2o8 MOSSES AND FERNS chap. 



but in Phascuvi it is much less conspicuous. Pleuridiuvi (Fig. 

 103, A) in its later stages corresponds exactly to Phascum, 

 except that the capsule is more slender. In both of these 

 genera the seta remains short, but is perfectly evident. 

 Whether the absence of a distinct operculum in the cleisto- 

 carpous Mosses is a primitive condition, or whether they are 

 reduced forms, it is impossible to determine positively from a 

 study of their embryogeny. 



BryinecE StegocarpcB 



Very much the larger number of Mosses belong to this 

 group, which is primarily distinguished from the foregoing by 

 the presence of an operculum. Of course among the 7000 or 

 more species belonging here there are many differences in 

 structure ; but these are mainly of minor importance morpho- 

 logically, and only the more important differences can be con- 

 sidered here. 



As we have already seen, there is great uniformity in the 

 growth of the stem, which, with the single exception oi Fissidens, 

 has always a three-sided pyramidal apical cell. In Fissidens 

 this is replaced by a two-sided one, but even here it has been 

 found ^ that the underground stems have a three-sided initial 

 cell, which is gradually replaced by the two-sided one after the 

 apex of the shoot appears above ground. In Fissidens the 

 leaves are arranged in two rows corresponding to the two sets 

 of segments, and are sharply folded, so that the margins of the 

 leaf are covered over by those of the next older ones, leaving 

 only the apex free. A similar arrangement is found in the 

 genus BryozipJiion {Eustichid), but here there is a three-sided 

 apical cell, and the two-ranked arrangement of the leaves is 

 secondary. The curious genus Schistostega shows also a two- 

 ranked arrangement of the leaves of the sterile branches, but 

 here they are placed vertically and the bases connivent, so that 

 the effect of the whole is that of a pinnatifid leaf The fertile 

 branches, however, have the leaves spirally arranged, and in the 

 sterile ones the three-sided apical cell is found. The leaves, 

 with few exceptions, e.g. Leucobryum, Fontinalis, have a well- 

 marked midrib, and the lamina is single-layered. Leucobryum 

 (Fig. 107, A) has leaves destitute of a midrib, and made up of 



1 Goebel (8), p. 371. 



