General Discussion 215 



first material, however, is dead and more nearly saturated with 

 copper; while living roots from soil culture, with proteids com- 

 bined to but a sinall per cent of their capacity for copper, do 

 not give satisfactory color tests. These reactions, therefore, do 

 not serve for qualitative determinations of toxic effects in field 

 material. 



Relations Between Amounts of Copper in Root Systems and 



Injury to Plants 

 An effort to establish relations between the amounts of 

 copper in parts per million of dry matter in root systems, and 

 toxic effects as shown in the condition of aerial portions -of the 

 plant, was only partially successful; but a sufficient number of 

 observations on samples of sufficient size produced under care- 

 fully regulated conditions would probably establish such rela- 

 tions. In the tables shown on the preceding pages there is a 

 fair degree of agreement between the members of each experi- 

 mental series, the copper found in root systems increasing in 

 most cases with the amount of copper in the soils of each particu- 

 lar series of cultures. In the case of beans and corn grown in cul- 

 tures containing copper in the form of precipitated carbonate, 

 beans show a somewhat higher resistance to toxic effects and also 

 contain larger amounts of copper in the root systems throughout 

 the series. The conditions under which the samples were grown 

 seem to have, within limits, more effect upon the copper content 

 of root systems than the amounts of copper in the soil, as is 

 indicated in the following tabular statement : 



TABLE XXXII 

 Toxic Concentrations of Copper in Soils and Root Systems 



Cu in root Cu in root 



Points at system at point system at 



which toxic not showing point showing 



effects toxic effects toxic effects 



Culture begin p. p.m. p. p.m. 



Corn, seven samples from field 

 soils 42 at .07% 



Corn in field plots containing 



Cu as sulphate 04% 245 at .025% 296 at .05% 



Corn in pot cultures containing 



Cu as carbonate 023% 748 at .02% 509 at .025% 



Beans in pot cultures contain- 

 ing Cu as carbonate 035% 950 at .025% 1358 at .05% 



