2 LOUISE DOSDALL 



SUMMARY OF VIEWS AS TO BOG XEROPHYTES 



(1) Much emphasis has been placed upon the physiological 

 aridity of bog habitats and its causes, but as yet an adequate 

 explanation has not been found. It is generally believed that 

 the water available for plant use is limited in certain zones 

 (Burns 1911: 106) characterized by xeromorphic plants. Clem- 

 ents (1916: 90), however, questions the theory of physiological 

 drought, while Gates (1914: 455) has shown that winter is the 

 deciding factor in the xerophily of the most typical of the bog 

 plants, namely the evergreen heaths. 



Schimper (1899: 6) pointed out the difference between physical 

 and physiological dryness. The latter is attributed to high con- 

 centrations of salt and humous acids in the water (1898: 691). 

 This suggested to Livingston "that if the physiological dryness 

 of the bog be due to humous acids or humous salts, these sub- 

 stances may check the absorption of water by plants either 

 physically by high osmotic pressure or chemically by toxic or 

 stimulation effects" (1904: 383). He therefore made a study of 

 the osmotic pressure of a series of bog waters. He found 

 practically no difference in the osmotic pressures, and concluded 

 that bog waters do not have an appreciably higher concentra- 

 tion of dissolved substances than do the streams and lakes of 

 the same region. 



Livingston (1905: 348) later made an investigation of the 

 physiological properties of bog waters by growing an alga 

 (Stigeoclonium) in them and found that "many bog waters act 

 upon the plant like poisoned solutions." He says, "The stimu- 

 lating substances are most markedly present in water from those 

 swamps whose vegetation is most definitely of the bog type. 

 They are absent from river swamps and large lakes ; in water from 

 swamps whose vegetation is of a character intermediate between 

 those of the river swamp and the bog, they are present to some 

 degree, their amount being roughly proportional to the extent 

 of the xerophilous character of the vegetation." 



Dachnowski (1910: 339) has extended the toxic theory. From 

 a study of physiological aridity, he concludes that "the real 



