446 



CHEMICAL AGENTS 



1.0 10 100 



Concentration of metallic ion, ppm 



1000 



Figure 3. Dosage-response curves for the action of heavy metals on Stempliylium 

 sarcinaeforme. Ordinate is probit of per cent inhibition of germination, abscissa is 

 logarithm of concentration. Redrawn from more extensive data of McCallan, 

 Wellman, and Wilcoxon (252) and selected to show different types of curve. Re- 

 produced by permission of the Boyce Thompson Institute for Plant Research, Inc. 



174, 244, 446). Representative curves of this type appear in Figure 3. 



Three characteristics of the log-probit curve are of greatest interest: 

 position, shape, and slope. Lateral position is, of course, indicative of 

 absolute toxicity; either the ED 50 or the ED 95 — the concentration re- 

 quired to inhibit germination by, respectively, 50 and 95 per cent — 

 may be determined from the curve and used in the rating of fungi- 

 cides. Position is affected by several factors in the assay — the presence 

 of nutrients, the age of spores, the concentration of spores, extremes of 

 temperature, the germination time, and the physical or chemical form 

 in which a toxicant is supplied (49, 91, 144, 174, 252, 260, 265, 436). 



Non-linear curves are not, however, uncommon; thus, a detailed 

 study of the action of copper sulfate on Monilinia fructicola and Alter- 

 naria oleracea shows for both fungi a J-shaped curve, concave upward 

 (242). Parker-Rhodes (323) proposed that this curvature represents the 

 cumulative effect of more than one chemical or physical process in the 

 inhibition reaction and also detailed an exponential transformation of 

 the independent variable which converts the J curve to a linear form. 

 Use of the transformation does not require acceptance of the multi- 

 process theory of fungistatic action, some of the difficulties of which 

 are discussed by McCallan (244). Non-linear curves of other types are 

 frequently encountered (242, 244, 252). 



The slope of the log-probit curve is its most interesting feature. 

 Since the ordinate is based on the distribution of sensitivity in the 

 population of spores, a steep slope means that there is relatively little 



