18 Scientific Proceedings, Royal Dublin Society. 



iu a bouse without any artificial heat, and having tlie air exceptioually dry. 

 These pots were not placed under bell-jars. An equal amount of water was 

 supplied to each of the six pots. The stems and leaves of the three plants 

 grown under conditions of higli temperature and much moisture were 

 attenuated and weak. The Phytophtliova first appeared on these plants six 

 weeks after planting ; and a fortnight later all three plants were blackened 

 and destroyed by the fungus. The potatoes grown in the cool, dry house 

 were perfectly healthy when two months old. At this time one of the plants 

 from the cool house was removed to the hot, damp house, and placed under a 

 bell-jar. Within nine days this plant was completely blackened and killed 

 by tlie fungus. A fortnight later a second potato plant, showing no indica- 

 tion of disease, was removed from the cool to the hot house, and placed under 

 a bell-jar ; within a week this plant was also killed by the Fhytophthora. The 

 third plant was allowed to remain in the cool house, and at the end of thirteen 

 weeks, when the experiment ended, showed no trace of disease." 



A& a piece of scientific evidence in favour of the theory promulgated, this experi- 

 ment is of course absolutely wortltless, owing to the simp)le fact that no control 

 plants derived from healthy tubers -were used for j^arposes of comparison. 



The details given of the experiment are unfortunately all too few. The 

 time of the year at which it was carried out is not stated, nor is the mode of 

 attack, whetlier from below upwards or from the foliage downwards, described. 

 There is absolutely no evidence produced to show that the plants were not 

 attacked by " spores " in the ordinary way. It is of course conceivable that 

 the three plants in the warm house did become diseased from mycelium from 

 the tubers; but six weeks is, in my experience, a longer interval than usually 

 elapses in such cases. But even if they did, this occurrence has no bearing 

 on the idea of dormant mycelium. With regard to the two plants brought to 

 the warm house from the cool one, what could be simpler than to suppose that 

 tliey became infected from " spores " previously produced by the first three 

 plants ? There is not a particle of evidence to show that they did not 

 become attacked from this source ; and the view that this may have occurred 

 is strengthened by the fact that the single plant which was left in the cool 

 house did not become diseased. There is a danger of laying far too much 

 stress on abnormal weather-conditions as a necessity for the development of 

 Phytophthora. For rapid growth and spread resulting in an epidemic un- 

 doubtedly a spell of warm moist weather is almost a necessity ; but slow 

 gradual development can occur under mucli less exacting conditions. I found, 

 for instance, that infection and a comparatively slow but decided spread of 

 the blight occurred on potted plants in an absolutely unheated greenhouse 

 with a dry atmosphere even in the month of April, when the temperature 



