and oblique below. Circular sori are seated on the anterior
branches of the parallel veins, and these are covered by circular
indusia, opening all round and remaining attached by a short central
stalk. The free venation and this peculiar form of the sorus and
indusium are the distinguishing characters of the genus. Fig. 58 shows a
sorus and indusium of Polystichum or Aspidium, which differ only in
venation. One of the spinous, serrated pinnæ of the Holly fern,
Polystichum Lonchitis, is represented in fig. 59; it is auriculate at
its base above and oblique below, with its sori disposed in regular
series on each side the mid-vein; these often become confluent at
maturity. This beautiful Alpine fern, which is of a deep glossy green,
has linear lanceolate, and simply pinnate fronds. The pinnæ are short,
arranged alternately and obliquely on the rachis, and extend nearly to
its base, which is rather densely clothed with reddish-brown chaffy
scales. The other British ferns of this genus are bipinnate. P.
aculeatum is rigid, but P. angulare is lax, and drooping. One of its
varieties, named proliferum, is abundantly viviparous, producing small
bulbils about the bases of the lower pinnæ and pinnules, which readily
reproduce the plant.
[Illustration: Fig. 59. Pinna of Polystichum Lonchitis.]
All the preceding genera, from Lastrea inclusive, belong to the
Aspidieæ, an enormous tribe, abounding in species, and almost
conterminous with the old genus Aspidium.
The Nephrolepis tuberosa and other species bear sub-translucent tubers
on the rhizome. They are subterranean, ovoid, an inch and a half long,
and filled with a nearly translucent mucus. The tubers have a circle of
vascular bundles forming a sort of balloon, proceeding from a common
base below, and converging to the apex.[73]
Most of the species are tropical, but Lastrea and Polystichum include
several European species, some of which are extremely variable.
The Onoclea is a remarkable genus from the pinnæ being contracted into
berry-like globes. Onoclea sensibilis is a handsome free-growing
American species and appears to have been named sensibilis, from the
particularly rapid withering of the fronds after being gathered.
The genus Cystopteris, which has species in both hemispheres, is the
type of the Cystopterideæ, a small group which approaches Aspidieæ
through Nephrolepis and Acrophorus. The British species of Cystopteris
are small, fragile ferns, growing on walls and rocks in Alpine and
Subalpine districts. Their fronds are for the most part erect,
lanceolate or deltoid, and bipinnate or tripinnate. They are generically
distinguished by having each round sorus covered by a hood-shaped
indusium, which is hollow and attached by its base, and opens towards
the apex of the segment, its free margin being elongated and fringed; in
this respect it bears some resemblance to Woodsia, in which, however,
the indusium projects from beneath the spore-cases equally on all sides,
becoming incurved in a cup-like form. The Cystopteris fragilis, or
Brittle Bladder fern, flourishes in most shady mountainous and rocky
districts, but is also found in the lowlands. It has a decumbent caudex,
which extends slowly, branching and forming new crowns around the old
one, often to the number of several during the summer and autumn. The
fronds rise in tufts from these crowns in April, rapidly attain their
maturity, and die away in succession as their place is supplied by
others, till the frost comes and destroys them all. The tufts vary in
height from two or three inches to a foot or more. The fronds differ
much in form and division even in the same crown, but both they and the
pinnæ are usually lanceolate; the pinnules are ovoid or oblong, always
deeply pinnatifid, and having the segments sharply toothed or serrated.
The Dicksonia group, Dicksonieæ, contains some of the finest tree ferns.
The Dicksonia antarctica sometimes attains a uniform girth of twelve
feet throughout its height of forty feet. In the stem of this fern, the
vascular bundles are symmetrically disposed round the axis so as to form
a closed cylinder. The New Zealanders slice the fibrous coating of the
trunk, and use it for constructing their houses. D. squarrosa reaches
the farthest south of the tree ferns. The D. lanata sometimes forms a
distinct stem but not always, for tree ferns vary much with regard to
the dimensions and elevation of their stem. The caudex of Cibotium
Barometz is covered with long tawny hair; each has its own peculiarity.
The species of Dicksonieæ belong principally to the tropics and southern
isles. The ferns of this group have globose sori, which are submarginal,
seated at the tip of a vein or veinlet. The indusium is lateral,
persistent, and bivalved; the lower valve is formed by the true
indusium, the upper by the altered tooth of the frond folded back. In
Dennstædtia the indusium is cup-shaped, and curiously deflexed; while in
Deparia, another section of the group, the cup-shaped indusium
containing the sorus is extrorse marginal, as shown in fig. 60.
[Illustration: Fig. 60. Sorus and cup-shaped indusium of Deparia
prolifera.]
The group Peranemeæ, or Woodsieæ, is represented in the British Flora by
two species of Woodsia, which are amongst the rarest and most curious of
our ferns, and inhabit the crevices in our highest mountain tops. Their
tufts of fronds are not more than two or three inches high, sometimes
less, and the fronds themselves are lanceolate, and pinnate; in Woodsia
ilvensis they have oblong and usually opposite deeply lobed pinnæ, whose
under-surface is clothed more or less with jointed hairs and long
attenuated scales. The sori in both species are circular, and situated
at the extremity of the lateral veins. When young they are covered with
an indusium, which opens at the centre, and forms a cup round the sori;
afterwards it is divided into numerous jointed and usually incurved
threads.
The Davallieæ are mostly tropical and subtropical ferns. Their sori are
submarginal, placed upon the tip of a vein or veinlet, and enclosed
within a tubular indusium, which is either short and approaching to
cup-shaped, or more or less elongated. In this group the stem assumes
the form of a rhizome, often creeping extensively, and the fronds are
articulated, falling off with a clean scar when they perish. Davallia
canariensis is cultivated in most conservatories, and is known as the
Hare’s-foot fern, from the stoutish scaly rhizomes resembling the foot
of a hare.
Somewhat allied to these are the Lindsæeæ, including Lindsæa and its
allies; these have linear marginal sori, for the most part continuous,
but sometimes regularly interrupted, in which state they approach some
of the Davallieæ, Microlepia to wit. The indusium opens towards the
margin of the frond, and the sori more or less connect the tips of the
veins and veinlets.
[Illustration: Fig. 61. Scolopendrium vulgare.]
The group of Asplenieæ is a very comprehensive one, including, besides
the extensive typical genus Asplenium, the cognate subdivisions of
Diplazieæ and Scolopendrieæ. The latter is represented by Scolopendrium
vulgare, the Hart’s-tongue fern, in which the sori are linear, and
situated in pairs on two parallel veins, so closely approximate that,
though really double, they seem to form but one straight line; they are
not covered with one indusium splitting down the middle, but each has
its own indusium opening in opposite directions. On the under-surface of
the long strap-shaped entire fronds of the common Hart’s-tongue, which
is a type of the genus, the double sori with their opposite indusia form
parallel equidistant straight lines, diverging on each side from the
mid-vein of the frond, as in fig. 61. The fronds are cordate at the
base; sometimes they are forked, and occasionally crisped and wavy, and
the leaf-stem is shaggy with narrow membranaceous scales. The caudex is
very compact and deep-rooted; it does not much elongate, but increases
slowly by the formation of new crowns round the older, attaining
considerable bulk. From this the fronds rise in circular tufts to the
height of a foot or more, and attain from two to three feet, in
favourable shady localities. At first the tufts are straight, but
ultimately they radiate and bend outwards. This plant is exceedingly
variable; hundreds of varieties have been found or raised in Great
Britain. Though twin sori are characteristic of the Scolopendrieæ, they
diverge somewhat in Camptosorus, a North American genus, sometimes
called the Walking fern, from its habit of throwing forward a bud on a
thread-like prolongation of the point of the frond, which becomes
established as a new growing centre, and thus carries the plant onwards.
The group of ferns called Diplazieæ, another section of the Asplenieæ,
typified by the genus Diplazium, is altogether tropical. The sori are
bilateral or double, but placed back to back on the vein, exactly the
opposite of Scolopendrieæ; so that the indusia, instead of opening face
to face, open in opposite directions. The species are rather numerous,
and embrace much variety of form and character.
To the Asplenieæ proper belongs one of our commonest species, the
Athyrium Filix-fœmina, or Lady Fern, which, when growing in moist shady
places, is one of the most elegant of British ferns. Its bipinnate frond
varies from a broad almost ovate outline to a linear lanceolate form.
The numerous fronds often spring from the caudex in a vase-like
arrangement, to the height of four or five feet, those in the centre
nearly erect, but the outer ones, drooping around in all directions,
forming a tuft of lovely feathery foliage. The sori have commonly a
semilunar outline, as in fig. 62; but they are occasionally so much
curved as to acquire a horse-shoe form, and more rarely they are linear,
and they become at length reflected by the growth of the sporangia. In
some varieties they are distinct, in others so close as to become
eventually confluent and completely cover the surface. The Filix-fœmina
is distinguished by the linear junction of the indusium with the frond.
It is one of the most variable of all the known ferns, and is remarkably
prolific, so that new forms may be raised from the spores. There are a
very large number found in this country.
[Illustration: Fig. 62. Athyrium Filix-fœmina.]
All the Asplenieæ have linear sori attached to the side of the vein, so
that they are said to be lateral. The indusium opens on its inward side,
and is sometimes ciliated. There are many tropical ferns of this group,
but the genus Asplenium, which has representatives in all parts of the
world, yields eight or nine species in Great Britain.
[Illustration: Fig. 63. Asplenium Ruta-muraria.]
The Asplenium Ruta-muraria, or Wall Rue, is an indigenous mountain plant
growing in tufts six or eight inches high in the clefts of the rocks,
but has diminished much in size in migrating to the plains, where it is
found on old brick walls and outhouses. The fronds are deltoid and
bipinnate, the pinnules are wedge-shaped and notched, or toothed, on
their upper margin; their colour is deep green, but in exposed
situations they are always covered with a glaucous secretion. The veins
diverge from the footstalk at the base of the pinnules, branch out
above, and extend to the teeth or serratures, as in fig. 63. The sori,
which are produced on the inner side of the veins, are linear,
elongated, and eventually become confluent, covering the whole
under-surface of each fertile pinnule. The indusium, which is only
traceable in the earlier condition of the fructification, is white, and
the free inner margin is somewhat jagged.
Changes as remarkable take place in the sori of the Asplenium
lanceolatum. At first, when the sori are covered by their white
indusium, they are oblong, but they become circular as they enlarge, and
eventually often confluent, so as to form a line round the whole
under-margin of the sharply toothed pinnule. This fern, which is
indigenous in the Atlantic islands, and is found in the Channel Islands,
as well as in the maritime counties of England and Wales, grows in the
crevices of rocks and old walls, or clothes the sides of wells and
deserted mines. It belongs to the section which has a mid-vein.
The genus Ceterach apparently belongs to that section of ferns which
have a vertical ring, and no indusium; but its affinities ally it
closely with Asplenium. Its want of an indusium is supplied, perhaps
occasioned, by the abundance of chaffy scales which cover the back of
the fronds. The lateral veins are alternate and irregularly branched,
with the branches anastomosing towards the margin. The sori are oblong
or linear, attached to the upper side of the anterior branches except
the last, which is on the opposite side of the lower branch. The British
species, Ceterach officinarum, or Scaly Spleen-wort (fig. 64), has
deeply pinnatifid, lanceolate fronds, with oblong, obtuse, alternate
segments, and linear sori. The whole of the under-surface is densely
clothed with brown, pointed, imbricated scales, finely serrated at their
margins, the outermost of which extend beyond the edges of the segments.
They completely cover the sori, and at first hide them entirely from
view. On the unexpanded fronds, the scales are white and silvery. ‘This
plant has an obscure indusium, only perceptible at an early stage of
fructification, and subsequently as a nearly erect membrane, attached to
the back of the vein. It is rendered unnecessary by the arrangement of
the scales, which are disposed in regular series along each side of the
veins and veinlets, pointing outwards, and concealing the sori with
their broad bases, which completely overlap them in their immature
condition.’
[Illustration: Fig. 64. Ceterach officinarum.]
It is well known that the seeds of tropical plants have been brought to
the western coasts of Scotland and Ireland by the great Atlantic
currents; it might therefore be expected, that the spores of Cryptogams
brought to our shores by the same means, and finding a congenial
climate, should become naturalized, as appears to have been the case in
ancient times, with regard to the Ceterach and Asplenium marinum, from
their migrations, as traced by Mr. Johnson. ‘The countries bordering on
the basin of the Mediterranean and the islands and eastern shores of the
North Atlantic appear to have been the original stations of this
remarkable fern. In the British islands its distribution is too partial
to admit of its being regarded as strictly indigenous, though probably
naturalized here at a period little subsequent to the arrival of
Asplenium marinum. It occurs here on limestone-rocks, but more
frequently on old walls and ruins, rooted deeply in the decaying mortar,
and often accompanying Asplenium Ruta-muraria and Asplenium Trichomanes.
Like other natural importations from the south, it is found most
abundantly on the western maritime counties that receive the more direct
flow of the tide, and has progressed slowly towards the northern and
central parts of the kingdom. In Scotland it has not yet traversed
beyond Perth, and is still regarded as a rare species; while in Ireland
its copious distribution seems to indicate an earlier arrival.’
The group Vittarieæ, consisting mainly of Vittaria and Tæniopsis, is
essentially equatorial or sub-equatorial. The plants have narrow
ribbon-like fronds, with naked sori immersed in the very margin of the
frond, in a more or less sunken furrow, there being no indusium.
The Blechnum Spicant, which may be taken as a type of the Lomarieæ, is a
very ornamental fern, with its dark green, linear-lanceolate fronds, of
two forms; the fertile ones erect, pectinate, pinnate, with distinct
narrow linear and acute pinnæ, while the barren ones are smooth,
spreading, and pinnatifid, with broad linear, blunt, approximate lobes.
The rachis is generally smooth, and of a dark purple hue, its leafless
portion being shaggy with membranaceous scales. The barren fronds lie on
the ground in the winter, while the fertile ones are erect, bear fruit
from May to October, and wither when they have shed their spores. In the
fertile fronds the lateral veins are alternate, and extend obliquely
upwards, about half way towards the margin of the lobe, when, by a
sudden turn, each runs parallel to the mid-vein, and anastomoses with
the one above it, thus forming an apparently longitudinal vein on which
the sorus is placed, so as to form a line on each side of the mid-vein;
this is covered with a continuous indusium like a hem, which opens on
the interior side (fig. 65). In the barren fronds the veins do not
anastomose at the margins of the lobe. This species is common in almost
every part of Great Britain, and extends on the Continent from Swedish
Lapland to the borders of the Mediterranean.
The species of Lomaria are marked by having the fertile fronds
contracted, so that the sorus is quite marginal; but there are many
species of Blechnum in which the elongated sorus, placed parallel to the
midrib, is quite distant from the margin, much more so than in Blechnum
Spicant.
[Illustration: Fig. 65. Blechnum Spicant.]
[Illustration: Fig. 66. Pteris aquilina.]
The genus Pteris is the type of another group, the Pterideæ, in which
the sori form a continuous marginal line covered by the attenuated edge
of the frond folded over it, and forming an indusium. This is the
structure in most of the species of Pteris, a large family abounding in
the tropics, and very widely distributed in almost every part of the
globe. In the Pteris aquilina, or Bracken (fig. 66), however, in which
the lateral veins of the leaf are divided two or three times before they
reach the margin, and the extremities of the branches anastomose and
form a vein at the exterior or extreme margin of the leaf, the sporangia
are produced on the upper-surface of the marginal vein, and are enclosed
by an extension of the skin from both surfaces of the leaf; so that the
fructification, which is folded back on the under-surface of the leaf,
is in the earliest stage of its development enclosed between two thin
membranes, both of which have their margins ciliated with jointed hairs,
while, under the microscope, their cellular structure will be found to
differ in accordance with that of the upper and under epidermis, from
which they are individually extensions. This fern is so universal in
Britain, as to exclude in some places almost every other kind of
vegetation, and is so well known, that it may appear superfluous to
mention that the fronds are deltoid, with an elongated, stem-like
petiole two or three times divided, the primary pinnæ opposite, the
ultimate segments oblong, obtuse, and confluent. There are exotic forms
of Pteris in every part of the world, which are mere varieties of the
Pteris aquilina, so that the Bracken may be said to rank as the most
universally distributed of all vegetable productions, extending from
West to East, over both continents and islands, in a zone reaching from
Northern Europe and Siberia to New Zealand, where it is represented by,
and perhaps identical with, the well known Pteris esculenta.
The Allosorus crispus, Parsley fern, or Curled Rock Bracken, which may
be taken as a type of its genus, is by some authors referred to the same
group of Pterideæ, though others regard it as one of the Polypodieæ. It
is a pretty little fern, growing in tufts in sheltered crevices of
mountain rocks, from Lapland to the Mediterranean. The fronds are
deltoid, twice or three times pinnate, and of two kinds. The ultimate
divisions of the barren fronds are wedge-shaped, cut, and toothed; those
of the fertile fronds are linear, oblong, and entire. Although the sori
are at first circular, and situated near the extremities of the lateral
veins, they become confluent in maturity, and being covered by the
reflexed margin of the pinnules of the contracted but scarcely altered
fertile frond, instead of by an indusium, they bear some resemblance in
fructification to Pteris aquilina.
The Adiantieæ form an exceedingly numerous and mostly tropical group of
ferns, in which the rhizome is creeping or globose. Their sori are
linear or oblong, straight or curved, and growing on the under-side of
the edges of the leaf, which are folded over with them, and thus serve
as an indusium. Adiantum has a free venation, but Hewardia, a related
genus, has a reticulated venation. The Adiantieæ have linear, oblong or
lunate sori, seated on the margin of the leaf. Of the seventy species of
Adiantum, only A. pedatum and A. Capillus-Veneris, the first American,
the latter British, are inhabitants of a cold climate. The A.
Capillus-Veneris, which is universally distributed in warm latitudes, is
believed to have migrated in ancient times to the mild and damp
south-western counties of England and Ireland.
The Adiantum Capillus-Veneris, or Maiden’s Hair, has a slender, black,
scaly, creeping, and branching rhizome, from the extremities of which
spring lax tufts of fronds growing from a few inches to a foot high. The
stems with their alternate branches and branchlets are slender,
hair-like, and of a blackish purple tint. The capillary branchlets bear
at their extremities thin, bright, but glaucous green, wedge-shaped
leaves, serrated at their edges. The fibro-vascular bundles which
traverse the stems and branches, on reaching the leaves, spread into a
palmate venation, which terminates at the margin in bifurcations; and
upon these, in the fertile leaves, roundish sori are placed, and covered
by the transverse oblong folds of the edge of the leaf, as in fig. 67.
These delicately graceful ferns flourish almost exclusively in damp
shady crevices of rocks under the spray of cascades, and still more
luxuriantly in deep tropical forests, where the air is loaded with warm
vapour.
[Illustration: Fig. 67. Adiantum Capillus-Veneris.]
The Acrosticheæ are among the most remarkable of ferns in having the
whole under-surface of the frond indiscriminately covered with naked
sori, and in a few species the upper-surface also. They are situated on
the veins and veinlets, from whence they extend into the interstices of
the fertile fronds, which are sometimes much contracted. The species are
almost all tropical or subtropical. A New Zealand species of Lomariopsis
climbs high trees by means of its stout rooting caudex, and has
different leaves on different parts of the plant.
The Cyatheineæ are chiefly arborescent ferns, and, with the exception of
some fossil species, contain the noblest representatives of the
cryptogamic flora. Their fructification consists of dot-like sori, in
which the usually compressed oblique sporangia are placed on an elevated
receptacle, which forms a raised point on the surface of the fronds when
the sporangia are removed. Of these the Alsophileæ are without true
indusia, while the Cyatheæ have a spherical indusium, bursting above or
below, and forming a cup round the sorus. The pulpy substance of the
stem of Cyathea medullaris was a common article of food with the New
Zealanders.
The Gleichenineæ have a creeping or climbing rhizome, and globose or
trigonal sporangia, few in number, and disposed in a radiating manner,
so that the narrow end is internal. They are often seated in a little
cavity, and are highly deciduous. The fronds are generally forked or
trifid, the middle division being sometimes supplied with a little
bulb-like body. They are mostly tropical or subtropical plants, but some
species grow in Chili, New Zealand, and Japan.
The Trichomanineæ are pretty well distinguished from other ferns by
their pellucid membranaceous texture. As regards their technical
characters, they have an oblique and complete ring to the sporangia,
which are of a lenticular form, and are collected about a more or less
elongated receptacle, which is free within either an urn-shaped cup or a
two-valved indusium, projecting from the edge of the frond. They are
chiefly inhabitants of the moist tropical forests, or extra-tropical
regions of damp and mild temperature. Great Britain only owns one
species of the genus Trichomanes, and two of the genus Hymenophyllum,
while species of these two genera constitute one-fifth of the fern
vegetation of our antipode islands of New Zealand. It is more than
doubtful even that the Trichomanes radicans, or Bristle fern, is
indigenous, as it is found in no other part of Great Britain except the
Irish counties of Cork and Kerry, growing on dripping rocks and
waterfalls, or depending from the walls and roofs of caverns; and as it
is also found in the West Indies and the North Atlantic islands its
transit may be accounted for, as in other cases.
[Illustration: Fig. 68. Trichomanes radicans.]
The fronds of the Bristle fern, Trichomanes radicans, are somewhat
deltoid, or of a more elongated form, flimsy, and beautifully
reticulated when viewed with a microscope. They rise at intervals from a
creeping rhizome, bristly, with narrow articulate scales, which often
covers the most precipitous rocks on which it grows with a dark-coloured
network. The rachis is branched and rebranched three or four times, and
the whole being distinctly winged on both sides in the plane of the
frond, the successive branches and branchlets running through the leafy
part become the veins of those parts, so that a segment of the frond is
merely a winged vein, the wings on the branches being however broader
than those on the rachis. The veins, which divide alternately, are hard,
woody, and wire-like, and, when barren, terminate before reaching the
ends of the segments; but, when fertile, they extend beyond the segment,
the tissue of which separates and distends in the form of a more or less
elongated cup around the prolonged vein. At the base of that vein the
sporangia form a small globular cluster; and, as they advance towards
maturity, the vein extends in the form of a bristle, far beyond the
mouth of the cup (fig. 68). The cup is winged in the more luxuriant form
of the plant, in consequence of the double layer of tissue composing the
segment to which it belongs not separating through its whole breadth.
The cup-like cylinder being slightly compressed in the plane of the
frond is indicative of its origin.
[Illustration: Fig. 69. Hymenophyllum tunbridgense.]
A slight difference in fructification separates the Hymenophyllum
tunbridgense, or Film fern (fig. 69), from the Trichomanes, for both
have a creeping rhizome, from which single fronds spring at short
intervals. The leafy parts of the fronds are merely winged veins, and
the wings consist of two layers of cellular tissue finely reticulated.
The Hymenophyllum is distinguished from the Trichomanes by a two-valved
cup spinously serrated, which completely conceals the fruit-bearing
vein, while, in the Trichomanes, the cup is more cylindrical,
smooth-edged, with the fertile vein projecting far beyond it (fig. 68).
The beautiful little Film fern, with its semi-transparent fronds not
more than two or three inches high, grows on shady wet rocks, among the
moss on the branches and roots of old trees, and on the ground near
lakes and rivulets. It is hardy enough to live in the Highlands of
Scotland; but the H. unilaterale, a far less beautiful plant, has its
limit in Unst, the most northern of the Shetland Islands. The
fructification in the latter plant, though the same in position, is
stalked instead of being sessile. The involucre is rounded and ovate,
instead of being cup-shaped, with much swollen convex valves, meeting by
their edges but not compressed towards the apex.
The Schizæineæ are many of them climbing ferns, and for the most part
tropical. Lygodium articulatum climbs trees in New Zealand to the height
of fifty or one hundred feet, and its tough wiry stems are used for
cordage. In one group of this order, represented by Lygodium, the
sporangia are disposed at the back of the frond in imbricated marginal
spikes, formed by a transformation of the ultimate pinnæ. In the other
division, represented by Schizæa, the fruitful spikes are really on the
under-side of the frond, but the frond being reversed they seem to be on
the upper.
The Ceratopteridineæ, sometimes called Parkeriaceæ, are tropical aquatic
ferns, whose sterile fronds, which are membranaceous, with a thick
vascular footstalk, float on the surface of the water, and by the time
these are nearly decayed the fertile fronds are perfected. The latter
are more erect, repeatedly divided and forked, the divisions being
linear. There is only a single genus, Ceratopteris, which has continuous
sori occupying the longitudinal veins at the edge of the frond, and
covered by the indusioid margin. The sporangia have a very broad
incomplete ring, and connect the ringless ferns with the group which
possess them. The spores are triangular, and marked with three sets of
concentric ridges.
The order Osmundineæ contains the Osmunda regalis, considered to be the
finest of all European ferns, which is common throughout Great Britain
in wet spongy soils, and appears to be the only species of the group
that is European. The fronds grow in tufts from a thick woody caudex,
which branches and extends widely by the formation of lateral crowns,
but, if impeded, it elongates and rises in an erect position to the
height of two or even three feet above the soil. The barren fronds of
the large luxuriant tufts are highly developed, and are from six to nine
or even eleven feet high; the fertile fronds are shorter and fewer in
number. The bipinnate character prevails throughout; the primary
divisions are opposite, the secondary alternate, and the pinnules are
oblong and opposite. The sculptured sporangia have a very rudimentary
ring, and, in the fruit-bearing fronds, four or five of the lower pairs
of pinnules have the leafy character, while the remainder develop
clusters of sporangia in place of pinnules. The sporangia on the
fruitful branches are at first pale green, but gradually become reddish
brown, hence the name of Flowering fern. The Leptopteris section of the
genus Todea, almost peculiar to New Zealand, has beautiful transparent
fronds, with naked sporangia placed upon the veins, forming very much
scattered sori.
The MARATTIACEÆ may be partly known by their huge globose rhizome
projecting above the ground, and rough with the processes from which the
leaf-stalks have fallen. This sends out a few large fibrous roots, and
consists of cellular tissue abounding in starch, with small bundles of
fibro-vascular tissue regularly distributed through it. The stipes have
a pair of stipule-like organs at their base; and the sori are either
oblong fronds of a double row of sporangia, which in some cases is
concrete, or they are circular with the sporangia annularly concrete, or
they are connate throughout the fertile portions. These peculiarities
respectively distinguish the smaller groups of Marattineæ, Kaulfussineæ,
and Danæineæ, while the group Marattiaceæ itself is distinguished from
Ophioglossaceæ by having its sori dorsal, that is, set on the back, or
under-surface, of flat leafy fronds. The leaves of Angiopteris evecta
are used as a perfume in the Sandwich Islands, and its rhizome serves
for food, as that of the Marattia salicina does in New Zealand.
The OPHIOGLOSSACEÆ, or Adder’s Tongue ferns, are few in number, and
present comparatively little difference in structure. They are known
among the exannulate series, from the Marattiaceæ, by having their
fertile fronds contracted, bearing their sporangia at the margin, so
that being entirely occupied by sporangia, the fertile fronds appear as
if they were an inflorescence distinct from the foliaceous organs, as
indeed they are analogically. In this group there is the further
difference that the fronds are not circinate, but straight, in their
æstivation.
These Adder’s Tongue ferns are dwarf herbaceous plants, differing
greatly in structure from the true ferns, though there is some
similarity to them in the fructification. They are divided into four
genera, of which Ophioglossum and Botrychium are best known. The base of
the stem is thick and bulbiform, and sends off spreading succulent
roots. The species of the genus Ophioglossum are chiefly plants of a
warm climate, but Ophioglossum vulgatum, or common Adder’s Tongue, is
distributed in almost every part of the globe. In England it is
exceedingly abundant in meadows and pastures, and varies in height from
a few inches to a foot, in moist soil. It has one barren ovate and one
fertile linear frond. The barren frond, which is of a yellowish-green
colour, invests the stem of the fertile frond like a spathe; its form is
ovate, varying to ovate lanceolate, and more or less obtuse, with a
complicated network of anastomosing veins. Bivalved sporangia form two
parallel series on the margins of the club-like terminations of the
fertile frond. Soon after the fruit is shed the fronds die, but one or
two buds are previously formed at the base of the plant, which remain
dormant till the following spring.[74]
The genus Botrychium has few species, about half of which grow in North
America. Botrychium Lunaria, or Moonwort, is the only species indigenous
in Great Britain. This plant, which is from five to six inches high, has
a rather long succulent stem, invested at the base by the dark-coloured
membranous remains of withered leaves. It has one barren frond, the base
of whose stem is a sheath, through which the tall, fertile frond rises.
The barren frond has from three or four to seven pairs of opposite
smooth pinnæ, of a lunate shape and glaucous green colour, overlapping
each other; they are smooth, crenated on the margin, and occasionally
lobed, which gives them a fan shape. The fertile frond is longer than
the barren one, and ends in a panicle or thyrse. The sporangia, which
are large and bivalved, are disposed in two regular series upon the
divisions of the panicle, and directed towards the upper or inner face
of the frond. This fructification bears a strong resemblance to that of
Osmunda regalis, the Flowering fern.
The stem of this plant has been assumed to be solid and branched, which
is by no means the case; for, upon dissection, it is found to be hollow,
and at its base the fronds of the following year may be detected more or
less perfectly formed, and the rudimentary bud of the succeeding year
within the latter; the position of the barren and fertile fronds being
reversed in the successive developments.