ON THE DEVELOPMENT AND STRUCTURE OF MOUSES. 15 



This nerve often projects beyond the surface of the leaf 

 and especially below, and is sometimes plaited beneath, as in 

 Dicranum scoparium (Plate 23, fig. 7), sometimes furnished 

 with close parallel laminae above, as in Pogonatum nanum 

 (Plate 19, fig. 1 c, d), and not unfrequently rough or echinulate. 



The cells vary greatly in size and form, those at the basal 

 angles being often larger than the rest and less filled with 

 chlorophyll. They also vary greatly in thickness, the walls 

 of two contiguous cells being sometimes so blended together 

 that their distinction is not visible, but occasionally very ac- 

 curately defined (Plate 18, fig. 5 c). The primary cell- wall is 

 not always distinct, but in some genera it is well-defined and 

 sometimes is very irregular in outline. 



The edge of the leaf is often serrated or crenulate through 

 the whole or a part only of its course, the serratures consist- 

 ing for the most part merely of cells projecting beyond their 

 neighbours, so as to give a toothed outline. Sometimes, how- 

 ever, the margin is distinctly thickened, and the serratures or 

 inequalities, if present, may then consist of two or more cells. 

 Indeed, this may be the case where there is no thickening. 

 The cell-walls of the bordering cells are sometimes thicker 

 than those of the rest. A portion of the surface of the leaf, 

 especially towards the edge or tip, like the nerve, is sometimes 

 rough with spine-like projections, arising from the protrusion 

 of individual cells. In some genera the walls of the cells on 

 either surface or on both are strongly granulated, but in such 

 cases the granulations belong to the thickened walls of the cells 

 (Plate 8, fig. 4). 



The leaves are generally symmetrical, but this is not always 

 the case, especially where they are equitant, as in Fissidens 

 (Plate 14, fig. 2, 3). In some genera, as Hedwigia (Plate 21, 

 fig. 8), they are strongly fringed, and in others, as Sphag- 



