422 BOTANICAL GAZETTE [DECEMBER 



the temperature of the subsoil is that of percolating water which has 

 been warmed at the surface of the soil. Because of the high water- 

 table and the stagnant condition of the underground water in bog 

 areas, this source of heat is relatively unimportant. 



The effects of these factors, resulting in low soil temperatures, 

 are far-reaching. As compared with well-drained soils, chemical 

 action is retarded, the rate of diffusion, solution, and osmosis is 

 greatly reduced, and the conditions for the existence of soil bacteria 

 made unfavorable. Plants which can successfully compete for the 

 occupancy of such areas must be able to withstand low temperatures 

 and late frosts. The difference between the temperature of the air 

 and that of the substratum favors plants having a low transpiration 

 ratio. 



However, in so far as the region of southern Michigan is concerned, 

 the temperatures prevailing in bog areas do not seem to be adequate 

 to account for the presence of the bog plants or their xerophilous 

 structures. It is to be noted that with the leafing-out of the trees, 

 about May 27, the temperature of the maple-poplar substratum falls 

 below that of the tamarack. But that the soil temperature is one 

 of the factors entering into the problem of competition between 

 species there can be little doubt. It is probable also that in the region 

 of optimum conditions for bog plants the conditions which occur 

 here only in the spring are prolonged through the summer. That 

 is, the difference between air and substratum temperatures is more 

 marked, and is a powerful factor in the selection of plants for bog 

 areas and in the production of xerophilous structures. 



3. Texture. This property of the substratum has already been 

 referred to in connection with the genetic changes in peat. The sedge 

 zone is developed upon a raft of interwoven rhizomes and roots. It 

 is a coarse meshwork; but since it lies at or below the surface of the 

 water, its texture is of slight importance except as a means of mechan- 

 ical support. As the bog develops, the admixture of moss and shrub 

 debris brings about the formation of a rather compact peat, overlaid 

 by a stratum of loose material. In some cases, as at Delhi and 

 Oxford, 45 miles (72 km ) northeast of Ann Arbor, the living sphagnum 

 makes up the bulk of this loose covering. Usually the water level 

 lies just beneath it. As a consequence, this covering becomes the 



