E. S. Salmon 307 



Seedlings which have been raised from Z 25 have shown interesting 

 deviations from normal susceptibility to mildew. The "seed" was 

 collected in 1917 from the plant of Z 25 growing in the hop-garden at 

 Wye College — the male parent being unknown. In 1919, 33 of these 

 seedlings (Ref. no. 4/17), 2-year-old and non-flowering, were grown 

 in pots in the greenhouse and constantly exposed throughout the season 

 to inoculation by conidia. The behaviour of the seedlings was as follows: 



Immune Seedlings Susceptible Seedlings 



24 9 



The immune seedlings were all entirely immune. 



The susceptible class comprised seedlings showing very different 

 degrees of susceptibility. One seedling was very susceptible and another 

 was normally susceptible — in both cases large "powdery" patches being 

 produced by the mildew soon after inoculation. The third seedling was 

 very slightly normally susceptible — that is to say, the few infection-spots 

 ])roduced consisted of normal "powdery" patches. The fourth seedling 

 might have been termed "semi-immune," except that occasionally 

 effused "powdery" patches were formed. The fifth, sixth, and seventh 

 seedlings were all similar, and were apparently "semi-immune," in the 

 sense in which this term is used above at p. 302. The last two seedlings 

 presented new types of susceptibility not previously found in seedlings 

 of the wild hop. In one seedling the slight susceptibility shown was 

 manifested by the production of scattered conidiophores diffused over the 



Bairn'. Temperature: In England South-East the departure from normal was as small as 

 + 0-la. The monthly aggregates of rainfall were below the normal. July. Cold generally ; 

 Dull in England. The mean monthly temperature was below the normal in all districts. 

 The rainfall was below the normal, although the shortage in England South-East was only 

 1mm. August. Many Hot days; Rainfall moderate; Sunshine abundant. Some warm days 

 during the first part of the month were nearly balanced by some cold ones during the latter 

 part, so that the mean for the month in most districts did not differ very largely from the 

 normal." 



The weekly deviation from Normal of the Mean of tlie Temperature of the Air (in 

 degrees Fahrenheit) at the recording stations in >S.E. England nearest Wye was as follows: 



There seems evidence then, for considering the temporary susceptibility shown by Z 25 

 at the beginning of August as an effect on the "constitution'' of the plant caused by the 

 abnormally low temperatures obtaining at the end of June and throughout July. 



Ann. Biol, vi 21 



