INSECTICIDES, FUNGICIDES, ETC. 187 



The efficiency of a fungicide in actually killing the fungus, or 

 in preventing the germination of its spores, appears from Clark's 

 work to be simply a question of the amount of soluble copper 

 present ; and in the case of every copper fungicide known there 

 is, either initially, or after it has been subjected to the action of 

 the air, sufficient soluble copper to kill the spores of most fungi. 

 Even in the case of the carbonate, malachite, the solubility may 

 exceed the average lethal strength of 0-0015 P er cent, found by 

 Clark (see p. 186) ; and, doubtless, the lethal dose would be 

 reduced if the action were prolonged for more than the 24 hours 

 which Clark allowed. 



In view of the presence in, or liberation from, the fungicide 

 of this soluble copper, it would appear unnecessary to resort to 

 any theory attributing its presence to the action of the fungus 

 itself. Yet the opinion that it is excretions from the fungus 

 itself which renders the copper soluble has often been put 

 forward, and has recently been pressed by Barker and Gimming- 

 ham. 1 There are certainly no reasons for regarding excretions 

 from an active fungus spore as being otherwise than probable, 

 but fungicides act on spores when they are in the dormant con- 

 dition, and can be excreting nothing; whilst it is highly im- 

 probable that they should ever start into activity, so as to 

 produce excretions, in a liquid containing a lethal dose of copper, 

 such as must, according to Clark's results, be nearly always 

 present in the case of these fungicides. 



The experiments at Woburn showed that there was no question 

 as to acid excretions from fungi (XI, 106), and that, even in cases 



1 Journ. Agric. Science, III, 69; VI, 220. Their main argument is based 

 on the fact that fungi are affected when in contact with the basic sulphate, 

 but not affected when in the supernatant liquid from which the basic sulphate 

 has been allowed to settle. This proves nothing. There is, of course, no 

 such thing as actual contact in such a case : the fungus and the basic 

 sulphate would always be separated by a film of liquid. On the one view, 

 the basic sulphate gives off particles which pass through this film and act 

 on the fungus ; on the other, the fungus gives off particles, which travel 

 to the basic sulphate, attack it, and then travel back to the fungus. Except 

 for there being a single journey in the one case, and a double one in 

 the other, the modus operandi is precisely the same, and the results 

 can not be invoked to settle between the two views. The reason why 

 the fungicidal action is inappreciable at some distance from the basic 

 sulphate, is very simple : the passage of the particles of copper salt from 

 the basic sulphate to the fungus must follow the laws of diffusion, and if 

 when the two are in so-called contact the distance between them is, say, 

 even as much as one-thousandth of an inch, their action on each other 

 would be a million times greater in a given time than if they were an inch 

 apart : if in the one case enough copper reached the fungus in one hour to 

 affect it, it would take (in the absence of convection currents) in years 

 for the same amount to reach it at the distance of one inch. No wonder 

 that no fungicidal action was recognised by Barker and Gimmingham in 

 the short duration of their experiments. 



