12 



HANDBOOK OF MOSSES. 



example occurs in Grimmia pulvinata (fig. 19), the little 

 hoary, cushion-like patches of which may be seen on wall- 

 tops and thatch. 



The stem and branches are more or less densely clothed 

 with leaves, which are always simple (undivided), and vary 

 in shape from awl-shaped to round, the most frequent forms 

 being lance-shaped, or oval. The leaves vary in structure, 

 but are usually formed of a single layer of cells \ exceptions 

 occur, as in Leucobryum ; in this case the leaves are formed 

 of three layers of cells. 



FIG. 4. Pottia truncata. i, plant slightly enlarged. 2, obliquely rostrate 

 operculum ; a, columella, which remains attached to lid, and falls away with it, 

 3, tip of leaf ; a, upper leaf cells ; 3 b, cells of base of leaf. 



The cells forming the leaf assume a variety of forms, but 

 may be referred to two types I. Parenchymatous (having 

 the cells placed end to end), as in Pottia^ etc. (fig. 4, 3 b) ; 

 II. Prosenchymatous (Jiaving cells which overlap one another 

 at their ends) ; these have pointed ends, and are longer than 

 broad, as in Hypnum (fig. 5,40, and fig. i, 4). The study 

 of these leaf-cells is one of great importance, as the generic 

 and specific differences of many mosses are often made out 

 by the character of the cells forming the leaf. Among other 

 forms assumed by cells we have round (fig. 34, 4 a), as in 

 Orthotrichum ; quadrate, as in Pottia (fig. 4, 3 b) ; hexagonal, 

 as in Tetraphis ; oblong, as in Isothecium ; rhomboid, as in 



