in this latter field, a preliminary chapter is devoted to the 

 existing data concerning the* composition of the soil air 

 and to the possibilities and probabilities in this con- 

 nection. 



The contradictions of evidence oiatlined in the 

 opening paragraph of this introduction imply that an ex- 

 planation may lie in the variability of different species 

 of plants with regard to oxygen needed by their roots; 

 some species requiring an ample supply, some species 

 being able to exist with little or none. It will appear 



below that this suggestion receives conclusive support 



fnotn 



*•» the results of the experimental work. 



