Page:First course in biology (IA firstcourseinbio00bailrich).pdf/210

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.

Fig. 260.—Part of Frond of Sword Fern. To the pupil: Is this illustration right side up?

Fig. 261.—Fertile and Sterile Fronds of the Sensitive Fern.

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.

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 dime 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.

Fig. 262.—Prothallus of a Fern. Enlarged.

Archegonia at a; antheridia at b.

On the under side of the prothallus two kinds of organs are borne. These are the archegonium (containing egg-cells) and the antheridium (con-