FERN ( AS. fr a ru, . Tarn ; probably connected with Skt. panto, feather, leaf, and with Russ. pa porot i, h, fern). A plant of the order Filieales, one of the three great living groups of pteridophytes. The group con tains about 4000 of the species belonging to the pteridophytes. and therefore is usually consid ered to be the representative grotty. 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 prevail ingly terrestrial plants, but some of them are aquatic, even floating; while there are numerons forms, especially in the tropics, which are ern phytic—that is, they grow upon other plants. The Filicales differ from the other groups of pterido phytes chiefly in having a few large leaves which do both foliage work and spore-hearing. The al ternation of generations (q.v.) is very distinct, the sexual plant (gametophyte) being represented by the prothallium, and the sexless plant (spore phyte) by the leafy plant. The prothallium is like a small liverwort, with a do•siventral body, and numerolls 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 arehegonia (fe male 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 tis sue of the prothallium and open on the surface, more or less of the neck of the arehegonia pro jecting. 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 taper ing to a long beak in front, where numerous cilia are developed.
The sexless, leafy plant consists in the main of a subterranean do•siventral stem. which gives out secondary roots from beneath and sends up char acteristic 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 di chotomous (q.v.), and by their vernation, which is coiled or The spore-vessels (spo rangia) are borne for the most part on the under surface of the foliage leaves, usually closely asso ciated with the veins, and organized into groups of definite form known as •so•i.' 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 sari 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 (St•uthioptcris), the climbing fern (Lygodium), the royal fern (Osmunda), etc. An ordinary fern sporangium (spore-vessel) consists of a slender stalk bearing a spore-ease. 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 por tion of the ease-wall yields the spring straightens violently, the case-wall is torn, and in the re bound the spores are discharged with consider able force.
The true ferns are often divided into two great groups on the basis of the origin of their spo rangia. 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' (Filiees) and the 'water ferns' (Hy dropteridte). Among the Filices six great families are ordinarily recognized, as follows: Osmun dacete, containing the royal ferns: Gleicheniacetr, which are tropical forms; Schizfeacefe, which in clude the climbing ferns as well as various other peculiar genera ; Hymenophyllacecc, which con tain the ferns with the most delicate bodies, often called the 'filmy ferns'; Cyatheacea., which in clude among other forms the tree-ferns; and finally Polypodiacete, the greatest and most high ly organized family, to which almost all of the true ferns of the temperate region belong.