E. S. Salmon 259 



On the other haiul, the difference may well be not in the fungus but 

 in the plant, which may change its power of resistance as the result of 

 changed conditions of growth. The chief differences between the con- 

 ditions in the hop-garden and in the greenhouse are the greater atmo- 

 spheric humidity and (particularly towards the end of summer) the 

 greater variation of temperature in the hop-garden, and the higher 

 temperature reached in the greenhouse. It is noteworthy that the 

 infection of the "immune" seedlings in the hop-garden is most evident 

 in the autumn when low temperatures^ occur at night. As regards the 

 effect of greenhouse conditions, the statement has been made by 

 Butler (5), p. 124, that "the mere growth of a plant under glass may 

 reduce its resistance. Several varieties of wheat are much less resistant 

 to black rust when grown in greenhouses than in the open, and the 

 same is true of some other rusts and mildews." Butler (5), p. 162, also 

 attributes the change from immunity to susceptibility sometimes shown 

 by Einkorn wheat in India to the effect of high temperature. In the 

 case of these hop seedlings, their transference to a higher temperature is 

 associated, apparently, with a change to immunity. 



SmiMARY. 



1.. Certain seedlings of the wild hop when grown in the greenhouse 

 are persistently immune to the attacks of the mildew Sphaerotheca 

 Humuli. This immunity has been shown by the same individual seedling 

 for three consecutive years. Under the same cultural conditions other 

 seedlings of the same parentage prove to be very susceptible. 



2. Certain seedlings (Group 2) which are immune when grown in 

 the greenhouse are also immune when grown in the open. These 

 seedlings have retained this immunity after four years' residence in a 

 hop-garden under normal conditions of cultivation and manuring. 



3. Certain seedlings (Group 3) which are immune when grown in 

 the greenhouse are susceptible when grown in the hop-garden; in some 

 cases the susceptibility shown is of the highest grade. 



4. "Cuts" taken from the seedlings of Group 3 in the same year in 

 which the seedling proved to be susceptible in the hop-garden are 

 immune in the greenhouse under cultural conditions in which "cuts" 

 taken from other susceptible seedlings in the hop-garden are very sus- 

 ceptible. 



^ It is possible that the low temperature increases the infection-powers of the conidia 

 of the mildew. In the Enjsiphaceae, as in the Uredineae, '"chilled" spores have increased 

 powers of germination. 



