General Discussion 219 



Physical attractions, or adsorptive effects, also account for 

 a very considerable lessening of the amount of dissolved copper 

 salts, in contact with soil particles. Jensen, for instance, finds 

 that a dilute copper solution is ten times as toxic in the free con- 

 dition as when it is mixed with an artificial quartz soil, that is 

 to say, the quartz reduces the toxic effects about nine-tenths. In- 

 asmuch as tlie reduction in toxicity is a function of the solid 

 surface to which the soluble salts are exposed, the finer the state 

 of division of a soil tlie more will be the adsorption and the less 

 will be the toxic effects of a stated copper solution.-*' 



The age of plant roots markedly affects their susceptibility 

 to copper salts. Young and tender roots, containing large 

 amounts of protoplasm, are much more quickly and easily 

 poisoned than old and comparatively fibrous structures contain- 

 ing a small proportion of protoplasmic materials. This may be 

 due to differences in the thickness of cell walls protecting the 

 cell contents from outside substances ; it may be due to a different 

 degree of permeability of the protoplasm of older roots to copper 

 salts; or it may be due to lessened reactivity due to changed 

 chemical character. In any case, this observation indicates a 

 distinctly greater resistance to copper in soils, of older, more 

 fibrous, and possibly intrinsically more resistant root systems. 

 Different species of plants also show varying degrees of resistance 

 to copper salts. In pot cultures, peas are distinctly more re- 

 sistant to precipitated carbonate of copper than corn. Different 

 plants of the same species also show a certain amount of indi- 

 viduality with reference to absorption of copper. 



Stimulation 



Not only do the various influences described above lessen the 

 toxic effects of copper upon plants, but it is possible, also, that 

 the amounts of copper may be decreased in the field to the 

 point at which stimulating effects occur. As shown in the dis- 

 cussion of water cultures on preceding pages, extreme dilutions 

 of copper salts in distilled water, for instance, 1 part to 100.- 



26 G. H. Jensen, Botanical Gazette, vol. 43, p. 11, Jan., 1907. 



