3i8 



MOSSES AND FERNS 



CHAP. 



but after the antheridium is burst open, the two lower ones 

 become so distended that they nearly fill the central cavity. The 

 opening is effected either by a central rupture of the cover cell, 

 or less commonly by a separation of this from the upper ring 

 cell. 



The development of the archegonium is intimately connected 

 with the apical growth of the large female prothallium. As 

 soon as the single apical cell has been replaced by the marginal 

 initials, the divisions in the latter become very definite. Com- 

 parison of cross and longitudinal sections shows that these are 



much like those of Marattia or, 

 among the Hepaticas, Dendroceros 

 or Pellia epiphylla. Each initial cell 

 has the form of a semi-disc (Fig. 

 175, A), and the growth is both 

 from lateral segments, which mainly 

 go to form the wings of the pro- 

 thallium, and basal, or inner seg- 

 ments, which produce the projecting 

 archegonial cushion. If this begins 

 S to form very early, it may develop a 

 midrib extending nearly the whole 

 length of the prothallium ; but usually 

 it does not form until relatively late. 

 Each basal segment of the initial cells 

 divides into a dorsal and ventral cell 

 (semi-segment), the latter the larger 

 of the two, and with much more 

 active growth. The latter alone is 

 concerned in the growth of the pro- 

 jecting cushion. Each ventral semi- 

 segment is first divided by a wall parallel with the primary 

 segment wall, and from the anterior of these cells, almost 

 exactly as in Notothylas, the archegonium is developed. It is 

 not possible to make out any definite succession of walls by 

 which the axial cell of the archegonium is cut out, but it soon 

 is recognisable by the granular cytoplasm and large nucleus. 

 As in Marattia, the first transverse wall separates the inner cell 

 from the cap cell, and the inner one then divides into the basal 

 and the central cells. The cover cell divides into the four 

 primary neck cells, and the central cell arching up between these 



Fig. 176. — Ripe archegonium of 

 O. struthiopteris in the act 

 of opening, X300; 0, the 

 egg. 



