1915] 



MERRILL ELECTROLYTIC DETERMINATION OF EXOSMOSIS 513 



They determined the amount of CO2 excretion per gram dry 

 weight of roots of wheat, oats, rye, and barley. The amount 

 varied for the different plants but a correlation was found 

 between the amounts of P 2 5 , K, and Na contained in the dry 

 roots of plants grown on gneiss and basalt and the amount of 

 CO2 excreted. 



"We now come to the work of various soil investigators 

 whose results have again focused attention during the past 

 decade upon De Candolle's original theory. The essential 

 features of this work have become so well known that for our 

 purpose it is not necessary to do much more than merely men- 

 tion it here. Though not considering directly the phases of 

 the subject with which we are dealing, yet the much-discussed 

 paper by Whitney and Cameron ( '03 ) is historically impor- 

 tant and bears an intimate relation to the later work of the 

 investigators in the Bureau of Soils of the U. S. Department 

 of Agriculture, the results of which led to the so-called toxic- 

 excretion theory. Among the workers most prominently con- 

 nected with the early studies along this line may be mentioned 

 Livingston, Britton, and Reid ('05); Livingston, Jensen, 

 Breazeale, Pember, and Skinner ('07); Schreiner and Reed 

 ('07); Schreiner, Reed, and Skinner ('07); Schreiner and 

 Reed ('07 a ) ; and others. As is well known, opinion is much 

 divided on the various phases of this subject, however. Among 

 those opposing the ideas or theories advanced along this line 

 by the investigators named above should be mentioned Hop- 

 kins ( '10) ; Hall, Brenchley, and Underwood ( '14) ; and others. 



That the question is one upon which investigations are still 

 being pursued is shown by the publications from various quar- 

 ters. As recent examples of these the work of Molliard ('13) 

 and Prianischnikov ('14) may be cited. The former found 

 that peas grown in water cultures in which previous crops of 

 peas had grown produced a smaller growth than the original 

 crops. This he attributed to the excretion of toxic substances 

 in the medium by the earlier plants. The latter, from his own 

 experimental work and from the results observed by him at 

 the Rothamsted Experiment Station, is inclined to believe that 

 the hypothesis of root excretion is not sufficiently demon- 



