BOG XEROPHYTES AND ACID SOILS. 127 



It seems certain, however, that the lack of oxygen is the primary- 

 cause, and that the acids secreted by the roots and the deleterious 

 substances produced by anaerobic fermentation and decomposition 

 are the consequences of it. 



In later studies (1909, 1910), Dachnowski has concluded that the 

 contaminated condition of the agricultural soils used and the de- 

 creased activity of plants in them indicates that xerophily can not be 

 due to acidity, lack of oxygen, or low temperatures, and that the 

 injurious substances present in bog- water and bog-soils are at least 

 in part the cause of xerophily and of decreased fertility. It is recog- 

 nized that fertility is restored through aeration, after sufficient time 

 has elapsed for the oxidation of the injurious bodies, and, in conse- 

 quence, decrease in toxicity always follows aeration and drainage. 

 Transpiration data obtained from wheat seedlings grown in solutions 

 inoculated with pure cultures of bog bacteria showed reduction in 

 varying degree, and this was confirmed by the growth, showing that 

 the residual products of many bacteria constitute in part the toxicity 

 of the bog habitat. 



As indicated in earlier sections, Hesselmann (1910) has found 

 that the water of peat-bogs and swampy forests is often completely 

 free of oxygen and rarely contains more than a small amount, while 

 it is high in streams and lakes and often approaches saturation. In 

 consequence, he finds that swamped pine forests sufi'er greatly on ac- 

 count of the lack of oxygen, while those watered excessively by spring 

 brooks are remarkably luxuriant, and concludes that oxygen is the 

 paramount factor. 



Burns (1911 : 105) accepted the view of Davis that xerophytic 

 areas in peat-bogs are chiefly due to the drying of surface layers and 

 the ability of peat to render large amounts of water non-available, 

 though he also regarded low temperature and low air-content as 

 secondary factors. 



From a study of the bogs of Cranberry Island, Dachnowski 

 (1911 : 147) confirmed his earlier conclusion that the limiting factor 

 was not evaporation or temperature, but the toxicity of the sub- 

 stratum. The effect of the latter appears chiefly when the water- 

 supply has become stationary, through the action of bacteria espe- 

 cially. The edaphic aridity of the central zone reduces absorption 

 by the roots at the time when transpiration and growth are making 

 greater demands, and the dwarfing of the roots diminishes it still 

 further. In a later paper (1912 : 513), the character of the obligate 

 bacterial flora and the nature of the organic compounds produced in 

 bogs are considered to explain the xerophytic conditions. The or- 

 ganic products of decomposition play the controlling role, and cause 

 the infertility of peat-deposits, even when these have abundant soil- 

 air and water-content, and favoring temperature and humidity. 



