. Principles of modern biology. Biology. Reproduction in Multicellular Plants - 205 ....... Fig. 12-3. Magnified vertical section of a fern leaf, passing through a sorus. Three of the clustered spo- rangia show quite clearly; and the ripening spores can be seen through the side walls of each sporan- gium. (From The Plant World, by Fuller and Carothers. Holt, Rinehart and Winston, Inc.) The prothallia grow in moist shaded places on the ground, or on decaying logs; and these plants are not recognized as "ferns," except by those who have traced out their origin (Fig. 12-5). The prothall
. Principles of modern biology. Biology. Reproduction in Multicellular Plants - 205 ....... Fig. 12-3. Magnified vertical section of a fern leaf, passing through a sorus. Three of the clustered spo- rangia show quite clearly; and the ripening spores can be seen through the side walls of each sporan- gium. (From The Plant World, by Fuller and Carothers. Holt, Rinehart and Winston, Inc.) The prothallia grow in moist shaded places on the ground, or on decaying logs; and these plants are not recognized as "ferns," except by those who have traced out their origin (Fig. 12-5). The prothallium of a fern usually meas- ures less than half an inch at the widest part. Nevertheless the prothallium grows inde- pendently like other green plants. All the cells of the prothallial body possess chloro- plasts; but the numerous rhizoids, which grow down into the soil from the underside of the prothallium, are colorless. During growth, the rhizoids absorb water and min- eral salts for the whole prothallium; and sev- eral weeks after a spore germinates, the pro- thallium is sexually mature. When mature, the gametophyte of the fern produces gametes. Both male and female or- gans develop on the undersurface of each prothallium. Usually the egg-forming organs, called archegonia, lie near the indented mar- gin (apical notch) of the prothallium (Fig. 12-4), and each archegonium contains a sin- gle egg. The egg lies in a hollow chamber, the venter, which communicates with the en- vironment through a short tubular channel, the neck. The sperm-forming organs, called antheridia, are usually situated among the rhizoids, nearer the other end and margins of the prothallium (Fig. 12-4). Each anther- idium is a hollow structure from which a number of delicate flagellated sperm are fin- ally liberated (Fig. 12-4). The sperm are usually liberated in rainy weather, and they must swim through water, underneath the prothallium, in order to reach the archegonia. However, the arche- gonia and anth
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