436 George E. Nicliols, 



will be discussed in detail later: suffice it to state here that in 

 addition to the sphagnums which form the groundwork o£ the 

 mass, the most prominent plants are low, ericaceous shrubs and 

 the sedge, Scirpus cacspitosus. 



For the development of a raised bog, the fundamental pre- 

 requisites are the presence of certain species of sphagnum and 

 of environmental conditions congenial to their growth, since 

 from start to finish in the evolution of such a bog these mosses 

 play an all essential role. Of foremost importance is a copious 

 water supply, and this is controlled partly by climatic, partly by 

 edaphic conditions. Of the water which falls on the earth's 

 surface in the form of rain and snow, part enters the ground, 

 forming the ground water supply ; part runs off over the surface 

 into streams and lakes; the remainder is evaporated or is 

 absorbed directly by vegetation. In so far as the development 

 of raised bogs is concerned, it is now generally recognized that 

 the chief source of their water supply is meteoric, rather than 

 telluric. In other words, while locally a limited amount of the 

 water needed may be derived from springs, on the whole their 

 distribution and growth is independent of the ground water 

 supply. Surface drainage from neighboring slopes may and 

 frequently does help out, but in the large it is the water pre- 

 cipitated directly upon the surface of the area occupied, in the 

 form of either rain or snow, which is most important. Swamps 

 which thus are dependent directly upon atmospheric precipitation 

 for their water supply have been designated precipitation-swamps, 

 by way of distinction from spring-swamps and lake-swamps (see 

 earlier remarks, p. 354). 



The importance of edaphic factors is seen in their influence 

 on water loss through surface runoff and downward percola- 

 tion. Given a substratum sufficiently impermeable to prevent loss 

 through percolation, a raised bog may originate under quite 

 varied topographic conditions. In general, it may be built up 

 either in and around a water-filled rock basin or over any 

 essentially flat, undulating or irregular surface from which the 

 rain and snow water tend to run off slowly or to accumulate in 

 local depressions. Such surfaces as the latter, considered in 

 their entirety, may be either approximately level or slightly 

 inclined. It is probably the lack of suitable areas of impermeable 

 substratum that is responsible for the observed absence of raised 



