i6 



THE LIFE-HISTORY OF A FERN 



[CH. 



archegonia upon the massive cushion. The latter develop in acropetal order, 

 the youngest being nearest to the incurved apex of the prothallus. The 

 position of the sexual organs is evidently favourable to their continued 

 exposure to moist air, or to fluid water which is necessary for carrying out 

 their function. 



The antheridium, which arises by outgrowth and segmentation of a single 

 superficial cell (Fig. 20, 2, 3), consists when mature of a peripheral wall of 

 tabular cells, surrounding a central group of spermatocytes (Fig. 20, 4, 5). 

 The antheridium readily matures in moist air, but it does not open except 





"ixi r-. 





Fig. 19. Mature prothallus oi Dryopteris Filix-inas, as seen from below, bearing antheridia 

 among its rhizoids, and archegonia near to the apical indentation. (After Kny.) 



in presence of external fluid water. This causes swelling of the mucilaginous 

 walls of the spermatocytes and increased turgor of the cells of the wall. 

 The tension is relieved by rupture of the cell covering the distal end, and 

 the spermatocytes are extruded into the water; in this the cells of the wall 

 assist by their swelling inwards, and consequent shortening (Fig. 20, 6). 

 The spermatocytes thus extruded into the water which caused the rupture, 

 soon show active movement, and the spermatozoid which had already been 

 formed within each of them escapes from its mucilaginous sheath, and moves 

 freely in the water by means of active cilia attached near one end of its 

 spirally coiled body (Fig. 20, 8). 



