42 S. E. A. McCallan 



been unable to find any such evidence. All the positive evidence seems 

 unanimous in showing that the sphere of action of the spores is decidedly 

 limited. As to the nature of the solvent excretions, if any, there appeal's 

 to be no experimental information. 



OTHER HYPOTHESES 



A hypothesis that has received but scant mention in literature is one 

 which might be called cumulative action. This applies to very difficultly 

 soluble or even so-called insoluble fungicides. It is based on the assump- 

 tion that absolute insolubility is impossible. Theoretically, all com- 

 pounds in the presence of water may be expected to enter into solution 

 to a certain extent. The actual amount in solution may be the merest 

 trace, but it will exist in equilibrum with the insoluble part. If the solute 

 be removed, it is assumed that more will enter into solution to maintain 

 the equilibrum. In this manner an appreciable amount of the compound 

 may eventually come into solution, provided always, however, that the 

 solute is constantly removed from solution. 



Swingle (189G) perhaps has this hypothesis in mind when he suggests 

 that the protective action of bordeaux mixture is due to copper in solu- 

 tion which so weakens the germ tube of the fungus as to render penetra- 

 tion impossible. Swingle cites much of Nageli's work (1893) on oligo- 

 dynamic action, or poisoning by exceedingly small quantities of metallic 

 substances in solution. Lutman (1916) is inclined to favorably consider 

 Swingle's suggestion. On the other hand, Butler (1914) was unable to 

 demonstrate any evidence of oligodynamic action. 



Bell and Taber (1907) hold that minute traces of copper are in solution 

 in bordeaux mixture, and that the concentration of this soluble copper is 

 constant. 



Barker and Gimingham (1911:86-87) state that the minute traces of 

 copper in solution in bordeaux mixture may act as a cumulative poison. 

 However, in a later paper (1913:59), they state that the minute traces 

 of copper " which are present in solution" did not "act as a cumulative 

 poison." 



Pickering (1912), in his studies on the action of iron rods on the solubility 

 of lOCuO, SO3, is inclined to this hypothesis. He was, as has been noted, 

 able to render small amounts of copper soluble, and in discussing the 

 low degree of solubility of lOCuO, SO3 he makes the significant state- 

 ment (page 280 of reference cited) : 



The greater part of the copper in it may pass into solution and be utilised within a com- 

 paratively short time, provided there is any body present, such as iron, to react with it and 

 remove it from the solution. This will occur equally if it be removed by absorption into a 

 fungus spore or the leaf of a plant, and the absorption in that case should be at a much greater 

 rate than in these experiments, because the reacting surface exposed to a given amount of 

 copper would be far greater Thus we may get a very energetic absorption or 



