E. S. Salmon 



W^ 



laterals bearing very mouldy h()|)s gi-ew in among the lateral shoots of 

 OD 17 which bore a crop of hops practically immune from mould." 



There appears to be evidence, then, that the physiological or '"con- 

 stitutional" characters which underlie the phenomenon of "semi- 

 immunity" remain constant for the particular seedling, just as do those 

 which account for complete immunity or susceptibility. 



The phenomenon of "semi-immunity" has been met with in other 

 seedlings of widely different parentage. 



In the case of two seedlings (now planted out in the hop-garden under 

 Ref. nos. 222 a and 236 a) studied in the greenhouse during 1919, 

 infection scarcely proceeded further than the formation of small " humps " 

 or "bUsters," over which a scanty growth of hyphae occurred (never 

 white in the mass), with a weak growth of conidiophores. The mildew 

 soon died away, and exposed to view a patch of the underlying epidermal 

 cells which had turned brown. Exactly the same phenomenon was 

 observed with one of the seedlings in 1918. Both these seedlings were 

 raised from seed collected by me in Sept. 1916 from a hop-plant growing 

 wild on the sea-cliff at Salcombe Regis, near Sidmouth, Devonshire. 



Another form of "semi-immunity" was met with (under greenhouse 

 conditions) in a seedling (now Ref. no. 256 a in the hop-garden) raised 

 from seed collected in 1916 from a plant of the variety neo-niexicanus 

 planted in the hop-garden at Wye College — the male parent being 

 unknown. Here inoculation is followed by the production of a weak 

 growth of mycelium, scarcely white or "powdery," which soon dies 

 away, leaving pale-green or yellowish, semi-translucent patches of leaf- 

 tissue at the places w^here the mildew had previously existed. I have 

 seen somewhat the same phenomenon, although not so marked, in the 

 case of the var. neo-mcxicanus itself, when grown in the green- 

 house. 



