SECTION VIII.FILICES, OR FERNS.

Ofall 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 FalklandIslands, and one grows in matted tufts in Kerguelenâ€™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 rutÃ¦folia 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. 51A) 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. 51B), 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 primarygreen leaf or marchantioid prothallus (fig. 51D) 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.

Fig. 51. Development of spores of Pteris serrulata:â€”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. 52shows the globular antheridium and the spermatozoids of Pteris serrulata. The archegonia are fewer in number, and contain the germ cell, represented infig. 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.

Fig. 52. Antheridium and Spermatozoids of Pteris serrulata:â€”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.

Fig. 53. Archegonium of Pteris serrulata:â€”A, as seen from above;B, side view, showing atAthe cavity containing the germ cell; atB, the walls of the archegonium made up of four layers of cells; and atC, 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, fromrhiza, 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 StenochlÃ¦na, 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 thetrunks of tree ferns, which rise sometimes sixty or eighty feet high, are formed. The common Lady Fern, Athyrium Filix-fÅ“mina, furnishes an example of the ordinary herbaceous caudex; as also does the Male Fern, Lastrea Filix-mas. In the Hartâ€™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 Hartâ€™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 pinnÃ¦, and the secondary or subsequent ones when present, pinnulÃ¦ 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. 54shows an oblique section of the footstalk of a fern leaf with its bundles of scalariform ducts, as determined by Dr. Carpenterâ€™s microscopic observations.

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.

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 calledthe stoma, where the horizontal bursting takes place. This form is seen infig. 56,a,b. In other groups it is, though vertical, somewhat oblique, as infig. 56c. Sometimes, though more rarely, it is transverse and complete; in which case the rupture is vertical, as infig. 56d. In a few cases it is apical,fig. 56e; and in a few others it is obsolete,fig. 56f. These are the true Ferns. In one or two small groups, sometimes called Pseudo-Ferns, the ring is altogether wanting.

Fig. 56. Sporangia of Polypodiaceous Ferns:â€”a,b, PolypodiaceÃ¦;c, CyatheineÃ¦;d, GleichenineÃ¦;e, SchizÃ¦ineÃ¦;f, OsmundineÃ¦.

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]andwhich agrees in its general features with that of most modern observers.

The two leading groups are theAnnulateand theExannulateFerns, the first being much the larger division, and consisting of the order PolypodiaceÃ¦, which comprises the True Ferns, while the second includes the two orders MarattiaceÃ¦ and OphioglossaceÃ¦, which are some times called Pseudo-Ferns.

ThePolypodiaceÃ¦, 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.PolypodineÃ¦, 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 striÃ¦ of the ring are elongated, and apparently weaker; CyatheineÃ¦, 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; MatonineÃ¦, 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; GleichenineÃ¦, 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; TrichomanineÃ¦, with the ring resembling that of the GleichenineÃ¦, 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; SchizÃ¦ineÃ¦, 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; CeratopteridineÃ¦, one or two aquatic species, the sporangia sometimes furnished with a very rudimentary ring, reduced, as in OsmundineÃ¦, to a few parallel striÃ¦, sometimes furnished with a very broad and more lengthened ring; and OsmundineÃ¦, with the spore-cases two-valved, bursting vertically at the apex, the ring very rudimentary, reduced to a few parallel vertical striÃ¦ 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 OsmundineÃ¦ they split at the top in two equal divisions.[72]

A large portion of the PolypodineÃ¦ 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 isdistinguished 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.

Fig. 57. Pinnule of Lastrea Filix-mas with sori.

The AspidieÃ¦ 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 pinnÃ¦. 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 LastreÃ¦. 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 Ã¦mula has globular glands, sessile over the whole under-surface of the fronds, whichsecrete 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.

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 pinnÃ¦ 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. 58shows a sorus and indusium of Polystichum or Aspidium, which differ only in venation. One of the spinous, serrated pinnÃ¦ of the Holly fern, Polystichum Lonchitis, is represented infig. 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. ThepinnÃ¦ 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 pinnÃ¦ and pinnules, which readily reproduce the plant.

Fig. 59. Pinna of Polystichum Lonchitis.

All the preceding genera, from Lastrea inclusive, belong to the AspidieÃ¦, 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 pinnÃ¦ 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 CystopterideÃ¦, a small group which approaches AspidieÃ¦ through Nephrolepis andAcrophorus. 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 pinnÃ¦ are usually lanceolate; the pinnules are ovoid or oblong, always deeply pinnatifid, and having the segments sharply toothed or serrated.

The Dicksonia group, DicksonieÃ¦, 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 treeferns 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 DicksonieÃ¦ 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 DennstÃ¦dtia 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 infig. 60.

Fig. 60. Sorus and cup-shaped indusium of Deparia prolifera.

The group PeranemeÃ¦, or WoodsieÃ¦, 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 pinnÃ¦, 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 DavallieÃ¦ are mostly tropical and subtropical ferns. Their sori are submarginal, placed upon the tipof 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 arearticulated, falling off with a clean scar when they perish. Davallia canariensis is cultivated in most conservatories, and is known as the Hareâ€™s-foot fern, from the stoutish scaly rhizomes resembling the foot of a hare.

Somewhat allied to these are the LindsÃ¦eÃ¦, including LindsÃ¦a 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 DavallieÃ¦, 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.

Fig. 61. Scolopendrium vulgare.

The group of AsplenieÃ¦ is a very comprehensive one, including, besides the extensive typical genus Asplenium, the cognate subdivisions of DiplazieÃ¦ and ScolopendrieÃ¦. The latter is represented by Scolopendrium vulgare, the Hartâ€™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 Hartâ€™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 infig. 61. The fronds are cordate at the base; sometimes theyare 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 ScolopendrieÃ¦, 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 DiplazieÃ¦, another section of the AsplenieÃ¦, 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 ScolopendrieÃ¦; 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 AsplenieÃ¦ proper belongs one of our commonest species, the Athyrium Filix-fÅ“mina, 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 havecommonly a semilunar outline, as infig. 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-fÅ“mina 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.

Fig. 62. Athyrium Filix-fÅ“mina.

All the AsplenieÃ¦ 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.

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 inexposed 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 infig. 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 attheir 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. â€˜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.â€™

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. â€˜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.â€™

The group VittarieÃ¦, consisting mainly of Vittaria and TÃ¦niopsis, 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 LomarieÃ¦, 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 pinnÃ¦, 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 everypart 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.

Fig. 65. Blechnum Spicant.

Fig. 66. Pteris aquilina.

The genus Pteris is the type of another group, the PterideÃ¦, 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 theupper 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 pinnÃ¦ 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 PterideÃ¦, though others regard it as one of the PolypodieÃ¦. 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 AdiantieÃ¦ 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 theedges 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 AdiantieÃ¦ 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 Maidenâ€™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 infig. 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.

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