BOG XEROPHYTES AND ACID SOILS. 



123 



of 232 for beech. As to leaf-surface and water-loss, von Hohnel's 

 results were as given in table 29. 



Table 29. 



CAUSES AND INTERPRETATIONS OF BOG XEROPHYTES. 



The early differences of opinion as to the effective causes of appar- 

 ent xerophytic adaptations in bog-plants still persist. However, a 

 critical study of the investigations in this field makes it evident that 

 some views have become invalid, while others are incomplete. 

 While further research is needed to determine quantitatively just 

 what species are xerophytes and what are the causal relations of the 

 various factors, it seems possible to anticipate some of these con- 

 clusions. In spite of the fact that none of the earlier studies measured 

 the various factors concerned, they advanced nearly all of the inter- 

 pretations among which choice must be made today. Although 

 frankly puzzled by finding xerophytic devices in bogs, Volkens 

 thought that these might be explained by periodic drought in the 

 upper layer of the soil. In his earher work. Warming regarded the 

 dry climate of polar regions as the major fatjtor in most cases, but 

 he explained the leaf-structure of certain swamp-sedges as a con- 

 sequence of inheritance, and hence independent of the habitat. 

 Schwendener hkewise appealed to a fixed heredity in explanation 

 of the leaf-structure of exotic grasses and sedges, and thought it 

 probable that the same interpretation would apply to the various 

 protective modifications found in bog-plants. 



Kihlmann regarded low soil-temperatures and strong drying winds 

 as the primary factors in producing the xeroid adaptations of bog 

 and swamp plants, but admitted that some species of the most 

 exposed situations lacked such devices, assuming in consequence that 

 their tissues must possess a specific resistance against cold. 



Stenstrom rejected Kihlmann's conclusion, and turned to fixity 

 of character in evergreen shrubs especially, and to transpiration 

 relations as affording the proper solution. On the contrary, Goebel 

 advanced the same explanation as Kihlmann, finding the cold soil, 

 strong winds, and rarefied air of the Paramos sufficient reason for 

 the presence of xerophilous species. 



