360 BOTANY. 



layer is called the columella (c, d , Fig. 246, (7), while the two or three 

 layers of cells exterior to it constitute the spore-sac. 



In all the members of this order, the capsule, when ripe, opens by the 

 falling away of a lid (operculum), which is composed for the most part 

 oi the epidermis covering the apical portion (Fig. 247). In most of the 

 genera, when the operculum falls off, one or two rows of teeth (the 

 peristome) are exposed, surrounding the opening of the capsule (Fig. 

 248). These teeth, which are always some multiple of four (4, 8, 16, 

 32, or 64), are in most cases formed respectively of the thickened outer 

 and inner walls of rows of cells which lie beneath and parallel to the 

 wall of the operculum, and converge toward its centre. Each tooth 

 is thus made up of parts of several cells, and the transverse lines seen 

 upon it are the thickened transverse walls which formerly separated 

 the original cell cavities. 



The peristome of Polytrichum and its allies is composed of bundles 

 of thickened cells, hence they are much firmer than in those genera in 

 which they are made of fragments of cell membranes. 



The Bryacese include many genera, which are widely distributed 

 throughout the world. The genera arrange themselves under two 

 groups (sub-orders), according as the sporogonia are terminal or 

 lateral, with reference to the main axis ; the first constitute the Acro- 

 carpce, including Funaria, Bryum, Mnium, Polytrichum, etc. ; those 

 with lateral sporogouia constitute the Pleurocarpce, and include Fonti- 

 nalis, Climacium, Hypnum, etc. 



In the Tertiary of Europe the order is represented by an Eocene spe- 

 cies of Musettes, and Miocene species of the modern genera Fontinalis, 

 Dicranum, Barbula, Polytrichum, Hypnum, etc. A single species of 

 Hypnum from the Tertiary of Colorado is the only American fossil of 

 this order yet detected. 



The most valuable systematic works for the student of the Bryo- 

 phytes of this country are " Musci and Hepaticse of the Eastern United 

 States," by W. S. Sullivant, 1871; " Icones Muscorum," by the same 

 author, 1864-74 ;>and "Catalogue of Pacific Coast Mosses," by L. Les- 

 quereux, 1868; " Manual of the Mosses of North America," by Leo 

 Lesquereux and Thomas P. James, 1884; " Descriptive Catalogue of 

 the North American Hepaticse North of Mexico," by L. M. Under- 

 wood, 1884. 



