BOG XEROPHYTES AND ACID SOILS. 125 



soil should promote rather than retard transpiration. He demon- 

 strated experimentally that Sagittaria latifx)lia and Ranunculus 

 sceleratus were hydrophytes, and concluded that this was true of 

 most bog and swamp species. Their xerophytic features were re- 

 garded as stable structures that had persisted since the much earlier 

 period when the habitat itself was xerophytic. 



While Whitford (1901 : 314) regarded the accumulation of humous 

 acids and insufficient aeration as the probable factors concerned, he 

 emphasized the importance of the latter, owing to its preventing the 

 healthy growth of the root-system and the presence of nitrifying 

 bacteria. These are necessary to convert organic matter into ni- 

 trates, and hence organic decay is retarded or ceases in their absence, 

 with the consequent accumulation so characteristic of peat-bogs. 



Livingston (1904 : 383) assumed that if physiological dryness were 

 due to humous acids or salts, these might check absorption physically 

 by high osmotic pressure or chemically by their toxic or stimulating 

 action. The osmotic pressure of bog-water from a number of lo- 

 calities was determined, and it was found that bog-waters do not have 

 an appreciably higher concentration of dissolved substances than 

 lakes and streams of the same region. Moreover, the same bog 

 showed practically no difference in the amount of dissolved material 

 found in the driest part of the summer and in winter. In further 

 studies (1905 : 348) the conclusion was reached that some bog- waters 

 contain chemical substances that produce the palmella stage of 

 Stigeoclonium, as do solutions of poisons and those of high osmotic 

 pressure, but such substances are not directly related to the acidity. 

 The response to bog-water and cold closely resembled that produced 

 in Rumex by Transeau. The stimulating substances were most 

 abundant in communities most definitely of the bog type, and the 

 amount was roughly porportional to the extent of xerophily in the 

 vegetation. However, the results are somewhat obscured by the 

 fact that some of the plants regarded as xerophytic, e. g., Eriophorum, 

 Typha, and Larix, are actually hydrophytes or mesophytes. 



Transeau (1905 : 408) has given a comprehensive account of the 

 factors in bog habitats, which is of exceptional value because of the 

 large amount of experimental evidence. His results showed that 

 bog- water itself had no tendency to produce xerophytic modifications, 

 but that low soil-temperatures and lack of aeration caused a reduction 

 in the growth of several plant organs. When these two factors acted 

 together, the effect was very marked. Experiments with Rumex 

 acetosella showed that nearly all the characteristics of bog-plants could 

 be developed by lowering the soil temperature, preventing proper 

 aeration, or by growing plants in dry sand. Thus, while an undrained 

 peat substratum may cause xerophilous structures, this is due to lack 

 of aeration rather than acidity, which may, however, be a factor in 

 the competition between different species for possession of the bog 



