30 S. E. A. McCallan 



Pickering attempts in a later paper (1912) to refute the criticisms of 

 Gimingham (1911) and of Barker and Gimingham (1911), and presents 

 new evidence. Air was passed through suspensions of the various basic 

 sulfates and copper carbonate in which bright iron rods were placed. 

 According to Pickering, the loss in weight of the iron rods should give the 

 amount of soluble copper abstracted from the suspension, and this is a 

 much more delicate test than that by the potassium ferrocyanide. Soluble 

 copper was detected, but in the case of bordeaux mixture there was none 

 until all the free lime had been carbonated. The respective amounts of 

 copper removed after three hours were: in the case of 4CuO, SO3 - 7.3 

 per cent; lOCuO, S0 3 - 4.2 per cent; lOCuO, S0 3 , 3CaO - 2.3 per cent; 

 and 2CuO, CO2 - 1.0 per cent. In another experiment, air containing 

 various percentages of CO2 was passed through a suspension of lOCuO, 

 S0 3 for three hours. The relative amounts of copper brought into solution 

 were: per cent C0 2 - 109; 0.03 per cent C0 2 - 100; 10 per cent C0 2 - 

 58 ; and C0 2 only - 834. The fact that, with the exception of an atmos- 

 phere of C0 2 only, the solvent action is correspondingly greater as the 

 amount of C0 2 in the air is reduced, reflects seriously on the possible role 

 of atmospheric C0 2 in bringing copper into solution. Pickering (1912: 

 274), however, maintains that the fungicide would be " bathed in an atmos- 

 phere of the carbon dioxide evolved from the leaves themselves." 



These conclusions of Pickering (1912) were further criticized by Barker 

 and Gimingham (1914 b). These workers point out that the amount of 

 C0 2 present in the atmosphere would retard rather than promote the 

 solution of copper, according to Pickering's own results. Also, they note 

 that the C0 2 from the leaves is rapidly dispersed. This assertion is based 

 on the results of an experiment in which a large strawberry plant was 

 covered with Woburn bordeaux paste (lOCuO, S0 3 ) and " placed in a 

 bowl of water under a bell-jar in order to keep the atmosphere saturated. 

 Under these conditions the action of any C0 2 respired by the leaves would 

 presumably be at a maximum " (page 221 of reference cited). After three 

 days no copper could be detected in the condensed water on the leaves. 

 The potassium-ferrocyanide test was employed. 



Groth (1910), studying bordeaux injury on peaches, was able to render 

 the copper soluble with distilled water and tap water as well as with rain 

 water. He found also that the lower surface of the leaf was apparently 

 more sensitive to injury than the upper. Peach leaves have no stomata 

 in the upper epidermis. In the laboratory, injury occurred only when 

 there was a covering of water on the leaves, a high relative humidity, and 

 some cloudy weather. Groth therefore offers the theory (page 19 of refer- 

 ence cited) : " An excess of C0 2 evolved in the shade, passes into the water 

 standing on the leaf during wet weather. C0 2 is dissolved, and the carbon- 

 ated water thus formed dissolves some of the copper." 



