186 



MOSSES AND FERNS 



CHAP. 



geb's, except as to the relation of the columella and outer spore- 

 sac. The first divisions in the embryo correspond exactly to 

 those in Andre&a and the Bryales, and for a time the young 

 embryo grows from a two-sided apical cell. The secondary 

 divisions in the segments, however, are quite different from that 

 observed in any other Moss, and are like those in the anther- 

 idium. Instead of the first wall dividing the segment into 

 equal parts, it divides it very unequally. The second wall 

 strikes this so as to enclose a central cell, triangular in cross- 



A. 



FIG. 96. Archidium Ravenelii. A, Median section through a nearly ripe sporogonium, 

 XQO; B, base of the sporogonium, X270. 



section, which with the corresponding cell of the adjacent seg- 

 ment forms a square. This square, the endothecium, does not 

 therefore at first show the characteristic four-celled stage found 

 in all other Mosses. The amphithecium becomes ultimately 

 three-layered, and between the second arid third layers an inter- 

 cellular space is formed, as in the Bryales, but this extends com- 

 pletely over the top of the columella. The most remarkable 

 feature, however, is that no archesporium is differentiated, but 

 any cell of the endothecium may apparently become a spore 



