E. S. Salmon r>()l 



the latter have grown for several years in a manured hop-garden are 

 immune to mildew when grown in the greenhouse. 



Although it seems clear from the above that the conditions of growth 

 obtaining in the greenhouse are more favourable for the phenomenon of 

 immunity than are those in the hop-garden, experiments have shown 

 that it is only with seedlings of a certain "constitution" that the en- 

 vironment has this effect on the plant. If cuttings are taken from other 

 susceptible seedlings (of the same parentage and age) in the -hop-garden, 

 and grown in the greenhouse, no approach is shown towards immunity. 

 In the winter of 1918-19 cuttings were taken from the following seedlings, 

 which had been affected with mildew during 1918 to the extent indicated : 

 Z 24 (S3). Z 26 (S3). Z 39 (S^), OA 26 (S^), OA 35 (S^), OA 53 (S^), OB 48 

 (S3), OD 16 (S3), II 11 (SI), II 31 (S3). These cuttings were grown, and 

 exposed to infection, in exactly the same way as the 53 cuttings of 

 the 19 '"immune" seedlings enumerated above at p. 298, and all quickly 

 became infected and continued so throughout the season. Most exhibited 

 what could be called normal susceptibility, but the seedlings Z 39, 

 OA 26, and II 31 showed extreme susceptibility under greenhouse 

 conditions. 



In no case has any seedling of the wild hop from Italy which has 

 proved immune in the greenhouse in any one season shown any sus- 

 ceptibility when tested in the greenhouse in other seasons^. Since this 

 immunity in the greenhouse is correlated with some degree of, if not 

 total, immunity when the plant is grown in the open, the index of re- 

 action to mildew of hop plants in the greenhouse becomes a useful guide 

 for the selection of mildew-resistant plants for the hop-garden. 



In a previous article ((3), p. 256) cases of "semi-immunity" were 

 recorded. Four seedlings were met with showing this phenomenon, viz. 

 Z 15, 0C6, Z23, and OA 33. During 1919 another "semi-immune" 

 seedling, OD 17, was found. The behaviour of all these seedlings was 

 studied both in the greenhouse and in the hop-garden. 



* In 1918 a certain seedling (Ref. No. 8/16 I) raised from a seed collected from a plant 

 of the "wild hop" of Canada (sent to me from Morden, Manitoba, in 1916 by Prof. W. T. 

 Macoun) growing in the hop-garden at Wye College, remained persistently immune in the 

 greenhouse throughout the season. This seedling was a 1-year-old plant of weak growth. 

 In 1919 the seedling, still kept in the greenhouse, was slightly more vigorous, and produced 

 two rather weak stems ( 1 f t. 9 in. and 1 ft. 6 in. in length), on most of the leaves of which 

 small "powdery" patches of mildew appeared — the plant being clearly susceptible to a 

 normal degree. The "wild hop" of Canada is probably H. americanus and the present 

 seedling is therefore most likely of hybrid origin. A similar case to the above, where the 

 seedling concerned was of hybrid (American) origin, has already been recorded ((3), p. 257). 



