370 BOTANICAL GAZETTE [NOVEMBER 



As to the influence of external factors, high temperatures increase 

 the rate of disintegration, while the presence of acids prevents its 

 continuance, due to the killing of the bacteria involved. It is to be 

 noted that the products of putrefaction, both intermediate and final, 

 can be of little use in furnishing food materials for the higher plants. 



With these two processes in mind, we may now consider the 

 matter of peat formation as it occurs in this region. We have already 

 seen how the substratum is being extended at the edge and renewed 

 at the surface by the plants forming the outer zone of the bog vege- 

 tation. It consists of sedges, especially forms of Carex and Erio- 

 phorum. Each year these plants send up stems and leaves from the 

 matted rhizomes. At the approach of winter these are killed, and 

 the snow later on aids in bringing them down to the water level. In 

 the spring the water covers almost the whole of this zone to the depth 

 of several inches. With the gradual lowering of the water level and 

 the coming of warmer temperatures, the conditions for eremacausis 

 are made favorable. If the water is approximately neutral in its 

 chemical reaction, the fungi and bacteria begin the work of disintegra- 

 tion, which if continued would result in the complete destruction of 

 the vegetable debris. However, on account of the great demand for 

 oxygen, the process can be carried on only near the surface of the 

 water. Even at a depth of a few centimeters the rate of oxygen 

 diffusion is so small, as compared with the demand for it, that practi- 

 cally all aerobic bacterial action is prevented. All of the surface waters 

 which I have examined have been found to be teeming with bacteria. 



Close upon the extension of the bog-sedge zone comes the 

 sphagnum-heath zone. Here the surface is characterized by hol- 

 lows and elevations, the latter frequently due to the upward growth 

 of the sphagnum beneath the shade of the heath plants, but in some 

 cases due to the building of mounds by ants. In the hollows the 

 water stands above the substratum throughout a large part of the 

 year and even during dry periods lies just at the surface. Unlike the 

 sedges, the principal plants of this zone are evergreen. The 

 sphagnum forms a continuous mat of living plants several centi- 

 meters in thickness, through which all of the oxygen must diffuse 

 before it can be available for the eremacausis of the dead plant- 

 material beneath. The cassandra, cranberry, and andromeda which 

 compose the bulk of the shrubby vegetation add to the debris largely 



