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which case the rupture is vertical, as in [fig. 56] d. In a few cases it is apical, [fig. 56] e; and in a few others it is obsolete, [fig. 56] f. These are the true Ferns.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.
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.
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.