28 



THE LIFE-HISTORY OF A FERN 



(Fig. 10) ; but conditions of crowded culture may lead towards a partial, 

 or even complete separation of the sexes. The flattened hermaphrodite 

 prothallus of the ordinary cordate outline, grown under normal circumstances 

 of moisture and moderate lighting, on a horizontal substratum, lies with 

 one of its flattened surfaces facing the substratum, and produces upon 

 that lower surface antheridia and archegonia, the former in the basal or lateral 

 regions, the latter upon the massive cushion : here they develop in acropetal 

 succession, the youngest being nearest to the emarginate apex of the 

 thallus. This position of the sexual organs is evidently favourable to 

 their continued exposure to moist air, or even fluid water: and indeed 

 the latter is necessary for the completion of their function. 



The antheridium, which arises by outgrowth and segmentation of a 

 single superficial cell, consists when mature of a peripheral wall of tabular 



K' 



K" 



Fit;. 12. 



Archegonia of Polyf>odinm vulgare. A, still closed : c = ovum. A" = canal-cell. A"" = ventral- 

 canal-cell. B, an archegonium ruptured. X240. (After Strasburger.) 



cells, surrounding a central group of spermatocytes (Fig. u. 4, 5). The 

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

 the 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, the cells of the wall 

 assisting by their swelling inwards, and consequent shortening (Fig. u. 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. n. 6 and 8). 



The archegonium also originates from a single superficial cell, and 

 grows out so as to project from the downward surface of the thallus. It 

 consists when mature of a peripheral wall of cells constituting the 

 projecting neck, and a central group, arranged serially : the deepest seated 

 of these is the large ovum, which is sunk in the tissue of the cushion, 

 and above this is a small ventral-canal-cell, and a longer canal-cell 



