2i8 MOSSES AND FERNS CHAP. 



smaller ventral ones, so that the plant resembles very closely a 

 foliose Liverwort. 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., Fontinalis, have a well- 

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

 (Fig. 121, A) has leaves made up of two or three layers of 

 cells, large hyaline ones, somewhat as in Sphagnum, and small 

 green cells. The hyaline cells, as in Sphagnum, have round 

 holes in the walls, but no thickenings. The midrib may be 

 narrow, as in Funarla, or it may occupy nearly the whole 

 breadth of the leaf, as in the Polytrichacese, where, owing to 

 the almost complete suppression of the lamina, secondary ver- 

 tical plates of green cells are formed (Fig. 121, B). 



The one-third divergence of the leaves found in Fontinalis 1 

 is replaced in most other genera by a larger divergence. 

 (Goebel (8) ) . Thus in Funaria hy.gr ome trie a it is I ; in Poly- 

 trichum commune^] in P. formosum^}. 



As the archegonia are borne upon lateral branches, or upon 

 the main axis, the stegocarpous Bryinese are frequently divided 

 into two main divisions, the Pleurocarpse and the Acrocarpae, 

 which are in turn divided into a number of subdivisions or 

 families. How far the division into acrocarpous and pleuro- 

 carpous forms is a natural one may be doubted, as probably the 

 latter are secondary, and it is quite conceivable that different 

 families of pleurocarpous forms may have originated inde- 

 pendently from acrocarpous ones. 



The simplest of the stegocarpous Mosses, while having the 

 operculum well marked, have no peristome. Thus the genus 

 Gymnostomum has no peristome at all, and in an allied genus, 

 Hymenostomurn, it is represented by a thin membrane covering 

 the top of the columella. In nearly related genera, however, 

 e. g., Wcisla, a genuine peristome is present. 



The Tetraphidese, represented by the genus Tetraphis 

 (Georgia) (Fig. 118), are interesting as showing the possible 

 origin of the peristome, as well as some other interesting points 



1 This seems to be strictly the case only in the smaller branches ; in the 

 larges axes the leaves are not exactly in three rows. 



