148 For7ns of the Hop resistant to Mildew 



All the above 53 "clone-plants" — cuttings taken from 14 seedlings — 

 became fully infected, and the mildew persisted on them until the end of 

 the growing season. 



The cuttings of both the "immune" and "susceptible" seedlings 

 enumerated above were all taken from the parent-plants, growing in 

 the Experimental Hop-garden at Wye College, in the winters of 1917, 

 1918 or 1919. In those cases where cuttings were taken in successive 

 years, no change from immunity to susceptibility, or vice versa, was ever 

 found. The 200 immune plants mixed in among the other seedlings (of 

 the same origin) white with mildew afforded the most convincing proof 

 of the different "constitutional" characters possessed by seedlings of a 

 wild plant. In many cases, e.g. with the cuttings of Z 24, Z 26, Z 41, 

 OA 35, OD 18, II 31, the "susceptible" parent-plant has been grown in 

 the hop-garden by the side, and within 3| feet, of the "immune" 

 parents (Z 25, Z 42, OA 34, OD 19, II 30), so that all these parent- 

 plants have thus been subjected to exactly the same soil, cultural, 

 manurial and weather conditions. It is clear that the differences in 

 susceptibility shown are inherent in the particular seedlings and not 

 induced by any special environmental conditions. 



The phenomenon of "semi-immunity^" was again shown by a few 

 seedlings when exposed to infection in the greenhouse. The details are 

 given below. 



Table III. 



List of Seedlings {of the same origin as in Tables 1 and II) showing "semi- 

 immunity," in the greenhouse, throughout the season of 1920. 



20 



The seedling Z 38 had not before been tested in the greenhouse; the 

 other seedlings had shown "semi-immunity" in previous seasons, 



A resume may now be given of the facts observed from 1914 to 1920 

 as to the susceptibility to mildew of the seedlings of the wild hop. From 



1 Described in Annals Appl, Biology, vi, 302 (1920). 



