AERATION AND AIR-CONTENT. 



INTRODUCTION. 



Recent studies of the so-called bog xerophytes have shown the 

 air-content of the soil to be an ecological factor of primary and often 

 of unique importance (Clements, 1916, 1920; Dosdall, 1919; Berg- 

 mann, 1920). In organizing a comprehensive investigation of bog 

 and swamp plants and of related problems, it has become desirable 

 to analyze much of the literature dealing with the respiration of 

 roots and with anaerobiosis. An attempt has been made to present 

 a complete digest of the results in so far as they have to do with 

 transpiration, growth, or movement, or serve to throw light upon the 

 mooted questions of bog toxins, acid soils, or toxic exudates. In 

 addition, a complete account is given of the development of views 

 upon bog xerophytes and soil toxins, primarily to afford a clear 

 grasp of existing opinion and to reveal the points to which further 

 study should be directed. 



In spite of the enormous amount of work done upon the respira- 

 tion of roots and the significance of soil-air, physiologists and eco- 

 logists generally have ignored this subject or have given it little 

 consideration. A cursory survey of a score of text-books shows that 

 the air-content of the soil is not even mentioned by the majority, 

 while it is given slight attention by a few, and accorded recognition 

 as a factor of primary importance by three or four only. This was 

 probably a natural outcome of the laboratory development of physi- 

 ology. At least, it appears certain that the basic importance of the 

 air-content of the soil could not be appreciated fully until ecology 

 had begun the instrumental measurement of the habitat. Even the 

 latter has found it difficult to distinguish between sequelss and causes 

 in this field (Plant Succession, p. 90), and general recognition of 

 air-content as a primary factor in many habitats, and a controlling 

 one in wet soils and water, is yet to come. 



In the following treatise, the development of our knowledge of 

 the respiration of roots and other underground parts is first taken 

 up in historical sequence, together with an account of studies deal- 

 ing with the nature and composition of soil-air. This is succeeded 

 by a digest of results in the field of anaerobic respiration, with 

 especial reference to its relation to germination, growth, and move- 

 ment. A section is devoted to bog xerophytes and swamp plants, 

 with which are considered bog toxins and acid soils, while soil toxins 

 and toxic exudates are discussed in a final section. 



