BOG XEROPHYTES AND ACID SOILS. 141 



placement of weak bases by strong ones, with resulting hydrolysis, 

 as primarily responsible for acidity, while Hanley, Truog, Gillespie, 

 Sharp and Hoagland, and Plummer ascribe it to the presence of 

 soluble acids, and Loew invokes both factors. 



The manner in which acid soils work injury to plants still demands 

 much study, but at least three methods seem highly probable. In 

 muck soils and in bogs it appears certain that the lack of oxygen and 

 the accumulation of carbon dioxid are the primary factors, while the 

 organic acids and salts resulting from anaerobiosis probably play 

 some part also. In some soils acidity brings salts of aluminum, 

 manganese, or iron into solution, and the toxic effect is then exerted 

 by these. This has been demonstrated by Blair and Macy and by 

 Ruprecht and Morse for all three elements, by Abbott, Conner and 

 Smalley, Daikuhara, Frear, Ruprecht, Miyake, Funchess, Hartwell 

 and Pember, and Mirasol for aluminum, and by Wilcox and Kelley, 

 Truog, White, and Johnson for manganese. In general, the harm- 

 ful influence of acidity is explained by Truog as due to its effect in 

 preventing certain plants from getting the calcium needed for neutral- 

 izing the organic acids produced in metabohsm at a rate sufficiently 

 rapid, thus leading to the lowering of both metabolism and growth. 



The frequent assumption that acidity itself is injurious is not borne 

 out by the results of Hoagland, who found that a large number of 

 plants gave excellent crops in soils with pH 5.4 to pH 4.5. Haas 

 found the acidity of cell-sap to be often as high as pH 3, and Truog 

 and Meacham as high as pH 4. The highest H-ion concentration 

 found in acid soils by Truog and Loomis was pH 4.5 and by Gilles- 

 pie, pH 4.4, while Sharp and Hoagland obtained a maximum of pH 

 3.7, and Plummer found greater concentrations only in muck soil. 

 Hence, the statement of Truog that the acidity of soils is not often 

 high enough to directly injure plant roots seems to be entirely war- 

 ranted. Even in muck soils it is probable that acidity is a conse- 

 quence of anaerobic respiration, and thus an effect rather than a 

 cause. 



CONCLUSIONS. 



Great divergence of opinion has prevailed with respect to the causes 

 of the xerophilous appearance of bog and swamp plants. The great 

 majority of the views have been derived from observation rather 

 than from the measurement of habitat factors and actual experi- 

 ment. With the increase of experimental study, the task of assign- 

 ing the proper value to each factor suggested has become much 

 easier, but further research is needed to determine the mutual re- 

 lations of the primary factors. The value assigned to humic acids 

 and humates by Warming, Schimper, and Whitford has not been 

 accepted by other workers, and little importance has been given to 

 periodic drought in bogs, as suggested by Volkens, Davis, and Burns, 



