The New International Encyclopædia/Fern

FERN (AS. fearn, OHG. farn, Ger. Farn; probably connected with Skt. parna, feather, leaf, and with Russ. paporoti, Ir. raith, fern), A plant of the order Filicales, one of the three great living groups of pteridophytes. The group contains about 4000 of the 4500 species belonging to the pteridophytes, and therefore is usually considered to be the representative group. Although known in considerable numbers in the temperate regions, its chief display is in the tropics, where ferns form a striking and characteristic feature of the vegetation. In habit ferns vary from those with delicate and filmy moss-like leaves to tree-like forms, rising to a height of 35 to 45 feet, and crowned by a rosette of leaves 15 to 20 feet long. The various species of ferns are prevailingly terrestrial plants, but some of them are aquatic, even floating; while there are numerous forms, especially in the tropics, which are epiphytic &mdash; that is, they grow upon other plants. The Filicales differ from the other groups of pteridophytes chiefly in having a few large leaves which do both foliage work and spore-bearing. The alternation of generations (q.v.) is very distinct, the sexual plant (gametophyte) being represented by the prothallium, and the sexless plant (sporophyte) by the leafy plant. The prothallium is like a small liverwort, with a dorsiventral body, and numerous rhizoids extending from its under surface. It is so thin that all of the cells contain chlorophyll, and it is usually short-lived. The antheridia (male organs) and archegonia (female organs) are usually developed on the under surface of the prothallium, and differ from those of the mosses in that they are sunken in the tissue of the prothallium and open on the surface, more or less of the neck of the archegonia projecting. The eggs are not different from those formed within the archegonia of mosses, but the sperms are very different. The fern sperm is a long spirally coiled body, blunt behind, and tapering to a long beak in front, where numerous cilia are developed.

The sexless, leafy plant consists in the main of a subterranean dorsiventral stem, which gives out secondary roots from beneath and sends up characteristic aerial leaves which have long been called ‘fronds.’ The leaves are recognized not merely by their ordinary habit of branching, but better by their venation, which is forking or dichotomous (q.v.), and by their vernation, which is coiled or circinate. The spore-vessels (sporangia) are borne for the most part on the under surface of the foliage leaves, usually closely associated with the veins, and organized into groups of definite form known as ‘sori.’ The sorus may be round or elongated, and is usually covered by a delicate flap known as the "indusium" (q.v.), which arises from the epidermis. Occasionally the sori are extended along the under surface of the margins of the leaf, as in the maiden-hair fern and common brake, in which case they are protected by the inrolled margin. While in most cases the leaves doing foliage work also produce sporangia, there are some forms in which the two kinds of work are separated, certain leaves doing only foliage work and others producing spores,

the latter being called sporophylls, as in the ostrich-fern (Struthiopteris), the climbing fern (Lygodium), the royal tern (Osmunda), etc. An ordinary fern sporangium (spore-vessel) consists of a slender stalk bearing a spore-case. This case has a delicate wall formed of a single layer of cells, and extending vertically almost around it from the stalk, like a meridian about a globe, is a row of peculiar cells with thick walls, forming the heavy ring called the ‘annulus.’ The annulus is like a bent spring, and when the delicate portion of the case-wall yields the spring straightens violently, the case-wall is torn, and in the rebound the spores are discharged with considerable force.

The true ferns are often divided into two great groups on the basis of the origin of their sporangia. In one case the sporangium is purely an external structure, being derived from a single epidermal cell, and such ferns are said to be ‘leptosporangiate.’ In other ferns the sporangium involves the deeper structures as well, and is really an internally developed organ; such ferns are ‘eusporangiate.’ The eusporangiate ferns are the more primitive forms, and probably were the only kind in the extensive fern display of the Carboniferous period. The leptosporangiate ferns are the modern and abundant forms. There are two great divisions of Filicales, namely the ‘true ferns’ (Filices) and the ‘water ferns’ (Hydropteridæ). Among the Filices six great families are ordinarily recognized, as follows: Osmundaceæ, containing the royal ferns; Gleicheniaceæ, which are tropical forms; Schizaeaceæ, which include the climbing ferns as well as various other peculiar genera: Hymenophyllaceæ, which contain the ferns with the most delicate bodies, often called the ‘filmy ferns’; Cyatheaceæ which include among other forms the tree-ferns; and finally Polypodiaceæ, the greatest and most highly organized family, to which almost all of the true ferns of the temperate region belong.

The water-ferns (Hydropteridæ) contain but few forms, and grow either in water or marshy places. They are of particular interest from the morphological standpoint, because they are heterosporous (q.v.). There are two distinct families. Marsiliaceæ, represented by the common Marsilia, contains semi-aquatic species with slender stems, which send down numerous roots into a mucky soil, and give rise to comparatively large leaves, each of which has a long erect petiole, and a blade of four wedge-shaped leaflets like a four-leaved clover. From near the base of the petiole another leaf-branch arises in which the blade is modified as a spore-bearing structure, which incloses the spore-case and becomes hard and nut-like. The other family is the Salviniaceæ, represented by Salvinia, whose species are floating forms.

. Leaves and stems of fossil ferns common in all geological formations above the Silurian, and they are especially abundant in the Coal Measures of the Carboniferous age. Indeed, ferns may be considered in have been the dominant forms of the Paleozoic land flora, not alone by reason of their abundance, but also because of their large size. Many of them were tree-ferns that grew in the vast forests along the swampy shores of later Paleozoic seas. A large number of the Paleozoic genera seem to have had their sporangia inclosed by the tissue of the fertile leaf, as seen in the modern adder's-tongue

fern (Ophioglossum), and its allies the Marattias. Because of this structure, these genera are included in a suborder, the Eusporangia.

Although fragmentary fern-leaves and some supposed stems are found in Lower Devonian rocks, well-preserved specimens are not found until the Upper Devonian (Catskill) sandstones are reached, where Archæopteris is obtained. In the Coal Measures, where ferns are abundant, the principal genera are Pecopteris, a tree-fern; Neuropteris, Archæopteris, Dictyopteris, Odontopteris, Alethopteris, etc., which are herbaceous ferns of large size; and Hymenophyllum and Sphenopteris, which are delicate ferns that must have formed the undergrowth of the forest vegetation. In the Permian and Triassic periods the most important genus was ../Glossopteris/ (q.v.); and an allied genus, Tæniopteris, is very common in the Jurassic rocks. During Cretaceous time the ferns began to resemble more closely those of to-day, and in Tertiary time a large number of modern species appeared.

Consult: Lyell, Geographical Handbook of All the Known Ferns (London, 1869); Baker, Summary of New Ferns Discovered Since 1874 (London, 1892); Hooker and Baker, Synopsis Filicum (London, 1874 ); Eaton, The Ferns of North America (Boston, 1880); Goebel, Outlines of Classification and Special Morphology of Plants, trans. by Garnsey (Oxford, 1887); Solms-Laubach, Fossil Botany (Oxford, 1891), contains a full bibliography of paleobotany, including works on ferns. Consult also: Zittel, Schimper, and Barrois, ''Traité de paléontologie, Part II. Paléophytologie'' (Paris, 1891); Bernard, Eléments de paléontologie (Paris, 1895); Gardner, “Monograph of the British Eocene Flora, Vol. I. Filices,” Monographs of the Palæontographical Society (London, 1879-82); Feistmantel and others, “Fossil Flora of the Gondwana System,” Palæontologica Indica, Series ''II. and XII.'' (Calcutta, 1876-81). The most important works describing the fossil ferns of North America are: Fontaine and White, “The Permian or Upper Carboniferous Flora of West Virginia and Pennsylvania,” Second Geological Survey of Pennsylvania, vol. PP (Harrisburg, 1880); Lesquereux, “Description of the Coal Flora of the Carboniferous Formation in Pennsylvania and Throughout the United States.” Second Geological Survey of Pennsylvania, Report of Progress P, vols. i., ii., and v. (Harrisburg, 1880-84); D. White, “Fossil Flora of the Lower Coal Measures of Missouri,” Monographs of the United States Geological Survey, vol. xxxvii. (Washington, 1899).