A. DachnowsTci — Problem of Xeromorphy. 35 



substratum is not in direct relation to acidity in the soil. Tests 

 on the reducing powers of peat soils show that the wind-driven 

 aeration has little effect on the peat substratum beneath the 

 two-foot level. A shallow, superficial zone of oxidation exists 

 in peat soils, and the debris below this is sometimes so charged 

 with injurious decomposition products and gases, and so far 

 unaerated as to be inhospitable to all organisms but anaerobic 

 bacteria. 



In the growing season the temperature of peat soil, in 

 the more xerophytic of the succeeding bog associations, is 

 not below that of other soils. Rapid and passing changes of 

 air temperatures and the occasional extremes do not affect the 

 substratum temperatures. Only average effects prevail and 

 the great periodic seasonal changes of the dominant climate. 

 The temperatures of the deeper peat strata indicate that there 

 is scarcely anything of a seasonal descent. The continued 

 growth and persistence of the closely related plant association 

 and the slow succession of vegetation types in a habitat of that 

 character is no longer incomprehensible if we remember that 

 the vegetation grows on top of the accumulating debris and 

 that the water table is always at a high level. The disturbance 

 of the balance produced in the soil is thus not unfavorable to 

 the dominance of the associations. There occur natural succes- 

 sions which are determined, however, not by a deficiency of 

 mineral nutrients, such as lime, potash, phosphoric acid, or 

 great distance from the mineral substratum, but by an exces- 

 sive or defective action in the substratum. The lack of mineral 

 constituents does not even render it difficult for mesophytic 

 shrubs and trees to invade and grow as the deposit is built up 

 and oxidation processes become prominent in the surface layer 

 of the substratum. Acidity, toxicity, and reduction action rep- 

 resent merely stages in the decomposition of organic matter. 

 Each plant association augments the efficiency of the soil as a 

 habitat. It cannot be too strongly emphasized that the soil 

 processes exhibit an efficient natural process for the mainte- 

 nance of productivity relations, and that the prime condition 

 determining bog forest successions is not an increase in the 

 mineral constituents in the peat soils. To what extent bog 

 plants require the organic compounds arising in peat soils is 

 still undetermined. The assimilation of such substances is 

 undoubtedly made less difficult on account of the number of 

 saprophytic fungi and the endotrophic mycorhiza usually 

 present. 



The characteristic foliage of heath and shrub bog plants 

 is distinctly an effect to a habitat with a moderate or scanty 

 physiological soil-water content. Extreme xeromorphy is 

 reached in the upper layer of open shrub associations ; here 



