126 PLANTS, THE FOOD OF ANIMALS 



divide, forming root-like hairs (rhizoids) and the embryonic 

 plant termed the protonema (Fig. 50). The end cells of the 

 protonema develop chlorophyll, divide, and ultimately form a 

 flattened plate of cells closely applied to the ground, to which it 

 is anchored by the rhizoids. This flat plate of cells, or thallus, 

 is the sexual generation of the fern and is called the prothallium 

 (Fig. 50). It is entirely unlike the fern plant, but when mature 

 it bears the sex cells which, after fertilization, develop into the 

 fern. It thus resembles the medusa of a hydroid, an organism 

 quite different from the hydranth from which it came, but the 

 sole agent in the formation of the male and female germ cells 

 which, on fertilization, give rise to the hydroid. 



The sex cells of the fern are formed in characteristic organs 

 on the under side of the prothallium. The oospheres or egg 

 cells are developed and contained in peculiar chimney-shaped 

 structures termed archegonia (Fig. 50); while the male cells 

 are formed in smaller rounded or hemispherical structures 

 termed antheridia (Fig. 50). The two types of structure are 

 each formed by continued division of an epidermal cell. In 

 the archegonium these divisions result in a solid column of 

 cells, with the oosphere embedded at the base of the column. 

 The central cells of the column undergo liquefaction, thus form- 

 ing a passage filled with a mucilaginous liquid from the apex 

 of the archegonium to the egg cell. The antheridia are formed 

 by divisions of similar epidermal cells which develop into a solid 

 hemispherical mound, the internal cells of which divide re- 

 peatedly; the final divisions form the male cells or anther o- 

 zoids, each, when mature, bearing a spiral filament covered 

 with cilia. 



The antherozoids usually develop first, and are distributed on 

 the ground where they make their way in the moisture on the 

 under side of the prothallium to the archegonia of the same or of 

 different origin. They are attracted toward the chimney-like 

 opening of the archegonia; one or more penetrate the gelatin- 

 ous passage to the egg cell, and one antherozoid unites with it. 

 The entire process of fertilization takes place within the tissues 

 of the prothallium. 



