124 AERATION AND AIR-CONTENT. 



The controlling cause of xerophytic modifications in marine 

 swamps and solfataras was stated by Schimper to be the salt-content, 

 and in glacial alpine soils the low temperature. Warming consid- 

 ered free humous acids as the chief factors in producing bog xero- 

 phytes, as these were thought to depress the root's activity, but he 

 included low soil-temperatures, poor aeration, water-retention by 

 the peat, chemical substances, and a transpiration optimum as 

 factors playing some part in the result. In connection with the 

 concept of physiological dryness, Schimper laid the chief emphasis 

 upon the presence of humous acids and salts and upon low tempera- 

 tures as the causes of xerophytism in bogs and swamps, though it is 

 significant that he carried out no investigations with reference to the 

 former. It has already been suggested that this was due to a typo- 

 graphic error in the case of humous acids, while the retarding effect 

 of humous salts upon transpiration was accepted upon the evidence 

 furnished by Burgerstein, which Livingston, Transeau, and others 

 have since shown to be insufficient, as indicated later. 



The error made in Pfeffer's Plant Physiology with respect to Bur- 

 gerstein's results has been so generally followed that it is desirable 

 to give a brief summary of these. 



Burgerstein (1876 : 191) demonstrated that 0.15 per cent nitric 

 acid increased the transpiration of corn plants about 10 per cent, 

 and 0.3 per cent, about 30 per cent in comparison with distilled water. 

 Oxalic acid in solutions of 0.25, 0.5, and 1 per cent produced increases 

 of approximately 65, 200, and per cent in the water-loss of corn, 

 while the two higher concentrations effected an increase of 60 and 

 100 per cent in the transpiration of branches of Taxus. Tartaric 

 acid in 0.25 per cent solution increased water-loss in corn 20 per 

 cent, while the 0.5 per cent decreased it about 15 per cent. As has 

 been indicated earlier, carbonic acid usually increased transpira- 

 tion. Potassium and sodium hydroxid, and ammonia in concen- 

 trations of 0.02 and 0.1 per cent regularly decreased transpiration 

 from 15 to 40 per cent. A number of salts, viz, calcium nitrate, 

 potassium nitrate, acid potassium phosphate, magnesium sulphate, 

 ammonium nitrate, ammonium sulphate, sodium chloride, and po- 

 tassium carbonate, regularly promoted transpiration in strengths of 

 0.05 to 0.25 per cent and as regularly retarded it in solutions of 0.33 

 to 1 per cent. Nutrient solutions of 0.05 to 0.26 per cent decreased 

 water-loss from about 7 to 45 per cent, while humous extracts con- 

 taining about 0.06 per cent of solids depressed transpiration to about 

 the same degree. 



It would appear that Friih and Schroter regarded the absence of 

 oxygen as the primary factor in bogs and the water-retaining power 

 of peat and the low soil-temperatures as contributing factors. 



Clements questioned the views of Schimper as to the importance 

 of humous acids and pointed out that weak solutions of acid in the 



