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.
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.
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.
