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 
Fic. 12. 
Archegonia of Polypodium vulgare. A,still closed: o=ovum, K *=canal-cell. X" =ventral- 
canal-cell. S, an archegonium ruptured. X2q40. (After Strasburger.) 
cells, surrounding a central group of spermatocytes (Fig. 11. 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. 11. 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. 11. 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 thee 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 
