REPRODUCTION OF THE FERN 123 



cells closely applied to the ground to which it is anchored by the 

 rhizoids. This flat plate of cells or thallus is the sexual genera- 

 tion 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 

 rrthe^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 forming the male cells or anthero- 

 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 penetrates the gelatin- 

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



FIG. 50. Germination of the spore and formation of the prothallium. A, 

 Young plant leaving the spore case; B, similar stage after one cell division has 

 occurred (p, protenema; s, spore case; r, root). Later stages in formation of the 

 young prothallium are shown on the left, and below a fully developed prothallium 

 with archegonia and antheridia. In the notch above is a figure (life size) of the 

 same prothallium. (From Sedgwick and Wilson after Summski.) 



