314 



MOSSES AND FERNS 



CHAP. 



gonial cushion. If this begins to form very early, it may form 

 a midrib extending nearly the whole length of the prothallium ; 

 but usually it does not form until relatively late. Each basal 

 seement 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 projecting 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 

 has the pointed apex cut off by a 

 curved wall from the central cell. 

 The primary neck canal cell, so 

 formed, is noticeably smaller than 

 that of Marattia. The neck cells, 

 which in the eusporangiate forms all 

 grow alike, here show a difference, 

 and the two anterior rows develop 

 faster than the posterior ones, so that these rows are longer and 

 the neck is strongly bent backward. In Onoclea there are 

 usually about seven cells in each anterior row and about two less 

 in the posterior ones. The neck cells are almost colourless, 

 with distinct nuclei, and a few small, pale chloroplasts. From 

 the central cell is now cut off the ventral canal cell, which is 

 quite small, and separated from the egg by a strongly concave 

 wall. The nucleus of the neck canal cell always divides, but 

 no division wall is formed, and the two nuclei lie free in the 

 cell. The basal cell divides by cross-walls into four, and with 



Fig. 159. — Ripe archegonium of O. 

 stnithiopteris in the act of open- 

 ing, X 300 ; o, the egg. 



similar cells cut off from the adjacent prothallial tissue forms 



