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 In a few cases it is apical, fig. 56 _e_; and in a few others it is obsolete, fig. 56 _f_.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.]