MOSSES — FERNS 



191 



and protect the young {^rowing capsule. It is finally torn loose and 

 carried up on the spore-case. The mouth of the capsule is closed by 

 a circular lid, the operculum, having a conical projection at the center. 

 The operculum soon drops, or it may be removed, displaying a fringe 

 of sixty-four teeth guarding the mouth of the capsule. 



This ring of teeth is known as the peristome. In most mosses 

 the teeth exhibit peculiar hygroscopic movements, i. e., when moist 

 they bend outwards and upon drying curve in toward the mouth of 

 the capsule. This motion, it will be seen, serves to disperse tne 

 spores gradually over a long period of time. 



Not the entire capsule is filled with spores. There are no elaters, 

 but the center of the capsule is occupied by a columnar strand of tis- 

 sue, the columella, which expands at the mouth into a thin, mem- 

 branous disk, closing the entire mouth of the capsule except the 

 narrow annular chink guarded by the teeth. In this 

 moss the points of the teeth are attached to the margin 

 of the membrane, allowing the spores to sift out through 

 the spaces between them. 



When the spores germinate they form a green, 

 branched thread, the jn-otonema. This gives rise directly 

 to moss plants, which appear as little buds on the thread- 

 When the moss plants have sent their little rhizoids into 

 the earth, the protonema dies, for it is no longer neces- 

 sary for the support of the little plants. 



FERNS 



The adder's tongue fern, Ophioglossum vulgatuni, 

 shown in Fig. 341, is one of a peculiar type of ferns be- 

 longing to the family Ophioglossaeete. This plant has a 

 short, subterranean stem from which a single frond un- 

 folds each year. The roots arise near the bases of the 

 leaves. The leaves are curiously divided into a sterile 

 and a fertile part, the latter being a sporophyll. The 

 sterile part has a tongue-shaped blade which is narrowed 

 to a petiole. The young leaves are inclosed by the Opiiio :lossum 

 sheathing base of the petiole. The growth is very vulgutum- 

 slow, so that it takes several years for each leaf to develop before it 

 is ready to unfold. During its development each leaf is sheathed by 

 the one preluding it. 



The sporopiiyll is elevated on a stalk arising near the base of the 

 sterile part of the frond. The upper part consists of a spike bearing 



