262 C.-E. A. WiNSLOW AND E. E. LOCHRIDGE 



own. The authors concluded that "in the case of plants the toxic 

 action of solutions of electrolytes, when dissociation is practically 

 complete, is due to the action of the ions present. When dissocia- 

 tion is not complete, the undissociated part of the electrolyte may 

 also exert a toxic effect." Heald (1896) extended the work of Kahlen- 

 berg and True to the seedlings of three other flowering plants, and 

 reached the same general conclusions. All these authors pointed 

 out clearly that the effect of the common mineral acids is due to 

 hydrogen alone, since their anions have almost no strong toxic action 

 when neutral salts are used. 



The next work along these general lines was carried out in another 

 field of botany by Stevens (1898), at the University of Chicago, the 

 measure of viability used being the germination of fungus spores. A 

 study had been made by Wiithrich (1892), at a much earlier date on 

 the toxicity of metallic salts and acids for the spores of fungi; and 

 Maillard (1898 and 1899) at about the same time reported experi- 

 ments on the inhibition of the growth of Penicillium by copper salts. 

 In Stevens' experiments the spores were inoculated into hanging-drop 

 preparations of the solutions tested, and examined for development 

 after 24 hours. The five organisms used exhibited marked differ- 

 ences in their susceptibility, although all were much less affected 

 than the phanerogamous seedlings, requiring a strength of 0.01-0.02 

 normal acid to inhibit germination. The relative toxic effect of 

 various substances was not unlike that observed by Heald and 

 Kahlenberg and True. Mercuric chlorid and various copper salts 

 proved most fatal, the acids and cyanides being less active. By 

 the comparison of various substances it appeared that of the anions, 

 CN, CrO^, Cr^O^, and OH are poisonous, and of the cations, Hg, 

 Cu, and H, while the halogens and SO^ in dilute solutions exert no 

 influence. 



A still more exhaustive study of the effect of toxic agents upon 

 the fungi was made by Clark in the next year (Clark, 1899). This 

 investigator followed the same general method as that of Stevens, 

 exposing spores in hanging-drop cultures to the activity of the agents 

 to be tested. The cultures were divided into four classes : those which 

 grew normally, those which showed irregular or retarded growth, 

 those which failed to develop in the medium tested, but grew after 



