Prominent among the spore-propagated plants are _ferns_. The common _Christmas fern_ (so called because it remains green during winter) is shown in Fig. 254. The plant has no trunk. The leaves spring directly from the ground. The leaves of ferns are called =fronds=. They vary in shape, as other leaves do. Some of the fronds in Fig. 254 are seen to be narrower at the top. If these are examined more closely (Fig. 255), it will be seen that the leaflets are contracted and are densely covered beneath with brown bodies. These bodies are collections of =sporangia= or =spore-cases=. [Illustration: FIG. 256.—COMMON POLYPODE FERN. _Polypodium vulgare_.] [Illustration: FIG. 257.—SORI AND SPORANGIUM OF POLYPODE. A chain of cells lies along the top of the sporangium, which springs back elastically on drying, thus disseminating the spores. ] [Illustration: FIG. 258.—THE BRAKE FRUITS UNDERNEATH THE REVOLUTE EDGES OF THE LEAF.] [Illustration: FIG. 259.—FRUITING PINNULES OF MAIDENHAIR FERN.] The sporangia are collected into little groups, known as =sori= (singular, sorus) or =fruit-dots=. Each sorus is covered with a thin scale or shield, known as an =indusium=. This indusium separates from the frond at its edges, and the sporangia are exposed. Not all ferns have indusia. The polypode (Figs. 256, 257) does not; the sori are naked. In the brake (Fig. 258) and maidenhair (Fig. 259) the edge of the frond turns over and forms an indusium. The nephrolepis or sword fern of greenhouses is allied to the polypode. The sori are in a single row on either side the midrib (Fig. 260). The indusium is circular or kidney-shaped and open at one edge or finally all around. The Boston fern, Washington fern, Pierson fern, and others, are horticultural forms of the common sword fern. In some ferns (Fig. 261) an entire frond becomes contracted to cover the sporangia. [Illustration: FIG. 260.—PART OF FROND OF SWORD FERN. To the pupil: Is this illustration right side up? ] The sporangium or spore-case of a fern is a more or less globular body and usually with a stalk (Fig. 257). _It contains the spores. When ripe it bursts and the spores are set free._ [Illustration: FIG. 261.—FERTILE AND STERILE FRONDS OF THE SENSITIVE FERN.] In a moist, warm place _the spores germinate_. They produce a small, flat, thin, green, more or less heart-shaped membrane (Fig. 262). This is the =prothallus=. Sometimes the prothallus is an inch or more across, but oftener it is less than a ten cent piece in size. Although easily seen, it is commonly unknown except to botanists. Prothalli may often be found in greenhouses where ferns are grown. Look on the moist stone or brick walls, or on the firm soil of undisturbed pots and beds; or spores may be sown in a damp, warm place. [Illustration: FIG. 262.—PROTHALLUS OF A FERN. Enlarged: Archegonia at _a_; antheridia at _b_.]