H. WORMALD AND L. K. WoK31ALD 201 



selected; the uppermost and lowest leaves were sprayed with freshly 

 made copper emulsion by means of an atomiser and the middle leaf left 

 unsprayed. When the sprayed leaves were dry, drops of distilled water 

 containing motile zoospores of Phi/(ophfhora infeslans were placed on all 

 three leaves by means of a platinum loop, two drops beinir placed on 

 each side of the midrib of the lower side of the terminal leaflet and the 

 second pair of leaflets in each case. The zoospores were obtained by 

 placing affected leaves with ends in water under a bell jar on the previous 

 day and leaving overnight. From one to two hours before the zoospores 

 were required, the portions of the leaves bearing sporangiophores were 

 brought in contact with a few drops of distilled water in a watch glass ; 

 numerous sporangia became free in the water and zoospores were liberated 

 within an hour. 



Three days later, on August 11, there were black spots on both 

 unsprayed leaves but none on any of the sprayed leaves. On August 13, 

 one of the sprayed inoculated leaves had fallen off so the observations 

 were not continued. 



On August 15, a second experiment was carried out on growing 

 potato plants in the garden. Three healthy leaves were selected on 

 successive nodes ; two of these, the uppermost and lowest, were sprayed 

 with freshly made emulsion and the other left unsprayed. About two 

 hours after the leaves had been sprayed, there was a heavy thunder- 

 storm lasting about ten minutes, so the leaves were not inoculated until 

 the next morning. These were inoculated in the way described in the 

 previous experiment. 



On August 20, the unsprayed leaf showed seven distinct diseased 

 spots though there were no spots on either of the sprayed leaves and 

 on September 11, when the whole of the unsprayed leaf was destroyed, 

 the whole of one of the sprayed leaves was quite uninjured; on the 

 other sprayed leaf, one leaflet, though not one of those inoculated, was 

 withered but otherwise the leaf was quite green and uninjured. On this 

 date, the shoot was removed from the plant and photographed (see 

 figure). 



The two lowest leaves shown in the photograph had become natur- 

 ally infected ; the two healthy leaves seen on the right had been spra3"ed 

 with the emulsion and then inoculated, and the diseased leaf occurring 

 between the two healthy ones had been left unsprayed but had been 

 inoculated. 



The accompan}'ing table shows in detail the observations made on 

 the inoculated leaflets. 



