452 George E. Nichols, 



bog becomes superseded by dry bog. Incidentally it may be 

 remarked that, like the mesophytic forms, the xerophytic cushion- 

 forming sphagnums sometimes predominate from the very out- 

 set, so that both the bog meadow and wet bog stages may be 

 eliminated. On the higher, drier parts of a bog, as elsewhere 

 indicated, the xerophytic sphagnums in turn may give way locally 

 to various lichens and mosses, but these never become sufficiently 

 abundant to constitute a distinct association-type. 



Throughout the successive steps in bog development, as just 

 outlined, sight must not be lost of the part played by various 

 seed plants. These fulfill a triple role in that they facilitate the 

 upward growth of the sphagnums and bind together the spongy, 

 otherwise incoherent matrix of sphagnum remains, beside con- 

 tributing in varying degree to the bulk of the deposit. Much of 

 the springiness and comparative firmness which characterizes 

 the surface of a mature bog is ascribable to the tangle of stems 

 and roots with which the ground is interwoven. With regard 

 to the rate at which the bog surface is built upward : in general, 

 upward growth is comparatively slow at first, during the bog 

 meadow stage, most rapid during the wet bog stage and during 

 the early part of the dry bog stage, from which point on there 

 is a gradual slowing down until, in the case of the older, higher 

 bogs, growth is practically at a standstill (but see quotation from 

 Weber on p. 456). 



From the observations recorded in the preceding pages it is 

 apparent that not only do the sphagnums as a class play an all- 

 important part in the development of raised bogs, but that differ- 

 ent groups of sphagnums are responsible for different phases in 

 the development. It is also certain that the formation and 

 upward grow^th of a bog is not dependent on the presence of any 

 preexisting water basin from which the required water is raised 

 by capillarity. The view expressed by Ganong ('98. p. 148) that 

 "The raised bogs are formed, as all students of them agree, by 

 the pure Sphagnum growing upward and carrying the water by 

 capillarity with it" has long since been exploded. To quote from 

 Warming ('09, pp. 200-201): "It is erroneous to suppose tliat 

 Sphagnum sucks up water from the soil ; it raises water only 

 for an inconsiderable distance. The movement of water in a 

 Sphagnum-moor is essentially a descending one. The depth at 

 which the water-table lies is dependent on the atmospheric precipi- 



