The spores may be collected by placing the spore-bearing leaves on sheets of paper and letting them dry, when the ripe spores will be discharged covering the paper as a fine, brown powder. If these are sown on fine, rather closely packed earth, and kept moist and covered with glass so as to prevent evaporation, within a week or two a fine, green, moss-like growth will make its appearance, and by the end of five or six weeks, if the weather is warm, little, flat, heart-shaped plants of a dark-green color may be seen. These look like small liverworts, and are the sexual plants (prothallia) of our ferns (Fig. 66, _F_). Removing one of these carefully, we find on the lower side numerous fine hairs like those on the lower surface of the liverworts, which fasten it firmly to the ground. By and by, if our culture has been successful, we may find attached to some of the larger of these, little fern plants growing from the under side of the prothallia, and attached to the ground by a delicate root. As the little plant becomes larger the prothallium dies, leaving it attached to the ground as an independent plant, which after a time bears the spores. [Illustration: FIG. 66.--_A_, spore of the ostrich fern (_Onoclea_), with the outer coat removed. _B_, germinating spore, × 150. _C_, young prothallium, × 50. _r_, root hair. _sp._ spore membrane. _D_, _E_, older prothallia. _a_, apical cell, × 150. _F_, a female prothallium, seen from below, × 12. _ar._ archegonia. _G_, _H_, young archegonia, in optical section, × 150. _o_, central cell. _b_, ventral canal cell. _c_, upper canal cell. _I_, a ripe archegonium in the act of opening, × 150. _o_, egg cell. _J_, a male prothallium, × 50. _an._ antheridia. _K_, _L_, young antheridia, in optical section, × 300. _M_, ripe antheridium, × 300. _sp._ sperm cells. _N_, _O_, antheridia that have partially discharged their contents, × 300. _P_, spermatozoids, killed with iodine, × 500. _v_, vesicle attached to the hinder end.] In choosing spores for germination it is best to select those of large size and containing abundant chlorophyll, as they germinate more readily. Especially favorable for this purpose are the spores of the ostrich fern (_Onoclea struthiopteris_) (Fig. 70, _I_, _J_), or the sensitive fern (_O. sensibilis_). Another common and readily grown species is the lady fern (_Asplenium filixfœmina_) (Fig. 70, _H_). The spores of most ferns retain their vitality for many months, and hence can be kept dry until wanted.