A MOSS 95 



parts dying each year and being pressed and packed down by the 

 weight of the younger growing branches above. The dead parts of 

 the moss plants, together with plants of other species growing among 

 them, and fallen leaves, twigs, bark, and the like, undergo changes 

 which result in the formation of beds of peat. Further changes in the 

 peat may in the course of ages lead to the formation of certain kinds 

 of coal. Such peat bogs, whose material is steadily heaped up, play a 

 large part in the filling in of swamps as well as of small ponds and 

 lakes. Many lakes in the northern United States are thus being 

 gradually filled by the ingrowth of beds of peat mosses from about 

 their borders. In some locations, for example on cliffs and rocky 

 hillsides, clumps of mosses of various kinds catch and hold soil-making 

 materials such as dust and fragments of animal and plant bodies. 

 As parts of the moss plants die and decay, their own substance is 

 added to the collection ; in this way, mosses, aided by the decay- 

 producing bacteria, prepare a soil in which larger and more highly 

 developed plants can in time establish themselves. 



124. Relationships of the Mosses. Most closely related 

 to the mosses are the liverworts. Some of these are leafy 



FIG. 49. A, a liverwort (Marchantia). B, part of a liverwort plant, 

 showing the upright branches (6) which bear sex organs (in the case shown, 

 archegones). 



plants quite similar to the mosses ; others are flat, creeping 

 plants which grow on damp earth and rocks (Fig. 49) . Liver- 

 worts also have distinct sexual and asexual generations, but 

 the asexual generation of a liverwort is smaller and much 



