192 F. T. Brooks. 
As regards the previous weather conditions, rain fell on June 
21st and 24th. 
June 30. One of the lower petioles of plant A part of which 
shewed a brownish discolouration, was placed in a collecting tube 
and despatched to London where it developed spores of Phy¬ 
tophthora infestans freely by July 3rd on the discoloured parts, 
thus shewing that this petiole had been affected by the blight 
fungus. 
July 4. There was no change in plant A. Other plants 
were as before. 
July 6. The brownish-black zones on one of the main stems 
of plant A had spread considerably. 
July 7. Another stem of plant A arising as a lateral from the 
main stem already mentioned, shewed discolouration all round at 
about the same distance above soil level and extending upwards 
about two inches. This black zone was not in continuity with the 
discoloured areas noted on June 25th. 
The plant A was now dug up and a second smaller main stem 
arising from the old tuber was found healthy throughout. On the 
main stem which shewed discoloured areas above ground no 
blackening was found below soil level, but three tubers were 
diseased. One of these, the largest, showed a large discoloured 
area which might have been infected via the stolon, another of 
intermediate size was attacked in more than one place and was 
certainly not infected via the stolon, and the third was a small 
tuber found at a lower level than the others shewing more than one 
diseased area, one of which was not connected with the stolon. 
The diseased parts of the two larger tubers were incubated and both 
developed Phytophthora in abundance by July 12th. 
Search was made on July 7th as before for any diseased shoot 
arising from the parent tuber of this plant or from a plant close by 
which might have been the source of infection of these blighted 
tubers, but without success. Spores may possibly have developed 
at some previous date on the discoloured areas already observed on 
plant A, but Phytophthora was not seen on them in the field. 
The old tuber of plant A was completely rotten by this date. 
Subsequent examination of the discoloured areas on the stems 
shewed that the fungus was confined to the cortex. 
There is no doubt that plant A was affected by Phytophthora , 
but the manner in which it became infected is obscure. The 
possibilities are discussed later. It appears certain that the fun¬ 
gus developed within the plot and was not brought there from outside. 
July 6-8. Seven other plants (Nos. 1-7) in this plot were 
