426 George E. Nichols, 



consequence this stage may be protracted for a long time. But 

 sooner or later other sphagnums may secure a foothold and 

 eventually gain the upper hand. 



In general, the elevation of the bog surface above water level 

 is accomplished very largely through the activity of species of 

 Sphagnum which possess to a more or less marked degree the 

 cushion-forming habit, but the process is greatly facilitated by 

 the concurrent activity of the vascular plants growing on the 

 sphagnous substratum, since these, in the manner already sug- 

 gested, bind together and consolidate the sphagnum cushions, 

 and in addition may form a sort of scaffolding which expedites 

 the upward growth of the mosses. Sphagnum pulchrum with 

 its semi-aquatic habit is a common pioneer on the muck mat, 

 and with its erect habit of growth and tendency to form loose 

 cushions is able to build up the surface to some- height. Fully 

 as important at this stage, and subsequently much more so, are 

 5". papillosum and S. magellanicum, species with a generally 

 more robust habit and a tendency to form denser cushions than 

 5. pulchrum. Any of these three species may act as pioneers, 

 and frequently all of them grow intermixed. 



Once the cushion-forming sphagnums have firmly established 

 themselves, the bog surface may be built up quite rapidly. A 

 measure of the rate at which this takes place is sometimes 

 afforded by shrubs which have been buried by the rising sub- 

 stratum. To cite a specific example, in an erect stem of Myrica 

 Gale which had been partially buried to a depth of eight inches 

 a discrepancy of seven years was found in its age near the 

 bottom of the deposit (ten years), and at the surface (three 

 years), a fact which would seem to indicate that here the 

 sphagnums had grown upward at the rate of about an inch a year. 



As the surface rises higher, the mesophytic sphagnums (S. 

 papillosum, S. magellanicum) may gradually crowd out their 

 more hydrophytic competitor {S. pulchrum). The height to 

 which these two species are able to build up the substratum 

 varies, being apparently conditioned in part directly by soil 

 moisture relations, but largely by competition on the part of 

 other species. For with its increasing elevation the substratum 

 naturally becomes somewhat drier and consequently less congenial 

 to the mesophytic species, while at the same time conditions 

 become more favorable for relatively xerophytic species, such as 



