50 S. E. A. McCallan 



the total amount of copper present, and not by the relative concentration. 

 Similar conclusions have been drawn by Burrill (1907:568-569). Briggs 

 (1923), and others. 



In order that an adequate comparison of the volume relations may be 

 made, the total copper present in these solutions has been calculated 

 on the basis of milligrams of copper per liter of solution. That is, the 

 total copper present is assumed to be concentrated into a volume of one 

 drop. Having the concentration of copper expressed thus, by a reference 

 to figure 2 the germination theoretically to be expected for 16 spores may 

 be found. These germinations are given in the fourth column of table 3. 



A comparison of the observed and the theoretical germination shows a 

 great difference. Over the narrow range of volumes at which germina- 

 tion occurred, it will be seen that the observed germination was approxi- 

 mately equivalent to the theoretical for a volume only about one-fourth 

 as great. The same total amount of copper present in a very large 

 volume of solution is therefore less toxic than that present in a small 

 volume. 



This will appear contradictory to the previous statement, namely, that 

 toxicity is dependent on the total copper present regardless of volume or 

 concentration. Thus it would seem that toxicity is not entirely inde- 

 pendent of volume, the total copper content being the same. This dif- 

 ference may be explained as being due to the rate of diffusion. In the 

 case of a large volume of a dilute solution, the particles, or ions, of copper 

 are presumably rarer in a given space than they would be in a small volume 

 containing the same number of copper ions. Thus, in the former case, 

 in the proximity of the spore there would be present fewer ions than in 

 the latter case. So the final toxicity would, in a sense, be the resultant 

 of the rate of diffusion of the copper ions and the rate of germination of 

 the spores. The toxicity of a dilute copper-sulfate solution toward a 

 given number of spores is therefore determined, not only by the total 

 amount of copper present, but also to a certain extent by the rate of dif- 

 fusion of the copper ions. 



The results of these preliminary studies have shown that the toxicity 

 of dilute copper-sulfate solutions to the conidia of Sclerotinia americana 

 is dependent, not only on the concentration of the solution, but also on 

 the age of the spores, the number of the spores, the total amount of copper 

 present, and the volume of the solution. With increasing age the spores 

 become more copper-sensitive. The toxicity, or inhibition from germin- 

 ation of a given number of spores, is directly related to the total amount 

 of copper present in a given volume of solution. With the total copper 

 content and other factors held constant, the solutions become less toxic 

 with increasing volumes, the limiting factor being presumably the rate 

 of diffusion of the copper. 



