of Botanical Research of the Carnegie Institution of 

 V/ashingir n for placing his unpublished material at my 

 disposal and for continuous advice and suggestion. Dr 

 Gannon's experiments will be reviewed in detail below. 

 In the actual conduct of the experiments 1 have been 

 assisted from time to time by Mr. Herbert JT. McCall, 



. Jam i 1 . Trelease, Mr. _. ... Hildebrandt, Mr, Frederick 

 .. itte and Mr. HTilliam a. .Schaefer. I.Irs. Grace J. Living- 

 ston has assisted importantly in the preparation 

 the bibliography and the verification of references. 



7he investigation of this problem might take 

 either one of two directions; the study of the composi- 

 tion of soil air, or the direct investigation of the oxy- 

 gen requirements of plant roots. In the experimental 

 part of the present investigation it is the second line 

 that has been followed. T-he precise study of the com- 

 position of the soil air is a matter of extreme difficul 

 and assurance in the correctness of results would be al- 

 most impossible to obtain without an impossibly large 

 amount of observation and experiment. In drawing air 

 samples from the soil there is always a chance of down- 

 ward leakage from the atmosphere, of abnormal soil per- 

 meability, of bacterial or chemical abnormality, or of 

 some other exceptional circumstance. On the other hand 

 experiments on the oxygen requirement of roots are com- 

 paratively simple and are susceptible of precise control. 

 .although the experimental work here reported is entirely 



